PlumbMate

Three different figures. 48°C is the ADG3 ceiling at a domestic bath. 43°C is the Water Regulations ceiling at outlets in public buildings. 41–43°C is the TMV2 factory setting — a starting point, not a limit.

Test condition notation: 7°C air entering, 35°C water leaving. Ground source units use B for brine, as in B0/W45.

The lowest possible temperature, −273°C. Everything above it contains some heat energy, which is why a heat pump can still collect heat from air that feels cold.

The plate inside a flat plate collector, of copper, aluminium or stainless steel, that takes up the solar energy and passes it to the fluid channels bonded to it.

A person qualified to commission and carry out the annual test of an RPZ valve assembly.

The approved code of practice carrying the duty to assess and control the risk of legionella.

A system that moves the heat with a pump.

An unobstructed, complete physical break between the lowest point of discharge and the spillover or critical level of the receiving vessel. Not less than 20 mm or twice the internal diameter of the supply, whichever is greater.

The 25mm visible break at the tundish that prevents back-siphonage and shows when a valve is discharging.

Collects from outside air and delivers warm air indoors. Provides no domestic hot water, so it rarely suits a whole-house UK retrofit alone.

Air collected at a high point, blocking circulation. The first thing to suspect when the pump runs but no heat reaches the store.

The share of incoming solar radiation reflected straight back to space by cloud and by surfaces.

A vessel supplying drinking water to livestock. Fluid category 5, requiring an air gap, inflow control and a servicing valve.

An additive that raises a fire protection system from fluid category 2 to fluid category 4.

A device with an air inlet that opens to admit air if a vacuum occurs at the inlet, breaking the siphon. Also called a vacuum breaker.

A resilient mounting breaking the path by which compressor vibration would travel into the building structure.

The area of a collector that admits sunlight. The figure used in the MCS energy calculation, and smaller than the overall area.

A person approved by the water undertaker for the area, or certified by an organisation specified in writing by the regulator.

The Building Regulations guidance covering hot water supply and safety, including unvented systems.

Conservation areas, AONBs, National Parks, the Broads and World Heritage Sites. Solar equipment on a flat roof on such land requires prior approval.

A local authority direction withdrawing permitted development rights in a specified area.

A local authority direction withdrawing permitted development rights in a defined area.

A bidet with a spray inlet below the spillover level. Fluid category 5, never supplied directly from a supply pipe, and notifiable anywhere.

The pressure of the surrounding air (about 1013 mbar at sea level), at which water boils at 100°C.

An opening to the air that admits air to break a siphon, as in a DB or DC pipe interrupter.

A servicing valve with a variable orifice cartridge that caps the maximum flow rate irrespective of supply pressure.

Water flowing back into the mains from an installation, carrying contamination with it. Prevented by devices appropriate to the fluid risk category.

Flow upstream — in a direction contrary to the intended normal direction of flow, within or from a water fitting.

Backflow caused by the pressure downstream becoming greater than the supply pressure, pushing fluid back.

Backflow caused by the supply pressure falling, creating a negative pressure that draws fluid back towards the main.

An input to the MCS calculation. A boiler running in summer for hot water alone works well below its rated efficiency, so each solar kWh displaces more than a kWh of fuel.

Setting each circuit's flow to match its emitter's designed output, so rooms reach temperature together rather than the nearest circuits taking more than they need.

Mass added to a flat roof array to resist wind uplift. It must exceed the uplift with a margin, and the roof must be confirmed able to carry it.

Plastic pipe with an oxygen barrier layer in its wall. Without it, oxygen diffuses in continuously and corrodes every ferrous component in the system.

Plastics pipe with a permeation-resistant layer, for use where the ground may be contaminated by hydrocarbons.

The brine circuit pressure recorded at commissioning. The only reference by which a future engineer can recognise a slow loss from a buried collector.

A certification body whose approval is used to show an unvented unit meets the required performance.

The commissioning record listing the cylinder details, pressures, pre-charge and confirmation the safety devices were tested.

A tap with a horizontal inlet, typically wall-mounted. One to which a hose cannot be connected presents a lower backflow risk.

The outdoor temperature below which a bivalent system's supplementary heat source must contribute, where the falling output curve meets the rising demand curve.

A heat pump paired with a second, different heat source that contributes below a defined outdoor temperature.

The temperature at which water turns to steam; it rises as pressure increases.

A cistern providing a physical air gap between the supply and a downstream system, usually with a pump to restore pressure.

The water and antifreeze mixture circulated through a closed-loop ground collector. Routinely runs below 0°C in normal operation.

The pump moving the water and antifreeze mixture through the collector and evaporator. A parasitic load counting against the system's SPF.

The standard for above-ground screwdown stopvalves.

The standard for float-operated valves. Part 2 is the adjustable brass-bodied valve; Part 1 valves are not acceptable because the close level cannot be adjusted.

The standard for identification of pipelines and services — the basis for colour coding non-wholesome pipework.

The standard for plug cocks, acceptable underground in sizes 15 mm to 50 mm.

The standard for draining taps of the screwdown type.

The standard for fixed agricultural water troughs and water fittings.

The standard for cast iron gate valves, used at 50 mm and above.

The standard for thermal insulating materials on pipes, ductwork and equipment.

The standard for underground screwdown stopvalves of 15 mm to 50 mm nominal size.

The standard for buildings and structures for agriculture; Part 42 covers pig buildings.

The code of practice for the installation of thermal insulation.

The standard for electric surface heating — self-regulating trace heating used as frost protection.

The standard for the design, installation, testing and maintenance of water supply services.

The standard for suitability of non-metallic products in contact with water intended for human consumption, with regard to their effect on water quality.

The standard for jointing materials and compounds; Part 5 covers compounds for sealing screwed water fittings.

The standard for unsintered PTFE tape used for thread sealing, which must also satisfy BS 6920 Part 1.

The code of practice for preparing, commissioning and maintaining system water: cleaning, inhibitor dosing, filtration and testing.

The wiring regulations to which the electrical work must be carried out. Part P of the Building Regulations creates the legal duty.

The IET Wiring Regulations. Sets the technical requirements for the electrical installation — cable sizing, protective devices, earthing, bonding and testing.

The standard covering heat loss calculation methods.

The standard for soft solder alloys — lead-free tin/copper or tin/silver for capillary joints on copper.

The building services research body whose hot water demand formula is 46 + (26 × occupants) litres per day. It disagrees with SAP, so be clear which you are quoting.

A vessel adding system volume, providing a reservoir for the defrost cycle and, in some arrangements, hydraulic separation.

The local authority function that checks and certifies compliance with the Building Regulations.

A way of notifying Local Authority Building Control of work before it starts.

Dissolved hardness in raw water. Admitted to a heating system through a failed float valve, it causes rapid scaling.

A soldered joint in which molten solder is drawn into the gap between pipe and fitting.

Vapour bubbles forming and collapsing in a pump. Air in the primary causes it, so the pump churns rather than circulates.

A wall of two leaves with a cavity between. A pipe may pass through it but must never run within the cavity.

The Construction (Design and Management) Regulations. They apply to domestic work too, with the client's duties normally passing to the contractor.

Loft insulation laid at ceiling level. Pipes below it are within the thermal envelope; pipes above it are in extreme exposure conditions.

A groove cut into a wall or floor to receive a pipe. A fitting enclosed in one is a concealed water fitting.

A valve permitting flow in one direction only. Single check valves give fluid category 2 protection; two in series give category 3.

A non-return valve that prevents backflow of hot or expanded water into the cold main.

A round overflow, as used in type AF and type AG air gap arrangements.

A fixed container for holding water at atmospheric pressure.

The manufacturer's minimum spacing around a unit, for airflow and for service access. They differ on each face, so one figure must not be applied all round.

An insulation structure with sealed cells that will not absorb water — the type required for water pipes.

