Diagnosing a Hot Water Fault: Three Signatures and Where They Point

The call comes in as three words: “no hot water”. By the time you have the van unloaded and the airing cupboard open you could have lost an hour — while the person in the kitchen has watched this system every day for years.

The short answer

On a reported fault, particularly on an unvented system, the first method of finding out what will be needed to put it right is to ask the customer — before starting work. Not replace the inlet control group. Not pressure test the whole installation. Not order spares from the manufacturer. Those may follow, and only if the answers point that way.

Then, at the cylinder, three observations do most of the rest: which valve, when, and how hot. And one rule halves the work in a single step:

Pressure faults discharge cold. Temperature faults discharge hot.

Feel the discharge, or feel the pipe, before doing anything else.

What to ask, and why it counts as evidence

Four questions:

• The immediate history. When did it first occur? How did they notice it? Constant or intermittent, mornings only, worse when the heating is on?
• Have they seen water where it should not be? An unusual discharge around the vessel, or from a pipe outside the wall, points straight at a relief valve.
• Has the flow rate or the pressure changed? A drop in either points at a blockage, a blocked strainer or scale — not at anything electrical.
• Has anybody attempted a repair? Earlier work often has to be undone before the real fault will show itself, and a part fitted by somebody else may be the fault.

Add one more: what was the result of the fault? Lukewarm water at every tap is a different fault from scalding water at one. Cold water at the taps with the heating still working is different again.

Approved Document G 3.24 requires an unvented unit to be indelibly marked with a warning telling the user that a flow of hot water from the discharge pipe is a fault, and to switch the heater off and contact the installer. A customer repeating that back to you has handed you half the diagnosis.

BS 8558 makes the point in its maintenance guidance: in a single dwelling, responsibility for maintenance normally rests with the householder, and that includes identifying and correcting leakages and any discharges from overflow pipes or regular discharges from valves. The householder is therefore the only person who has been watching. Their description of a discharge — how often, how hot, how long — is data you cannot recover once the system has cooled down.

BS EN 806-5 clause 9.1 treats the user's senses as a diagnostic instrument too. Where damage or malfunction changes the odour, taste or colour of the water, corrective action is required; where immediate action is needed to prevent serious damage or contamination, the system is shut off at the service stop valve and the water supplier informed. A customer saying "the water has gone brown" is reporting a regulated condition, not a decorating problem.

Asking so that you get an answer

Ask in words the customer uses. "Is the expansion vessel discharging?" gets a blank look; "have you seen water dripping out of a pipe anywhere, inside or outside?" gets the answer. Ask them to show you rather than describe. Ask when the last service was and who did it.

Then write the answers down. They are the first entries in the job record, they are what you check your diagnosis against at the end, and if the fault is intermittent they are the only record of what it did on the day it was reported.

One warning. The customer reports symptoms, not causes. When somebody says "the thermostat has gone", hear it as "the water is too hot" or "the water is cold" and start from there. Their theory is worth testing, but it is not the diagnosis.

Where the procedure lives

You are in front of an unvented cylinder you have never seen before. The expansion valve is dribbling and you want to check the vessel pre-charge. What pressure should it be?

The step-by-step procedure for checking and recharging an expansion vessel is in the manufacturer's information. Not Approved Document G3, not the Water Supply (Water Fittings) Regulations, not BS EN 806 or BS 8558. Those documents say what must be achieved; only the maker of that cylinder gives the pre-charge figure, the order of dismantling and the part number of the replacement.

BS EN 806-5 clause 12 says it directly: routine maintenance on pipes, fittings, valves and appliances is carried out in accordance with the manufacturer's instructions, and procedures and frequencies shall be followed in accordance with them.

Clause 5 requires that all information relevant to the installation shall be readily available: the manufacturer's Technical Product Information for operation and maintenance retained and followed, the commissioning report as part of the documentation, and maintenance recorded and stored so the data is auditable. BS 8558 adds that the owner should have maintenance instructions and an accurate drawing particularly showing where pipe runs are concealed, with control valves clearly labelled and the labels renewed after redecorating.

Reading a fault diagnosis flow chart

Manufacturer fault-finding information usually takes three forms: known problems with their symptoms, a flow chart for identifying the problem, and the technique for replacing the component. Order parts by the model number of the equipment and the part number from the list, so what arrives fits.

