Reading Heating Wiring Diagrams, and the Tests That Prove Them

Somebody hands you a wiring diagram with two motorised valves, a programmer, a room thermostat and a cylinder thermostat, all landing on a ten-way terminal block. Two valves means S-plan fully pumped — and that identifies the system before you read another line.

The short answer

Count the valves, then count the cores.

• Two two-port valves → S-plan. Three or more → S-plan plus.
• One three-port valve with five cores → Y-plan, a mid-position valve.
• One three-port valve with three cores → W-plan, a diverter.

And on every one of them, the same requirement: boiler interlock — when there is no demand for space heating or hot water, the boiler and the pump are switched off.

S-plan and the five cores

S-plan uses two two-port motorised zone valves: one in the heating circuit, one in the cylinder primary. Each valve is opened by its own thermostat, and each valve tells the boiler and pump when it has opened. Because each circuit has its own valve, each circuit can be controlled and isolated on its own — which is why a fault on one circuit rarely takes the other out with it.

Inside the valve are a small synchronous motor and a microswitch — the auxiliary switch or end switch. When the thermostat sends live down the brown core, the motor drives the valve open. Only when the valve has reached the fully open position does the microswitch close, passing the permanent live on the grey core out through the orange core to the boiler and pump.

That orange wire is the whole point. On an S-plan diagram you achieve interlock by taking the boiler and pump live from the orange wires of both valves, joined together. If neither thermostat is calling, neither valve opens, neither microswitch closes, and there is no live at the boiler. The boiler cannot fire into a system with no route for the water, and it cannot cycle on its own thermostat while the house is warm.

BS EN 12828's National Annex says the same from the cylinder side: any valve fitted in the primary flow or return of the cylinder for actuation by the cylinder thermostat should be capable of switching to control the boiler and the pump. That is the auxiliary switch, described in a standard rather than on a diagram.

Reading the rest of the diagram: the supply arrives from a switched fused connection unit, 3 A rated. The programmer takes a permanent live and gives two switched outputs. The heating output feeds the room thermostat, whose output drives the heating valve motor; the hot water output feeds the cylinder thermostat, whose output drives the cylinder valve motor. Both orange wires feed the boiler and pump live. The cylinder sensor sits one quarter to one third up the cylinder, normally set to about 60 °C.

Y-plan against W-plan

A three-bedroom house where somebody showers while the heating is on. Both valves sit in the common flow, both have three ports, and on a drawing they are almost the same symbol. What separates them is what they can do at once.

• A mid-position valve can sit part way, allowing shared flow to the cylinder and the emitter circuit at the same time. This is Y-plan.
• A diverter valve passes the whole flow to one circuit or the other, never both. This is W-plan, a hot water priority system.

NA.4.6.3.1.2 recommends a mid-position valve where a three-port valve is fitted in the common flow, with a diverter used only if the system design is intended for a priority flow arrangement. So for a house where heating and hot water are often wanted together, the mid-position valve is the answer.

A three-port mixing valve is a different device again: it blends return water into the flow to control circuit temperature, and does not choose which circuit is served. Get those three mixed up on a specification and the system is wrong before a single pipe is cut.

The valve rests in the hot-water-only position with no power on it. From there: hot water only, it stays put; heating only, it is driven hard over to the heating port; both, it is held part way in the mid-position and flow is shared. The boiler and pump live comes out of the valve's own switch, exactly as from the auxiliary switch of a two-port valve.

S-plan plus and multi-zone

A two-storey house, 180 m², one gas boiler and a stored cylinder, with upstairs and downstairs on different times.

Approved Document L (5.14) requires new dwellings of 150 m² or more to have at least two space heating zones, each with an independently controlled heating circuit — and a heating circuit means a pipework run controlled by its own zone valve. Each space heating circuit then needs independent time control plus a room thermostat in a reference room served by that circuit, with TRVs on radiators outside the reference rooms.

So at 180 m²: two space heating zones, each with its own zone valve and its own independent time and temperature control. TRVs alone do not count as a zone — they give room-by-room trimming, not an independently controlled circuit. A single programmable thermostat in the living room does not make two zones either.

Hot water is separate. A stored hot water circuit needs independent time control and electric temperature control — a cylinder thermostat with a zone valve or a three-port valve. Hot water is a circuit in its own right, not a third space heating zone.

NA.4.6.4 sets out what each valve can do: a two-port valve opens or closes a single circuit supplying one zone; a three-port valve controls two zones only; and full independent control can only be achieved by interlocking electrical switching.

So three circuits — upstairs, downstairs and hot water — need a two-port zone valve on each, every valve interlocked electrically to the boiler and pump. That is S-plan plus. A single three-port valve cannot do it: three ports serve two circuits, not three.

The Annex adds a warning for systems using a separate pump per circuit: suitable valves must prevent flow in the other circuits when only one pump is running. Non-return valves do that job.

The wiring is S-plan with an extra leg: each zone gets its own time control, its own room thermostat, its own two-port valve, and its orange switched live joined with the others to the boiler and pump. All three orange wires land on the same terminal. Any one calling brings the boiler and pump on; none calling switches both off — the interlock again.