A sealed ground collector circulating a water and antifreeze mixture through buried pipework, exchanging heat with the ground by conduction.

Insulation that does not absorb water, so it keeps its value outdoors. Open-cell foam, fibreglass and mineral wool hold water against the pipe and are unsuitable externally.

Foam insulation whose sealed cell structure does not absorb water, so it retains its performance outdoors. Needs UV protection and must be continuous.

Heat output divided by electrical input at one stated set of conditions. A working heat pump always exceeds 1; a COP of 3.6 means 3.6 kW of heat per 1 kW of electricity.

The pressure the system is filled to when cold, taken from the static height of the system plus a margin so the highest point stays positively pressurised.

The system pressure when cold. The expansion vessel's gas charge is set to match it, and must be checked with the water side depressurised.

The sensor in the manufacturer's pocket at the collector flow outlet, reading the fluid actually leaving the collector. On the casing, in the return or taped to a pipe it means nothing.

Marking of pipes and cisterns to BS 1710 so non-wholesome pipework is easily distinguished from a supply or distributing pipe.

A single fitting delivering blended hot and cold water, subject to the same tap gap requirements as separate taps.

A cistern supplying cold water to a hot water system without a separate expansion cistern.

A valve capable of performing the function of both a temperature relief valve and a pressure relief valve.

Bringing an unvented system into service through a set sequence, proving every control and recording it.

The completed record MIS 3001 requires, produced by following a documented commissioning procedure.

The record of measured values and settings applied at commissioning. The baseline that makes later diagnosis a comparison rather than guesswork.

The route by which most domestic work is self-certified: the registered installer certifies and the scheme notifies the local authority.

A scheme allowing a registered installer to self-certify Building Regulations compliance, with the scheme notifying the local authority on their behalf.

A scheme whose registered members can self-certify their own notifiable work.

Raises the pressure of the refrigerant vapour, and its temperature with it. The only component consuming significant electricity, and where the upgrade happens.

A fitting installed below ground, passing through or under a wall, footing or foundation, enclosed in a chase or duct, or otherwise inaccessible or difficult to access.

The load-side heat exchanger. Hot high-pressure vapour gives up its latent heat to the heating water and condenses back to a liquid.

An area designated for its special character. An air source unit keeps its permitted development there, but may not go on a wall or roof that fronts a highway, or nearer a highway than the house.

Any reduction in the chemical or biological quality of water due to a change in temperature or the introduction of polluting substances.

The signed certificate an approved contractor must furnish on completion, stating whether the fitting complies with the Regulations.

The everyday thermostat that holds the storage temperature, about 60–65°C.

The written record of the commissioned controller values. It matters because settings drift, customers adjust them, and controls reset after a power failure.

The deliberately uninsulated pipework feeding the expansion vessel, so the diaphragm is not cooked by fluid arriving straight off a stagnating collector. The one place on a solar circuit you insulate less rather than more.

A chemical additive named in the fluid category 3 definition.

The multiplier converting a catalogue emitter output to its output at the actual design conditions. Divide the room's heat loss by it to find the catalogue size required.

A chemical dosed into system water to protect ferrous components. Depletes over time and is diluted by draining or topping up, so it must be verified at each service.

A chemical dosed into a heating circuit to protect it, and part of why primary water is not wholesome.

The regulations governing hazardous substances. For a heat transfer fluid the assessment is built from the manufacturer's safety data sheet for that specific product.

Regulations requiring assessment and control of risks from hazardous substances. Glycol falls under them on heat pump work.

The level of fluid in a receptacle under fault conditions, measured at least two seconds after closing the water inlet.

The heat exchanger inside a cylinder. Heat pump cylinders need a much larger coil — typically about 3 m², following the heat pump and cylinder makers' figures — because the primary temperature is far lower.

The discharge pipe from a safety valve to the tundish. Metal, not less than the nominal outlet size of the valve, and no more than 600 mm long.

The discharge pipe from the tundish to a safe, visible termination. At least one size larger than the valve outlet, with a 300 mm vertical drop below the tundish.

A length of pipe between the circulating hot water and an outlet. The water in it cools, so a long dead leg means a long wait.

A length of pipe holding water that does not move — a stagnation and bacterial risk, and on hot water a source of undue consumption.

The permanent weight of collectors, frames and any ballast.

A circuit serving the heat pump alone, correctly rated and protected. A plug and socket is unsuitable for a fixed appliance of this load.

The volume of the store served by the solar coil alone: at least 25 litres per net square metre of collector area.

Consent treated as granted unconditionally where the undertaker gives no notice within ten working days.

Briefly reverses the refrigerant cycle to melt frost off the outdoor coil. Frost blocks airflow and collapses heat absorption, and is worst in damp weather around 0–6°C.

A connection that consumes electricity, as a heat pump does. Distinct from generation connections, which are what G98 and G99 govern.

The vertical distance from finished ground level to the top of a buried pipe — not less than 750 mm and not more than 1350 mm.

A regional figure representing a cold but realistic winter condition, typically −1°C to −5°C in the UK. Not the coldest ever recorded.

The circulation rate specified for the primary: 0.5 L/min/m² at the low end, 0.6 typical and 0.7 at the high end, per square metre of collector.

The rate of flow that should be available at an outlet when only that outlet, or a group of outlets not exceeding 0.3 l/s, is discharging.

The water temperature the system is designed to run at. Unit output must be read at this condition, not at the headline test condition, or the installation is quietly undersized.

The site wind pressure derived under BS EN 1991-1-4 from the building height, the site altitude, the wind zone and the distance from the coast.

Breakdown of a fitting caused by the water reacting with the material itself, leaching zinc from brass and leaving a porous copper structure. Needs only one metal.

Working from the symptom to the cause rather than replacing components on suspicion. Gather information, verify, read the code, compare with the baseline, then test.

The controller comparing the collector sensor against the store sensor, running the pump only when the collector is meaningfully hotter than the part of the store it heats.

Sunlight scattered by cloud and atmosphere rather than arriving as a direct beam. It still carries usable energy, which is why UK systems work on overcast days.

An evacuated tube through which the heat transfer fluid itself passes — a wet connection.

An open-loop arrangement in which the potable water itself passes through the collector. Rare in the UK because of freezing, scaling and the absence of separation.

A system in which all hot and cold fittings are supplied from the supply pipe at mains pressure, with no cold storage cistern.

An unvented unit with its own gas or oil burner and factory-fitted controls.

A storage vessel in which the heat source, such as an immersion heater, is in direct contact with the stored water.

The metallic pipe from the safety valve to the tundish; at least the valve outlet size and no more than 600mm long.

The pipe from the tundish to a safe termination; 300mm straight drop then a 1 in 200 fall, one size larger than the valve outlet.

A device venting an intermediate pressure zone to atmosphere when backflow conditions occur — the CA device is a non-verifiable disconnector.

Treatment of a system after testing and flushing, required for new installations, major extensions, underground pipework, suspected contamination, or a system not in regular use.

Any pipe, other than a warning, overflow or flushing pipe, conveying water from a storage cistern or from hot water apparatus supplied from a cistern.

The company owning the local electricity cables and network, distinct from the energy supplier. Must be notified of the additional load a heat pump represents.

Switches flow between the heating and hot water circuits, usually with hot water taking priority.

Wider than 'house' — a hotel counts as domestic for the submerged inlet and shower hose rules. Healthcare premises never do.

Two check valves in series, giving fluid category 3 protection against both backpressure and backsiphonage. Type EC verifiable, type ED non-verifiable.

A hot water storage vessel with a separate feed and expansion cistern serving the primary circuit.

In the same direction as the flow of water — away from the source.

Dezincification resistant to approved quality — not less than 200 microns depth in any direction, marked CR or DRA and tested to ISO 6509.

A system in which the fluid falls out of the collectors under gravity into a drain-back vessel whenever the pump stops. Needs no antifreeze and no expansion vessel, and is inherently protected against freezing and stagnation.