A flow chart is a decision tree. It exists because a component such as a pressure reducing valve or an expansion vessel has several ways of failing that look identical from the outside.

Take excessive hot water from the taps on an indirectly heated cylinder. Is the thermostat set too high? If yes, reduce it. If no, is the motorised valve closing when the cylinder reaches temperature? If not, check the cylinder thermostat wiring, and check whether the thermostat is wired as the installation instructions require for that plan. If the wiring is right and the valve still does not close, the thermostat is faulty and is replaced.

Two rules make a chart work: start at the top, not in the middle, even when you think you know the answer — and change one thing at a time, so the chart is still telling you something on the next step.

What the service history tells you

The commissioning record and the service log are the system's memory. Between them they give the incoming static pressure when it was new, the expansion vessel pressure it was set to, what has been replaced since, and whether this fault has happened before.

A pre-charge that has fallen at every service is a vessel on its way out. A cut-out that has been reset before, with no cause recorded, is a thermostat that has been failing for a year. Where there is no history, treat the installation as unknown and inspect it as if you were commissioning it — nothing about it has been proved to you.

Reading the discharge

The rule works because the valves are designed for different jobs.

BS 5546 6.11.3 d): the expansion relief valve is there to discharge excess pressure if either the expansion vessel or the pressure reducing valve fails. It has no idea what temperature the water is; it opens on pressure alone, and on an unvented cylinder that pressure rise happens while the water is heating — so what comes out is the cold water sitting in the pipe and cool water from the cold side of the cylinder.

The temperature relief valve is a different device with a different job. Schedule 2 paragraph 23 of the Water Regulations requires one on every unvented vessel over 15 litres, located directly on the vessel, with enough capacity to ensure stored water never exceeds 100 °C. Typically it opens around 95 °C. When that valve lets go, scalding water and steam come out. It cannot be anything but a temperature fault.

BS 8558 puts the pressure half in one sentence: discharge from an expansion valve, or from a cistern warning pipe, indicates a possible malfunction of a pressure reducing valve, pressure limiting valve, expansion vessel or control valve. Four suspects, all on the pressure side.

Identify the valve first. The expansion valve sits on the cold inlet close to the vessel; the T&P valve is factory fitted into the top of the vessel itself. They may share a manifold to one tundish, so trace the discharge back rather than guessing.

Then the timing:

• Appears only while heating, stops when it cools → expansion.
• Runs continuously, hot or cold, heating or not → a valve or the PRV passing.
• Shows up only when water is drawn off → not a discharge fault at all. It is a flow fault.

Why the tundish has to be where you can see it

Approved Document G 3.54 requires the tundish to be vertical, in the same space as the unvented system, as close as possible to and lower than the safety device, with no more than 600 mm between valve outlet and tundish. Clause 3.55 adds that any discharge should be visible at the tundish, and Schedule 2 paragraph 19 requires discharges to be made in a safe and conspicuous manner.

Those requirements exist for the diagnosis as much as for the hydraulics. A discharge nobody can see tells nobody anything at all. Put the tundish in a loft or behind a bath panel and the cylinder can discharge for a year: the customer never reports it, the vessel never gets recharged, and the first anyone knows is when something worse fails.

Where a discharge might not be apparent — a dwelling occupied by someone with impaired vision or mobility, for example — paragraph 3.55 says consideration should be given to a device that warns when a discharge takes place.

One way to hold the whole skill: fault finding is commissioning backwards. Commissioning proves the system in order — fill, vent, pressure, temperature, controls, safety devices, record. Fault finding walks that order backwards from the symptom until you reach the thing that is not doing its job.

The three fault signatures

Almost every unvented complaint arrives as one of three patterns. Learn the three and you will name the fault from the doorway.

One: it dribbles while heating, and stops when cold

Water expands as it is heated and that expansion has to go somewhere. If it has nowhere to go, the pressure climbs past the valve setting and the valve does exactly what it exists for. When the system cools the pressure drops back and the dripping stops.