Two advantages worth stating in an answer: independent control of each zone, so unoccupied parts of the house are not heated; and further zones can be added later with minimal disruption — another valve, another thermostat, another pair of wires into the same wiring centre.

The wiring centre, pump overrun and frost protection

Six cables arrive at the same corner of an airing cupboard. Where they meet decides whether the next engineer can work on this system at all.

A wiring centre is a manufacturer's enclosure with a terminal block whose terminals are printed with what belongs on them. The alternatives are worse: a fused spur is a supply accessory, not a place to gather six cables; a cylinder thermostat has room for the three cores of its own flex and nothing else; and crimped connectors wrapped in insulating tape fail Regulation 526.5 outright.

The programmer is a two-channel time switch: a permanent live in, a heating-on switched live and a hot-water-on switched live out. Those feed the room thermostat and the cylinder thermostat respectively, which drive their valves. The whole lot is fused to 3 A through a double pole switch or fused connection unit.

Pump overrun

Some boilers need the pump to keep running after the burner stops. That is pump overrun, and its purpose is to dissipate residual heat from the heat exchanger so the boiler's energy cut-out does not operate.

The awkward moment is when both circuits are satisfied and both zone valves have shut — there is nowhere for the water to go. So an automatic bypass valve is fitted from the flow just after the pump back to the return, opening when the TRVs are closed and the hot water circuit is satisfied.

On the diagram, a pump overrun boiler is wired with separate terminals marked PL (permanent live) and SL (switched live), plus neutral and earth. The permanent live keeps the boiler's internal overrun circuit alive after the switched live has gone. A boiler wired with only a switched live cannot overrun, because everything dies the instant the valve closes.

Frost protection

A frost thermostat protects pipework in an unheated space. It should always be fitted in conjunction with a pipe thermostat: the frost stat senses air temperature, the pipe stat senses water temperature, and both must be closed before the boiler will fire. That combination stops the boiler firing on a cold night when the water is already hot.

A frost thermostat overrides all other controls — wired to bring the boiler and pump on regardless of the programmer. And it produces one of the classic dangerous faults: a frost thermostat fed from a different circuit means that isolating the heating spur does not make the boiler terminals dead. The whole control system should have one means of isolation; where a rogue supply is found, the frost thermostat is re-installed as part of the controlled system with a single point of isolation.

The dead tests, in order

Every dead test starts from the same place: the circuit is isolated at the consumer unit and locked off with an approved device, and a warning notice fitted. Switching off at the spur and taping over the switch is not isolation — the spur is not the point of isolation, and anyone can peel the tape off.

Regulation 643.1 requires the tests to be carried out in order, before the installation is energised: continuity of conductors, insulation resistance, protection by SELV/PELV or separation, floors and walls, and polarity. For a domestic radial circuit that reduces to three:

1. Continuity of protective conductors (and of the live conductors on a ring final circuit).
2. Insulation resistance.
3. Polarity.

Each test makes the next safe or meaningful. Continuity comes first because a broken protective conductor leaves everything downstream unearthed — you want to know before you apply 500 V to anything. Insulation resistance comes next because it finds a breakdown that would otherwise show up the first time the circuit is energised. Polarity comes last, on a circuit already proved continuous and sound; doing polarity first proves nothing you can rely on.

Regulation 643.1 also says that if any test shows a failure, that test and any preceding test whose result may have been influenced must be repeated once the fault is put right.

Earth fault loop impedance is not in this list. It is a live test under 643.7.3, after energising. So is RCD operation, and so is prospective fault current.

Insulation resistance with electronics connected

Table 64 requires a circuit up to and including 500 V to be tested at 500 V DC, with a minimum insulation resistance of 1 MΩ, and 643.3.2 requires current-using equipment to be disconnected.

A modern heating circuit is full of electronics. Regulation 643.3.3 deals with it: where a circuit includes electronic devices likely to influence the results or be damaged, only a measurement between the live conductors connected together and the earthing arrangement shall be made.

So you either disconnect the electronic equipment, or link line and neutral together and test that pair to earth at 500 V, looking for at least 1 MΩ. What you do not do is test at 1000 V with everything connected, or record the test as not applicable.

Some tests are about one component, not the installation. To assess how the motor in a zone valve is performing, the test is a dead test for resistance: an ohmmeter across its leads should give a reading, and an infinity or OL reading means the winding is open circuit and the motor is finished. Loop impedance testers, RCD testers and prospective fault current testers all measure the supply and the protective arrangements — none of them tells you anything about a valve motor.

Test leads follow HSE guidance GS38: where leads are used with a multimeter they should carry a high breaking capacity fuse, usually not exceeding 500 mA. The fuse is there so a wrongly set instrument, or a probe that slips, cannot deliver the full fault energy into your hands.

What polarity and earth continuity each prove

An engineer tests an existing heating circuit and finds the fuse and the switch of the fused connection unit sitting in the neutral. Everything works. The boiler fires, the programmer clicks. The circuit is also badly dangerous.