A tap provided to drain supply and distributing pipes. Screwdown to BS 2879, or an approved spherical type only in a frost free location.

A drinking water facility whose jet nozzle must be at least 25 mm above the spillover level, with a screen or hood.

Holds a desiccant to absorb moisture and a filter to trap debris. Moisture can freeze at the expansion valve and forms acids that attack the compressor windings.

A flushing device offering two volumes, the lesser not exceeding two-thirds of the greater. A single flush cistern may not be replaced by one.

An enclosure carrying a pipe. Duct ends at a building entry must be sealed against gas and vermin.

Where the system flow rate, the solar loop pressure drop and the pump characteristics meet — the point at which the chosen pump will actually run.

Where the required flow rate and required head meet on a pump curve. Choose the speed curve passing on or above it.

Impose the duty to work safely on electrical systems, including isolating and proving dead before work begins.

A jointing method fusing plastic pipe electrically. The only acceptable method for a joint that must be buried.

Any device giving heat off into a room — radiator, underfloor circuit or fan convector. Output depends on the difference between its surface temperature and the room.

A device that shuts off the heat source if the water overheats. Must be non-self-setting where it substitutes for a dedicated cistern.

The resistance of a pipe run expressed as an equivalent straight length, counting bends — the basis for sizing a D2 discharge pipe.

An energy label allowing products to be compared on a consistent basis, showing an efficiency class alongside rated output and sound power level.

Inaccurate metering of water supplied by the undertaker. One of the five Regulation 3 offences.

Referring a fault on rather than improvising — for structural concerns, electrical work beyond your competence, unvented components without G3, or a suspected manufacturing defect under warranty.

Referring a fault to the manufacturer or a specialist because it lies beyond your competence or authorisation — refrigerant circuit work being the clearest case.

A substance of low toxicity named in the fluid category 3 definition.

A collector built from sealed glass tubes under vacuum. The vacuum removes conduction and convection losses, so the advantage shows in cold conditions and at higher working temperatures.

The source-side heat exchanger. Low-pressure liquid refrigerant boils here, absorbing heat from the air, ground or water.

Recovers heat from ventilation air being extracted from the dwelling. Output is capped by the ventilation rate, so it suits small, airtight properties.

A safety valve that discharges water to the tundish if pressure rises too far; typically 4 bar (copper) or 6 bar (steel).

A controlled restriction that drops the pressure of the liquid refrigerant, lowering its boiling point so it can absorb heat from a cold source. Also regulates superheat.

A pressure-activated valve designed to release expansion water from an unvented water heating system. Fitted close to the vessel with no intervening valves.

A vessel accepting the expansion of the fluid as it swings from cold fill to stagnation. Fitted on the return, with its connecting pipework left uninsulated as a cooling leg.

Accepts the volume increase of the system contents as they heat. Sized on total system volume, temperature range, fill pressure and relief valve setting.

A vessel connected to a water heating system to accommodate the increase in volume of water when it is heated from cold.

A diaphragm vessel with a gas charge that takes up the increased volume of water as it heats; fitted on the cold side.

Fluorinated gases used as refrigerants. They have a high global warming potential, are heavier than air, and decompose into toxic products when heated, so their handling is regulated.

The competence legally required to work on a refrigerant circuit. Needed for split system installation and for any recovery or recharging.

Heat conducted out through walls, roof, floor, windows and doors. Calculated element by element as U × A × ΔT.

An emitter using a fan to move air across a coil, giving useful output at low water temperatures from a compact unit.

A manufacturer-specific code indicating a detected fault. The same number means different things on different makes, so the manufacturer's documentation is essential.

The cistern serving a vented primary circuit, sized to accommodate the expansion of the circuit when heated.

A garden chemical which, applied through a hose, presents a fluid category 3 risk.

The equipment used to circulate fluid fast enough during filling to sweep air along with the flow to the vents.

A temporary connection with backflow protection used to fill a sealed system. Must be physically disconnected after use.

A temporary connecting pipe used to fill a sealed heating system, which must be disconnected after filling. Leaving one connected is an offence.

Proprietary sealing of a service penetration to restore the fire resistance of the wall or floor. Plastic pipe requires an intumescent collar.

A unit that runs at full output or not at all. Depends heavily on adequate system volume to absorb surplus heat in mild weather.

The near-instant conversion of superheated water to steam when pressure is suddenly lost, expanding about 1600 times.

The weathering detail at a penetration or abutment. Dressed under the tiles above and over the tiles below, so water discharges onto the covering.

Full load equivalent run hours — annual energy demand expressed as hours of running at full output. Higher FLEQ means a more demanding duty on the ground and a longer collector.

A valve controlling the inflow of water to a cistern, closing as the float rises. Must be adjustable and securely fixed.

The volume of water that must circulate to carry the heat, found from kW ÷ (ΔT × 4.18). Inversely proportional to the temperature difference.

Confirming the flow rate indirectly by measuring the temperature difference across the heat pump at full output and comparing it with the design figure.

One of the five categories of fluid described in Schedule 1, running from wholesome water to a serious health hazard.

Wholesome water supplied by a water undertaker, complying with the drinking water regulations.

Water whose aesthetic quality is impaired by a change in temperature, or by substances or organisms changing its taste, odour or appearance. Not a health hazard.

Fluid representing a slight health hazard because of the concentration of substances of low toxicity.

Fluid representing a significant health hazard because of the concentration of toxic substances, including pesticides and carcinogens.

Fluid representing a serious health hazard because of pathogenic organisms, radioactive or very toxic substances — including faecal, human or animal waste.

The quantity of water delivered by a flushing device — not exceeding 6 litres for a single flush.

Running water through each length of pipe after installation to remove swarf, jointing compound and excess flux.

A cistern provided with a valve or device controlling the discharge of stored water into a WC pan or urinal.

An acidic paste used when making capillary joints. Excess left in the pipe corrodes copper internally, which is why flushing matters.

A buffer with separate flow and return on each side, giving full hydraulic separation at the cost of mixing within the vessel.

The point at which water discharges to atmosphere — one of the datums used for setting pipe interrupter heights.

A cold burn caused by escaping liquid refrigerant, which evaporates instantly and draws heat from whatever it touches. Skin and eyes are both at risk.

A sealed primary that remains full at all times, so it must be specified for stagnation and needs antifreeze, an expansion vessel and a pressure relief valve.

Controls that protect the water supply and keep the system working properly.

A non-mechanical safety device that melts at a set temperature. Where fitted, the relief valve must operate at least 5 °C below it.

The qualification legally required to work on unvented hot water storage. MCS registration does not cover it.

Engineering Recommendations governing the connection of generation — solar, batteries, anything exporting. They do not apply to heat pumps, which are demand.

Corrosion driven by an electrical cell formed between two dissimilar metals in contact with water, attacking the less noble metal.

Zinc-coated steel pipe. Connecting it downstream of copper drives galvanic corrosion, because zinc is the less noble metal.

A valve closed by a sliding gate, used for cast iron stopvalves of 50 mm and above.

The statutory instrument that grants permitted development rights in England. Amended from time to time, so published guidance often lags behind.

A commercial appliance with a submergible spray hose, presenting a fluid category 4 risk.

The antifreeze used in ground collector circuits. A hazardous substance under COSHH, and extremely slippery when spilt. Ethylene glycol is far more toxic if ingested than propylene glycol.

Water recovered from baths, basins and showers for reuse. Fluid category 5, and restricted to uses that do not need wholesome water.

The heat that must come from the ground: output minus the compressor's electrical input. Sizing an array on full output makes it a third too large.

The HSE guidance on test equipment: probe tips not exceeding 4 mm, with 2 mm strongly recommended, and lead fuses typically not exceeding 500 mA.

A copper alloy resistant to dezincification, required for concealed fittings and mechanical backflow prevention devices.