The cause is that the expansion vessel has lost its gas charge or is waterlogged — or, on a bubble-top cylinder, the air gap inside the cylinder has been lost. This is the commonest unvented fault of the lot, and the discharge is cold. It is not the check valve passing, not the thermostat failed high, and not a blocked strainer.

On a bubble-top cylinder there is no external vessel: expansion is taken up by an air gap held at the crown. That gap slowly dissolves into the water and disappears. Draining down and refilling re-forms the bubble.

Two: scalding water and steam

This is a temperature control failure, and both layers have already let go. AD G 3.17 requires two independent safety devices in addition to the control thermostat, and BS 5546 6.11.2.2 sets the order in which they act as temperature rises: thermostat, then energy cut-out, then temperature relief valve.

So when you see the third device working, the first two have already failed. The valve is reporting, not failing — replacing it fixes nothing.

It is also not normal operation. A properly working cylinder never relieves on a heat-up cycle. Anyone who tells the customer "they all do that" is describing a system with one layer of protection left.

Three: a burst of flow, then a trickle

The tap runs well for a few seconds, then dies back to a weak stream. Nothing is discharging. This is not a discharge signature at all — it is a flow one.

What you are watching is the stored hot water leaving the cylinder at full pressure, followed by the refill rate limiting the flow once the store is empty. The refill rate is limited by a blocked in-line strainer on the cold inlet. It is not the vessel collapsing under demand and not a failed thermostat — the water that did come out was hot and at good pressure.

Two more patterns worth carrying. Cold water running constantly from the expansion valve is usually incorrect pressure due to a malfunction of the pressure reducing valve — check and replace the PRV. Cold water running constantly from the T&P valve usually means a joint failure of the pressure reducing valve and the pressure relief valve, so both are checked and replaced.

Four defects that hide on a working system

Some faults are not failures. They were installed. The system runs, the customer is happy, and nothing announces the problem until the day something else goes wrong and the protection that should have caught it is not there. That is why a service or fault visit starts with a look round, not a test.

• Balanced cold from the wrong place. It must come from the cold feed to the vessel, after the pressure reducing valve and before the single check valve. The PRV then holds hot and cold at the same pressure, which mixer taps, shower mixers and TMVs need to blend safely. Taken from anywhere else: unbalanced pressures at the mixers, and a scald risk.
• No strainer. The in-line strainer keeps solid matter out of the PRV and everything downstream. Note which way round that is — fitting the strainer upstream of the PRV is correct practice, not a defect. Leaving it out is the defect.
• Missing or wrongly placed check valve. The single check valve stops hot water backflowing into the cold main, and it is what forces expansion into the vessel or the bubble rather than back up the supply pipe. Without it, expansion escapes up the main and a failed vessel produces no symptom at all — until the mains pressure rises one night and the cylinder has nowhere to expand into.
• Relief valve fitted too low. BS 5546 6.11.2.1 c) requires the temperature relief valve within the top 20 per cent of the water in the cylinder, preferably within 150 mm of the top. AD G 3.38 requires it directly on the vessel; 3.39 says it should be factory fitted, not disconnected other than for replacement, and not relocated into any other device or fitting. Fitted lower down it senses cooler water and lets the crown of the cylinder go past the temperature it exists to catch.

What to look at on a cistern, and on a vessel

On a vented system, one of the most important checks when inspecting an existing cistern is that the bearers and base are sound and fit for purpose. AD G 3.15 requires the platform to be flat, level and rigid, capable of safely withstanding the weight of the cistern filled to the rim, supporting the whole of the bottom, and extending at least 150 mm in all directions beyond it. A cistern full of water is a heavy thing sitting on old timber. Replace a metal cistern, or fit a larger plastic one, and the support is upgraded to suit.

On a hot water storage vessel, one of the most important checks is that the high limit thermostat is protected by a 3 amp fuse. Three related points come with it, each a distinct fault if wrong:

• An immersion heater is supplied through a fixed connection, not a 13 amp plug and socket.
• The cylinder thermostat is set to store at 60 °C for control of bacteria — not turned down to 45.
• The high limit thermostat must not reset itself. AD G 3.18 a) and BS 5546 6.11.2.1 b) both require a non-self-resetting energy cut-out.

Write down what you find. A defect you noticed and did not record is one you did not find.