The polarity test checks that live and neutral are wired correctly, and a failure means they have been crossed. Regulation 643.6 verifies that every fuse and single-pole control and protective device is connected in the line conductor only, that wiring is correctly connected throughout, and that centre contacts of Edison screw lampholders are on the line conductor.

Put the fuse and the switch in the neutral and the consequence follows immediately: switch off at the spur, or blow the fuse, and the boiler and its controls are still connected to line potential through the neutral path. The system looks off. It is not off. Anyone who opens the boiler case on that basis is working live without knowing it.

An earth continuity test confirms that all appropriate parts are earthed. It does not check polarity, insulation or supply voltage. It answers one question: is there an unbroken, low-resistance path from every piece of exposed metalwork back to the main earthing terminal? Without it, the earth fault loop is broken, the protective device will not disconnect quickly, and the casing of a faulty pump stays live.

The method on a radial circuit

The standard method proves continuity and polarity together and gives you R1+R2. With the circuit isolated:

1. At the origin, link the line conductor and the cpc together with a short lead. While doing it, check visually that the line conductor is correctly identified and connected into the protective device.
2. At the furthest point — the fused connection unit — remove the accessory and check visually that the line conductor is in the correct terminal.
3. Null the leads of the low-resistance ohmmeter, then connect between the incoming line and cpc terminals. A very low reading proves continuity, and that reading is R1+R2.
4. Remove the link at the origin and test again. The reading should now be high, which proves you tested the right circuit.

Step 4 is the one people skip, and it is the one that catches a reading taken through a parallel path or a neighbouring circuit.

A main bonding conductor is tested end to end with a low-resistance ohmmeter after safe isolation, with one end disconnected so parallel paths do not flatter the result. Take the reading from the disconnected end to the bonded metalwork rather than to the clamp, so the clamp itself is proved. Subtract the lead resistance unless the instrument has been nulled, and reconnect as soon as the test is finished.

📝 10-Question Self-Test

Straight from the Level 3 course question bank. Click an option to see whether you got it right — the explanation appears instantly, and there is nothing to submit.

Your score: 0 / 10

Question 1 of 10

What is the purpose of the wiring arrangement in the diagram shown?

ATo provide a pump overrun BTo give frost protection CTo allow weather compensation DTo stop the pump running dry

Question 2 of 10

The wiring diagram below identifies which type of heating system?

AC plan semi-gravity BW plan fully pumped CS plan fully pumped DOne-pipe gravity heating

Question 3 of 10

On the diagram below, what is the arrow pointing to?

AA main switch BA surge protection device CAn RCBO DA time-delayed RCD

Question 4 of 10

In a standard fused connection unit, what type of terminal is normally provided for the supply cable?

APush-fit spring terminals BScrew-down clamp plates CPillar terminals DCrimp lugs on studs

Question 5 of 10

A central heating radial circuit is to be dead tested. Which order of tests is correct?

APolarity, insulation resistance, continuity of protective conductors BContinuity of protective conductors, insulation resistance, polarity CContinuity of protective conductors, polarity, insulation resistance DInsulation resistance, polarity, continuity of protective conductors

Question 6 of 10

Under HSE guidance GS 38, test leads must be fused. What rating should the fuse have?

A100 mA B500 mA C1.0 A D3.0 A

Question 7 of 10

On an S-plan wiring diagram for a fully pumped gas-fired system, the two-port valve in the cylinder primary is shown being driven by the cylinder thermostat. According to the BS EN 12828 National Annex, what must that valve also be capable of doing?

ADiverting flow to the radiators when the cylinder is satisfied BOpening automatically on electrical failure to allow gravity circulation CSwitching the immersion heater off when the boiler is firing DSwitching to control the boiler and the pump

Question 8 of 10

A designer is producing a Y-plan wiring diagram for a house where heating and hot water are often required at the same time. Which three-port valve does the BS EN 12828 National Annex recommend in the common flow?

AA mid-position valve, which allows shared flow to both the cylinder and the heating circuit BA diverter valve, which passes the whole flow to either the cylinder or the heating circuit at any one time CA three-port mixing valve, which blends return water into the flow to control the circuit temperature DA three-port valve with the manual lever locked in the open position so both circuits are always fed

Question 9 of 10

A new gas-fired wet central heating system is to be installed in a two-storey house with a total floor area of 180 m2 and a stored hot water cylinder. Following Approved Document L, what is the minimum number of independently controlled space heating zones the wiring diagram should show?

AOne, with thermostatic radiator valves on every radiator providing all of the room-by-room control BOne, provided a programmable room thermostat is fitted in the living room as the reference room CTwo, each with its own zone valve and independent time and temperature control DThree, one for each storey plus a third space heating zone dedicated to the hot water cylinder

Question 10 of 10

When drawing the wiring diagram for a new S-plan system on a gas boiler, what does the Domestic Building Services Compliance Guide say the boiler interlock wiring must achieve?

AThe boiler runs continuously on its own thermostat and only the pump is switched by the room and cylinder thermostats BWhen no demand for space heating or hot water exists, both the boiler and the pump are switched off CThe room thermostat switches the boiler directly while the cylinder thermostat switches the pump on and off DThe programmer switches the boiler on and off and the motorised valves switch the pump through their auxiliary contacts