Showing the customer the system, explaining not to tamper with the relief valves, and leaving the documentation and service advice.

The documentation given to the customer: commissioning record, manufacturer's instructions, compliance certificates, warranty registration and a schematic.

The resistance a pump must overcome, measured in metres. Comes from the pipework, fittings, valves and emitters on the hardest circuit.

The vertical overlap of one course of tiles or slates over the course below. Inadequate headlap lets water track underneath, worse at shallow pitches and on exposed sites.

The relationship a compensation control follows between outdoor temperature and flow temperature. Set as low as comfort allows.

How much heat a ground collector can draw per metre of pipe, in W/m. Depends on ground conditions and on the annual run hours.

Room-by-room calculation of how fast a building loses heat at design conditions. Sizes the unit and every emitter, so errors propagate through the whole design.

The a₁ and a₂ terms in a collector's efficiency curve, describing how quickly efficiency falls as the collector runs hotter than ambient. a₂ dominates at high temperature differences.

An evacuated tube containing a sealed working fluid that evaporates, rises to a condenser bulb in the manifold and condenses. A dry connection, so individual tubes can be replaced without draining.

The water and glycol mixture circulating in the primary. It carries less heat per litre than water and is more viscous, which affects flow rate and pump sizing.

A mechanical self-sealing waste valve that allows a discharge into a soil stack when fitted vertically with high-temperature pipe.

A non-self-resetting safety thermostat (about 85–90°C) that cuts the energy source if the control thermostat fails.

A protective shutdown when the condensing side cannot reject heat, usually because water flow has fallen. Points to the water side.

The drum orientation of a standard domestic washing machine, to which the 27 litre per kilogram limit applies.

Ground collector pipe laid in trenches at around 1.2 m depth. Cheapest to install but land-hungry, typically needing two to three times the heated floor area.

A first-aid firefighting outlet, fluid category 2 where the system contains no additives.

A tap to which a hose can be connected. Category 3 in a house garden; likely category 5 in non-domestic premises.

The return leg of a secondary circulation system.

A single dwelling. The Regulations treat work in a house differently for notification, and assign lower fluid categories to several of its appliances.

A heat pump and boiler under one controller that chooses between them based on outdoor conditions and relative running cost.

A fire main outlet. Fluid category 2 where the system contains no additives.

A urinal control triggered by use of a washbasin tap on the same circuit. Fails if the user does not wash their hands, or uses the hot tap.

Oil, diesel, petrol and similar substances, which permeate through plastics pipe and taint the water without any leak.

The deliberate gap between the switch-on and switch-off differentials, which stops the pump short-cycling on and off around a single threshold.

An electric heating element fitted directly into a hot water storage vessel.

The watertight lining or membrane every pond, fountain or pool must have to prevent leakage or seepage of water.

An area of covering is removed and the collector is weathered into the roof with a proprietary flashing kit, becoming part of the weathering.

The circuit with the greatest total resistance. Sizing the pump for it satisfies every easier circuit automatically.

An approved alternative to a warning pipe, giving visual or audible warning of impending overflow. Requires the float valve to close 50 mm below overflowing level.

An unvented cylinder heated by an external boiler through an internal coil.

A closed-loop arrangement in which a sealed primary of water and glycol carries the heat to a coil in the store, never meeting the drinking water. The UK domestic standard.

A system in which outlets are supplied from a storage cistern, usually with the kitchen sink kept on the supply pipe for drinking water.

An electronic presence detector used to flush a urinal only after use, typically requiring around ten seconds of presence.

The corrosion-inhibiting additives in the heat transfer fluid. Repeated stagnation exhausts them, after which the fluid turns acidic and attacks the system from within.

The jet arrangement in a type AD air gap, sometimes called a jump jet.

A pesticide. Applied through a hose it raises the risk to fluid category 5, unlike fertilizer at category 3.

The solar energy accumulated on a surface over a period, measured in kilowatt hours per square metre. A quantity, not a rate.

A heater raising water temperature on demand rather than storing it. Exempt from the unvented control requirements at 15 litres or less.

A valve limiting the temperature of pre-heated water presented to a downstream appliance that cannot accept it.

An expansion method using a floating baffle that traps and compresses a pocket of air; needs periodic recharging.

An overflow discharging into the back of the WC pan instead of an external warning pipe. Accepted conditionally, with a strainer at the float valve.

A cistern placed between the supply and an appliance, forming a type AUK1 arrangement — as in a conventional WC suite.

A unit that varies compressor speed to match demand, so it modulates rather than stopping. Gives long continuous running and the best seasonal efficiency.

The rate at which solar radiation arrives on a surface, measured in watts per square metre. A rate, not a quantity.

The test method for dezincification resistance, to which DRA fittings are tested.

A valve shutting off flow to part of a system. Where a urinal cistern fills automatically, one on a time switch plus a lockable one is required.

A valve used to shut the system off from the mains for maintenance.

Used with hemp on solar primary joints. PTFE tape is not dependable at solar temperatures, and soft solder cannot be relied on at stagnation.

A sealing paste for screwed water fittings, complying with BS 6956 Part 5.

A roof finish in which each element overlaps the one below, so water runs over the joints rather than through them. The lap itself is the weatherproofing.

The energy absorbed when a liquid boils into a vapour, released again when it condenses. Far larger than the energy involved in simply warming a fluid, which is why the circuit uses a change of state.

Formal written confirmation from the local planning authority that proposed work is permitted development. Worth obtaining on any borderline job.

A material prohibited in fittings conveying water for domestic or food production purposes, because it leaches into the water.

Bacteria that can proliferate in stored water within a range of temperatures. Managed by periodic pasteurisation rather than by continuous hot storage.

Bacteria that thrive in water between roughly 20 and 45 °C. The reason hot water is stored at 60 °C, reaches the outlets at 50 °C within a minute (55 °C in healthcare), and cold water is kept below 20 °C.

Bacteria that can grow in warm stored water; controlled by storing hot water at 60°C or above.

A filter fitted immediately upstream of a backflow prevention device required for fluid category 4, protecting the device seats from debris.

A stainless steel gauze filter that removes debris to protect downstream components.

A building of special historic or architectural interest. Permitted development does not apply; listed building consent is required for external alterations.

A slab laid over a pipe trench to spread imposed loads, extending not less than 250 mm either side, used where cover falls below 750 mm.

A washer plate not fixed to the valve spindle. Prohibited on stopvalves, servicing valves and draining taps because of unwanted non-return behaviour and noise.

A device hydraulically separating two circuits so they can run at different flow rates, while holding only a few litres. Solves flow matching, not volume.

Heat that is plentiful but at too low a temperature to be directly useful. A heat pump's job is to upgrade it to a temperature that will warm a building.

A protective shutdown when the evaporator cannot absorb enough heat. Points to the source side — a blocked or iced coil, or a collector problem.

An in-line filter capturing magnetite. Fitted on the return before the heat pump, and cleaned at every service.

The black iron oxide sludge formed by corrosion in a heating system. Blocks the narrow waterways of a plate heat exchanger far more readily than a boiler's.

Where multiple collector circuits come together, with isolation and flow measurement for each. The only point at which a buried array can be diagnosed.

Avoid, assess, reduce. Ask first whether the handling can be avoided altogether; good lifting technique is a control applied last, not first.

Warning tape laid above buried pipework during backfilling, to alert anyone excavating in future.

A change in the purpose for, or the circumstances in which, premises are used — notifiable in its own right.

The limit the roof structure can carry. Collectors, frame, ballast and access loads together must fall within it.

The greatest pressure a system is designed to see in operation, taking account of night-time static pressure and any pumps.

The Microgeneration Certification Scheme. A quality scheme rather than a legal requirement — but the gateway to grant funding.

Product certification for solar heating products. The CEN Solar Keymark is the alternative route.

Certification for mounting and flashing kits, with declared wind uplift resistance and weathertightness — valid only within the limitations of the certificate.

The MCS planning standard. Sets out how to calculate the sound level from a proposed unit at the assessment position, to demonstrate the noise condition is met.

The solar domestic hot water energy calculation.

Sets out the safe sequence of work putting the risk assessment's control measures into practice.

A voluntary quality scheme. Not a legal requirement, but the gateway to government grant funding and a recognised consumer code.

A garden watering system such as a porous or perforated hose, treated as fluid category 5 where laid at or near the soil surface.

The rate of flow that must still be achieved at an outlet when simultaneous discharge occurs elsewhere in the system.

The water content a manufacturer requires for a given unit at its permitted starts per hour. Met by pipework, emitters and any added vessel.

The MCS installation standard for solar heating, covering design, sizing, installation, commissioning and handover.

The MCS installation standard for heat pump systems, governing design, sizing, installation, commissioning and handover.

Water used for a purpose to which it is not properly put and then discharged. One of the five Regulation 3 offences.

An air source unit with the entire refrigerant circuit sealed and factory-charged outdoors. Only water pipes enter the building, so no on-site refrigerant work is needed.

A system with one heat source, sized to meet the full design peak heat loss with no supplementary heat at any outdoor temperature.

The nearest habitable room window of a neighbouring property, where the sound level from a proposed unit is assessed. The condition protects neighbours, not the installing property.

A check valve preventing the circuit thermosyphoning backwards on a clear night, which would carry heat out of the store, up to the roof, and radiate it to the sky.

A safety cut-out that stays tripped until it is reset by hand, so the fault must be found and fixed before resetting.

Describes a thermal energy cut-out that will not reset itself, so a fault cannot cycle indefinitely.

An unvented hot water system with a capacity of 15 litres or less.

Giving notice to the water undertaker before beginning work listed in the Regulation 5 table, and not starting without consent.

The moving element that seals against a seat in a valve — a ceramic disc or a washer.

Brackets pass through the covering to the structure and the collector sits proud of the tiles.

A ground or water source arrangement that abstracts water, passes it through a heat exchanger and returns or discharges it. Requires an abstraction licence.

A hot water system fed by gravity from a cold cistern, with an open vent pipe to atmosphere.

A fitting such as a stopvalve, servicing valve or draining tap that must be readily accessible for operation and maintenance.

A bidet with taps above the spillover level and no ascending spray or flexible hose. Needs only a type AUK2 tap gap.

The complete outside dimensions of a collector including its frame. Used for layout and for wind loading — never for the energy calculation.

A pipe from a cistern in which water flows only when the water level exceeds a predetermined level.

The level at which water would overflow a cistern. Float valves close 25 mm below it, or 50 mm where an alarm replaces the warning pipe.

Obstruction of the collector by buildings, trees or roof features. An input to the MCS energy calculation, and far more damaging to yield than imperfect orientation.

Selecting a unit larger than the design heat loss requires. Causes short cycling rather than providing a safety margin.

Oxygen diffusing into system water through non-barrier plastic pipe. Drives continuous corrosion of ferrous components and produces magnetite sludge.

The backflow prevention paragraph of Schedule 2 — requiring an adequate device against backflow from any appliance, fitting or process.

The cold water services paragraph of Schedule 2 — cistern inlets, outlets, warning pipes, covers and insulation.

The WCs, flushing devices and urinals paragraph of Schedule 2.

A collector pipe configuration, sometimes called a harp, where several parallel risers run between a top and bottom header.

Building Regulations covering structural stability — relevant when forming openings for pipework or supporting the weight of units and filled cylinders.

Building Regulations covering fire safety, including fire stopping where services pass through a compartment wall or floor.

Building Regulations covering resistance to contaminants and moisture — weatherproofing and making good penetrations through the external envelope.

Building Regulations covering hot water storage and safety, including unvented systems and protection against scalding.

Building Regulations covering insulation, system efficiency, controls and commissioning. Remember it as L for Loss of heat.

Building Regulations covering electrical safety in dwellings. Creates the legal duty; BS 7671 describes how to satisfy it.

A valve that leaks past its seat when it should be shut — for example a pressure-reducing valve passing lets the system pressure creep up.

A system relying on natural circulation rather than a pump.

Raising the whole stored volume to at least 60 °C to control legionella. Where solar cannot achieve it, the auxiliary source must.

A periodic heating of the full cylinder contents to 60°C to manage legionella risk, needed because heat pumps store water at temperatures within the growth range.

A disease-causing organism. Its presence places a fluid in category 5.

The property-specific annual energy figure produced by the MCS calculation and given to the customer before they commit. Any later dispute is measured against it.

A property-specific statement of expected output, efficiency and running cost, given to the customer before they commit. The benchmark for any later dispute.

The passage of contaminants through the wall of a pipe into the water, without any leak. Plastics are permeable to hydrocarbons.

Planning permission granted nationally by statutory instrument. Solar equipment must project no more than 200 mm from a pitched roof slope, or sit no more than 600 mm above a flat roof.

Planning permission granted nationally by statutory instrument, so no application is needed provided every condition is met. The conditions are amended periodically.

A small orifice controlling the slow fill of an automatic urinal cistern. Prone to blocking.

A non-mechanical backflow device with an atmospheric vent that admits air to break a siphon. No control valve may be fitted downstream.

The unit by which dishwasher water consumption is measured — not more than 4.5 litres per place setting.

A quarter-turn tapered plug valve, acceptable underground in 15 mm to 50 mm sizes.

A concentrated weight bearing on one part of a structure, such as a filled cylinder on an upper floor. Must be checked against the structure's capacity.

Backflow protection at the individual appliance, fitting or process. Required by paragraph 15, and never replaced by zone or whole-site protection.

An ornamental water feature requiring an impervious lining, with no permanent or direct connection to a supply or distributing pipe.

A permeable hose used for irrigation. Category 5 where laid at or near the soil surface.

The gas-side pressure set in an expansion vessel. It must be set and checked with the system isolated and the water side depressurised to zero, or the reading means nothing.

The air pressure set in an expansion vessel with the system depressurised, sitting just below the expansion-valve setting so the vessel can take up expansion.

Confirming both that the system genuinely works as intended and that all documentation and certificates are complete and ready to issue.

The structured set of checks completed before work begins, producing a recorded decision on whether the job can proceed as specified.

The building or land to which water is supplied. Whether premises are a house or not determines many fluid categories.

A WC flushing device using supply pipe pressure to compress air and increase the pressure available for flushing.

A self-closing valve supplied directly from a supply or distributing pipe which discharges a predetermined flush volume when activated. Not permitted in a house.

A valve holding a constant downstream pressure irrespective of the upstream pressure, used to control excessive static pressure.

A valve that sets and stabilises the working pressure regardless of the mains pressure.

The last line of defence against overpressure, discharging safely and visibly. A valve quietly passing is how a system loses pressure and freeze protection unnoticed.

A pressure-activated valve which opens automatically at a specified pressure to discharge fluid.

A protective device that shuts the compressor down at abnormal pressure. Which one has tripped — high or low — tells you which side of the machine to investigate.

A cheaper, less precise alternative to a PRV, used on higher-pressure steel units.

Water fittings in which water circulates between a boiler or other heat source and a primary heat exchanger inside a hot water storage vessel, including any space heating system.

A lighter planning submission than a full application, but still one that must be decided — required for solar equipment on a flat roof on Article 2(3) land.

Tilting a surface away from the incoming beam spreads the same energy over a larger area, so the energy received per square metre falls. The reason orientation and tilt matter.

The antifreeze normally specified for solar primaries, chosen because it is far less toxic if ingested than ethylene glycol.

Unsintered polytetrafluoroethylene tape for thread sealing, to BS 6974 and satisfying BS 6920 Part 1.

A manufacturer's chart showing what a pump delivers at each speed. Each curve slopes downwards, because moving more water leaves less pressure available.

The assembly carrying the circulator, isolation, check valve, flow indication, pressure gauge, relief valve and vessel connection, usually mounted near the cylinder.

Circulating until the fluid runs clear and free of bubbles and flow through the meter is steady rather than surging.

Removing trapped air from a filled circuit. A ground collector needs high-velocity circulation from a powered rig, because air vents cannot reach air trapped mid-array.

A hybrid panel producing both heat and electricity from the same unit.

A system collecting rainwater for reuse. Fluid category 5, requiring separation, identification and a category 5 air gap on any mains back-up.

Setting up and proving the affected parts of a system after a repair, confirming the fault is cleared and nothing was disturbed in the process.

An outdoor unit drawing back its own discharged cold air because clearances are inadequate. Lowers the source temperature it sees and collapses output.

Water recovered for reuse rather than supplied by the undertaker. Fluid category 5, and colour coded green, black, green.

The intermediate zone in a BA device, held below the upstream pressure and vented by a relief valve if backflow conditions arise.

The instrument used to measure glycol concentration and read off the freezing point achieved. The only honest way to verify freeze protection.

An instrument measuring antifreeze concentration in a sample. Used to verify freeze protection rather than calculating it from what was added.

The working fluid in a heat pump's sealed circuit, chosen because it boils at a very low temperature. It changes state twice on every lap of the circuit.

Enforcement — placing the duty to enforce on the water undertaker for its area of appointment.

Application of the Regulations — what they cover, the temporary supply exemption, and the protection for work lawfully installed before 1 July 1999.

The restriction on installation and use — the regulation creating the five offences of waste, misuse, undue consumption, contamination and erroneous measurement.

Requirements for water fittings — appropriate quality and standard, suitability for the circumstances, and installation in a workmanlike manner.

Notification — the table of work that must be notified to the water undertaker before it begins.

Contractor's certificate — the requirement for an approved contractor to certify compliance on completion.

Penalty for contravening the Regulations, and the defence available to an owner or occupier.

The Secretary of State, or in Wales the National Assembly — the body that approves specifications and authorises relaxations.

The approved specification for backflow prevention arrangements and devices, setting out the types and their fluid category ratings.

An authorisation from the regulator permitting a Schedule 2 requirement to be relaxed in a particular case. Only the undertaker may apply.

A chemical fixing dispensed under pressure. The resin is an irritant, so eye protection is required, and drilling produces dust and debris.

Swaps the roles of the two heat exchangers, so the outdoor coil temporarily becomes the condenser. This is how the defrost cycle works.

Tapping an abrasive wheel with something non-metallic and listening for a clear ring, to check for cracks before use.

Identifies hazards, evaluates who might be harmed and how, and records the control measures. Comes before the method statement.

The part of the roof an array occupies — centre, edge or corner — which determines the pressure coefficients applied to the design wind pressure.

Reduced pressure zone valve assembly — the type BA device. Three pressure zones with a relief valve that opens rather than allowing backflow. Requires annual testing by an accredited tester.

A magnesium anode in a glass-lined mild steel cylinder that corrodes in place of the steel. Checked and replaced typically every two to three years.

A replaceable metal rod that corrodes in place of the cylinder to protect it from corrosion (used in copper cylinders).

Identify, inform, switch off, secure and tag, prove the indicator, test dead, then re-prove the indicator — the last step catching a tester that failed during the test.

The seven-step NICEIC procedure: check, isolate, secure, label, prove the indicator, test dead, re-prove. Securing happens at the moment of isolation, before testing dead.

Controls that protect the user from overheating and scalding.

The manufacturer's document giving a product's hazards, PPE, first aid, spillage procedure and disposal route. The primary document for any COSHH assessment.

The manufacturer's document giving a substance's hazards, required PPE, first aid, spillage procedure and disposal route. The source for a COSHH assessment.

The Standard Assessment Procedure. Its hot water demand formula is (25 × occupants) + 36 litres per day at 52 °C.

The schedule setting out the five fluid categories.

Requirements for water fittings — the technical body of the Regulations, carrying their full legal force.

A mesh-protected air inlet allowing a covered cistern to remain at atmospheric pressure while excluding insects.

A valve closed by screwing a washered head down onto a seat. Required for underground stopvalves and, normally, draining taps.

A heating system pressurised without an open vent or feed and expansion cistern.

The primary heating circuit used as a heat source; not the unvented hot water supply.

Efficiency calculated across a whole heating season and a range of outdoor temperatures. A more realistic comparison figure than a single-point COP.

Total useful heat delivered divided by total electricity consumed by the whole system over a year, as installed. Counts pumps, controls, defrost and any immersion top-up.

Water fittings in which water circulates in the supply or distributing pipes of a hot water storage system.

A return leg with circulator and check valve keeping hot water moving past distant outlets, so it arrives without a long draw-off. Must be insulated. On a heat pump it is a last resort, and the risk assessment then usually calls for more frequent pasteurisation of store and loop.

A hot water return circuit keeping the distribution charged with hot water, removing the dead leg but requiring the return to come back at 50 °C or more (55 °C in healthcare).

The cold feed pipe, hot water storage vessel, water heater and pipework from which hot water is conveyed to all points of draw-off.

The absorber surface treatment that takes in a high proportion of incoming solar radiation while re-radiating very little infrared back out.

A tap that shuts automatically when released. A basin fitted with them needs no plug, because it cannot be left running.

Premises in distinct occupation, the trigger for whole-site backflow protection and for shared stopvalve access.

A collector pipe configuration where a single continuous pipe snakes back and forth across the absorber.

A raised protective enclosure over an inlet arrangement, used on agricultural troughs where it does not compromise the air gap.

The record of a service visit: date, checks made, readings obtained, parts fitted, defects found and next service due. Readings are what reveal a developing problem.

Space to reach a component for maintenance, as distinct from space to install it. The check most often forgotten, and the one that causes trouble for years.

A valve for shutting off the flow of water to a fitting for the purpose of maintenance or service.

Repeated starting and stopping over short periods, caused by the unit producing more heat than the system can absorb. Wears the compressor and depresses seasonal efficiency.

Water passing between two cistern connections without circulating through the vessel, leaving stagnant volume behind.

A terminal fitting. One that is fixed, or on a hose that cannot be submerged, requires no backflow protection.

A window on the liquid line. Clear liquid at steady state is normal; persistent bubbles suggest an undercharge or non-condensable gases.

One check valve permitting flow in one direction only. Fluid category 2 protection in both modes.

A hot water storage vessel in which primary and secondary water are separated by an air bubble rather than a second cistern. Requires a permanent vent to atmosphere.

The traditional valveless WC flushing mechanism — inherently reliable at not leaking, since there is no seal to fail.

Describes the valveless flushing mechanism relying on a siphon, required for WC cisterns installed before 1 January 2001.

The liner around a pipe passing through a wall or roof, letting the pipe move with thermal expansion without abrading or being bonded into the structure.

Passing a pipe through a sleeve where it crosses the building fabric, so it can move with thermal expansion without abrading against the masonry.

Overlapped collector pipe laid in a trench, fitting more pipe into a shorter trench. Saves trenching length rather than ground area.

Chloros and common disinfectants — named in the fluid category 3 definition.

The share of annual hot water demand supplied by solar. UK designs target 50–60%; pushing beyond it plateaus in benefit and increases stagnation.

The CEN certification scheme for solar thermal products, an alternative to MCS 004.

Photovoltaics — a different technology producing ELECTRICITY rather than heat. Frequently confused with solar thermal.

A system using solar energy to produce HEAT for domestic hot water. A well-designed UK system supplies around 60% of annual hot water, strongly seasonal.

An electrically operated valve. Being a control valve, it must never be fitted downstream of a DB or DC pipe interrupter.

A floor of solid construction. No water fitting may be embedded in one.

The manufacturer's stated noise output for a unit, used as the input to the MCS 020a planning calculation. Not itself the limit that must be met.

A pressure test to confirm pipework is leak-free, carried out before anything is insulated or concealed.

The hydraulic pressure test proving an installation before it is first used, at not less than 1½ times maximum operating pressure.

The naming convention for heat pumps. The first word is where heat is collected from, the second is what it is delivered into — so air-to-water collects from air and heats a wet system.

The pipe delivering flushing water into a urinal bowl. The datum for the 150 mm pipe interrupter measurement.

The energy needed to raise one kilogram of a substance by one degree. For water it is 4.18 kJ/kg·K, the constant in the flow rate formula.

A quarter-turn ball valve. Acceptable above and below ground, but only with an integral handle or lever when used as a stopvalve.

The level at which the contents of a receiving vessel spill over its top edge when inflow exceeds outflow. The datum for measuring air gaps — the overflow is ignored.

An air source unit with the evaporator outdoors and the condenser and compressor indoors, joined by refrigerant lines charged on site. Requires an F-Gas qualified operative.

A tap delivering a fine spray at low flow. An appliance whose only taps are spray taps needs no plug.

A fire protection system. Category 2 without additives; category 4 where anti-freeze is used.

The clearly identified, correctly rated fixed supply to the solar controls, typically 3 A, with local isolation and a label saying what it feeds.

The collector receiving solar energy while the pump is not running and no heat is being removed, so it heats until its losses equal the energy arriving.

Water standing without movement, allowing bacterial growth. The reason for bottom outlets, cross-cistern circulation and eliminating dead legs.

The temperature a collector reaches under stagnation — roughly 150–200 °C for a flat plate, and above 250 °C for evacuated tubes.

The surge drawn as a compressor starts, higher than running current. Matters for supply adequacy, particularly on fixed-speed units.

The manufacturer's limit on compressor starts, commonly three to six. Fewer permitted starts means longer runs, which requires more system volume.

The pressure in a system when no water is flowing. It peaks at night when demand on the main is lowest.

A noise or other nuisance a local authority can investigate under environmental legislation. Passing the planning noise assessment is not an absolute defence.

Stable running conditions, reached after warm-up. Commissioning readings taken before this point do not represent normal operation.

The initial resistance to movement in a valve at high static pressure, common with ceramic disc taps.

A valve, other than a servicing valve, used for shutting off the flow of water in a pipe. Its symbol includes a dot in the centre.

A cistern for storing water for subsequent use, not being a flushing cistern.

An unvented hot water system with a capacity of more than 15 litres.

The sensor low in the cylinder, in the zone the solar coil heats. Fitted high it reads boiler-heated water, the controller sees no useful difference, and the pump never runs.

The natural layering of hot water above cold in a store. Mixing it presents the solar coil with lukewarm water instead of cold, so transfer and collector efficiency both fall.

The compacted layer beneath a concrete base, spreading the load and preventing settlement. A 100 mm slab on poor sub-base will still move.

An inlet discharging below the spillover level of an appliance. Category 3 in a house or domestic situation, category 5 elsewhere.

How far the refrigerant vapour has been heated beyond its boiling point. Maintaining it protects the compressor from liquid carry-over.

Water heated above 100°C but kept liquid because it is under pressure.

So much of a service pipe as is not vested in the water undertaker — the customer's side of the boundary.

A transient pressure increase, also called water hammer, generated by the rapid closure of a valve. Not accounted for by the test pressure.

A floor with a void beneath. A pipe entering through one needs insulation at any distance, because the void is ventilated and cold.

Metal filings and debris left in pipework by cutting and installation, removed by flushing before the system is used.

A pool requiring an impervious lining. One over 10,000 litres replenished automatically is notifiable.

The smaller temperature difference at which the controller stops the pump, typically 2–4 K.

The temperature difference at which the controller starts the pump, typically 6–8 K.

Efficiency measured across the whole installation rather than the unit alone — including auxiliaries, distribution, controls and storage losses.

The drawing of the installed system handed over to the customer as part of the documentation pack.

The vertical distance between the lowest part of a tap outlet and the spillover level of the appliance it discharges over.

A last-line safety valve that discharges hot water at about 95°C to stop the store overheating.

The gap between flow and return temperatures. Heat pump systems are designed around about 5K, against 20K on boiler systems.

The gap between the source temperature and the flow temperature delivered. The larger the lift, the harder the compressor works and the lower the efficiency.

A valve which opens automatically at a specified temperature to discharge fluid, located directly on the storage vessel.

A water outlet device — a tap, shower head or similar.

The plastics soundness test in which pressure is held by pumping for 30 minutes, deliberately reduced to one third, then must not drop over the following 90 minutes.

The plastics soundness test in which pressure is noted after 30 minutes of pumping, then must not drop by more than 0.6 bar over 30 minutes and a further 0.2 bar over 120 minutes.

A tapping on a verifiable device allowing its operation to be checked in situ.

Not less than 1½ times the maximum pressure to which the installation is designed to be subjected in operation.

The heated part of a building. Fittings outside it, or outside the building, need protection against freezing.

A separate vessel heated by solar that pre-heats the feed to an existing cylinder or appliance.

A vessel storing heated primary water. Mains cold water passes through a heat exchanger within it and is heated on demand as it flows.

A valve blending stored hot water with cold at the outlet, so a store hot enough to disinfect is not hot enough to scald.

Blends stored hot water down to a safe delivery temperature at the outlet, and shuts off if the cold supply fails. Store hot, deliver safe.

A valve blending hot and cold water to a controlled temperature at the point of use — required at public outlets so discharge does not exceed 43 °C.

A valve that blends hot and cold to a safe outlet temperature (max 48°C at the bath) to prevent scalding.

Controls one emitter's output by room temperature. On heat pump systems, TRVs closing reduce circulating volume and can drive cycling.

Circulation driven by density difference alone. The passive system case, requiring the store above the collector — and the effect a non-return valve exists to prevent running backwards at night.

In a drain-back system, the internal volume of the collectors plus all primary pipework above the drain-back vessel — everything that empties on a stop and refills on a start. It sizes the vessel and sets the pump's lift and priming duty.

A clock-operated control regulating when an automatic urinal cistern may fill. Required with a lockable isolating valve.

The manufacturer's stated water content for a unit at its permitted starts per hour. Subtract the emitter circuit volume to find the buffer size needed.

Self-regulating heating cable to BS 6351, used with insulation as an accepted method of frost protection.

A pipe fitting, or part of a sanitary appliance, that retains liquid to prevent the passage of foul air.

A funnel in a discharge pipe providing a visible air break between the safety valve (D1) and the discharge to outside (D2).

An open funnel that gives a visible 25mm air gap in the discharge, showing when a valve is discharging.

A cylinder with two heating coils — the solar coil low and the auxiliary coil above it, so the collector meets the coldest water in the store.

A cylinder with two internal coils so it can take heat from two sources (e.g. solar to the lower coil, boiler to the upper).

A buffer connected in series with the circuit. Simple, with no temperature blending, but no hydraulic separation either.

Opens or closes one heating zone in response to that zone's own demand.

An upward flowing supply or distributing pipe surmounted by an anti-vacuum valve, with any part of the outlet not less than 300 mm above the spillover level of an appliance.

Air gap with unrestricted discharge above the spillover level. Fluid category 5 against both backpressure and backsiphonage.

Air gap with weir overflow. Fluid category 5 in both modes — the practical category 5 arrangement, since cisterns must have lids.

Air gap with vented submerged inlet. Fluid category 3 in both modes.

Air gap with injector, sometimes called a jump jet. Fluid category 5 in both modes.

Air gap with circular overflow. Fluid category 4 in both modes.

Air gap with minimum size circular overflow. Only fluid category 3 — despite beginning with A. Created by a BS 1212 Part 2 float valve in a cistern.

Air gap with interposed cistern. Category 3 against backpressure but category 5 against backsiphonage — the only arrangement with a split rating.

The domestic tap gap. 20 mm up to G½, 25 mm above G½ up to G¾, 70 mm above G¾. Category 3 backsiphonage; no backpressure protection.

The higher risk tap gap — not less than 20 mm or twice the inlet bore, whichever is greater. Category 5 backsiphonage. Sinks anywhere, non-domestic appliances, healthcare.

A branch pipe serving an appliance where the branch connection to the vented distributing pipe is not less than 300 mm above the spillover level, or the highest discharge point served, whichever is higher.

Verifiable backflow preventer with reduced pressure zone — the RPZ valve. Fluid category 4 in both modes, and the only mechanical device that reaches category 4.

Non-verifiable disconnector with pressure zones differing by not more than 10%. Category 3 in both modes, and may be left permanently connected.

Anti-vacuum valve, or vacuum breaker. Category 3 backsiphonage only, fitted on a 300 mm Type A upstand with no valve on the outlet.

Pipe interrupter with atmospheric vent and moving element. Category 4 backsiphonage only. No control valve may be fitted downstream.

Pipe interrupter with permanent atmospheric vent. Category 5 backsiphonage only. Vertical pipe, flow downwards, no control valve downstream.

Anti-vacuum valve combined with a single check valve upstream. Category 2 backpressure, category 3 backsiphonage.

Verifiable single check valve. Fluid category 2 in both modes.

Non-verifiable single check valve. Fluid category 2 in both modes.

Verifiable double check valve. Fluid category 3 in both modes.

Non-verifiable double check valve. Fluid category 3 in both modes — the everyday category 3 device.

Hose union backflow preventer. Permitted only on existing hose union taps in house installations. Category 2 backpressure, 3 backsiphonage.

Diverter with automatic return, integral to some bath/shower combination fittings. Returns the bath outlet to atmosphere if a vacuum occurs. Category 3 backsiphonage.

Hose union tap incorporating a double check valve. Permitted only for replacing existing hose union taps in house installations.

How readily a building element conducts heat, in W/m²K. A lower figure means better insulation.

A blending valve and pump fitted between the primary and the underfloor manifold, blending the flow down to the underfloor design temperature.

A large-area emitter delivering the load at a mean water temperature around 35°C. The best match for a heat pump because of that low flow temperature.

Selecting a unit too small to meet the design heat loss. The system runs continuously in cold weather and relies on backup heat at a COP of 1.

Using more water than the Regulations permit — one of the five things Regulation 3 exists to prevent.

A collector with no glazing and no selective coating, used where the required temperature is low — principally swimming pool heating.

Discharge to atmosphere with nothing impeding the flow or the spillover — the defining feature of a type AA air gap.

A cylinder storing domestic hot water at mains pressure. Covered by Building Regulations Part G, with defined discharge arrangements.

A hot water storage vessel that does not have a vent pipe to the atmosphere.

A stored hot water system with no open vent pipe, fed at mains pressure and sealed to atmosphere.

Water that does not meet the wholesome standard, and which must be identified and kept separate from supply and distributing pipes.

Against the flow of water — back towards the source.

An individual urinal appliance. Not subject to the suite approval requirement that applies to WC pans.

A continuous urinal surface, counted in 700 mm units for the purposes of the automatic cistern filling rate.

A valve that admits air to prevent a vacuum forming and imploding the cylinder.

An external layer preventing moisture reaching insulation. Saturated insulation loses most of its effectiveness.

The four-stage process every heat pump uses: compress, condense, expand, evaporate. Compressing the refrigerant raises its temperature enough for the heat to become useful.

A pipe open to the atmosphere which exposes the system to atmospheric pressure at its boundary. Not less than 19 mm internal diameter.

A cylinder fed from a cold water storage cistern rather than directly from the mains.

A heating or hot water system open to atmosphere through a vent pipe. Deemed the higher risk of vented and sealed.

The energy needed to warm incoming outside air, calculated from room volume, air change rate and temperature difference.

Describes a backflow prevention device with test ports upstream of and between its mechanical elements, so it can be tested in situ. Verifiability does not change the fluid category rating.

Confirming before commissioning that the design being worked to matches what was actually installed. Commissioning to the wrong figures produces a system that runs but underperforms.

A drilled ground collector reaching deeper, more thermally stable ground from a small footprint. The answer where land area is the constraint.

A glass lining baked onto a steel cylinder during manufacture to prevent corrosion.

An approved instrument for proving a circuit dead. Must be proved on a known live source both before and after the test. A neon screwdriver is not one.

An insulated vessel plumbed in series purely to add water content. Everything flows through it, so there is no mixing penalty and no hydraulic separation.

An overflow pipe whose outlet is located in a position where the discharge of water can readily be seen. Not less than 19 mm internal diameter.

A combined washing and drying machine, permitted 48 litres per kilogram because the drying cycle uses additional water.

A communal washing appliance. Exempt from the plug requirement where its outlet cannot accept one and flow does not exceed 0.06 litres per second.

The unit by which washing machine and washer-drier consumption is measured — litres per kilogram of washload.

Water escaping and lost — a leaking fitting, a fractured pipe, or a tap left running. One of the five Regulation 3 offences.

The obligation on whoever removes waste from site to pass it to an authorised person or facility with proper documentation.

The audible symptom of a pressure surge travelling through pipework that is not adequately restrained.

An attenuation device absorbing the pressure spike created by rapid valve closure.

An appliance reducing water hardness. Softened water may only be used for drinking where the treated water remains wholesome.

The regulations keeping inhibited, glycol-bearing water out of the mains. Filling must be through a temporary backflow-protected connection, disconnected afterwards.

The regulations protecting the wholesome water supply from contamination by backflow. Byelaws apply in Scotland.

A licensed retailer supplying water in the non-household market. The Regulations apply to its supplies as they do to an undertaker's.

The company appointed to supply water for an area, and the body that enforces these Regulations there.

An expansion vessel whose diaphragm has failed or lost its charge, so it fills with water and can no longer accept expansion.

The online directory of approved plumbing businesses covering the recognised contractor schemes across the UK.

The sanitary appliance receiving the flush. Approved together with its flushing device as a suite.

Varying the flow temperature according to outdoor temperature, following a set curve. The single biggest control lever on seasonal efficiency.

The arrangements covering disposal of waste electrical and electronic equipment, including old appliances removed during an installation.

A pressure relief valve passing slightly and continuously. One of the three usual causes of a system losing pressure, alongside a leak and a failed expansion vessel.

The rectangular slot overflow that distinguishes a type AB air gap, sized to discharge fully under fault conditions.

Backflow protection on the supply or distributing pipe serving a whole site, required where a pipe serves two or more separately occupied premises.

Water supplied by a water undertaker complying with the requirements of the drinking water regulations — fluid category 1.

The Water Industry Approved Plumber Scheme, administered by WRAS and recognised by all water suppliers.

The suction force wind exerts on an array. The critical structural risk for roof-mounted collectors, and highest at roof edges and corners.

Any place where a person could be injured by falling from it. There is no minimum height, and it includes places at or below ground level.

The set outlet pressure the system is designed around, held by the pressure-reducing valve.

The standard required for installation, connection, alteration, repair or disconnection — conforming to an appropriate standard, an approved specification, or a method approved by the undertaker.

The Water Regulations Advisory Scheme — publisher of guidance, information and guidance notes, and the approved fittings directory.

Also optical efficiency, written η₀. A collector's efficiency when there is no temperature difference between it and ambient — the ceiling it works down from.

Backflow protection covering a part of a building, provided in addition to protection at each point of use.

Dividing a dwelling into separately controlled heating areas. In a new dwelling of 150 m² or more, Approved Document L (5.14) asks for at least two independently controlled heating circuits, each zone with its own time and temperature control.