Most hot water work is not installation. It is somebody ringing to say there's no hot water, or the shower has gone cold halfway through, or the pipes bang when the boiler fires.
This outcome is worth 5 questions, 12.5% of the 506 paper, and it splits neatly in two: the indicators a customer describes, and the faults underneath them. Learn both lists and the marks are straightforward, because the exam mostly asks you to connect one to the other.
For the systems themselves see the hot water revision guide, and for the controls that fail, temperature control and commissioning.
The five indicators
An indicator is a symptom — what somebody notices and phones about. It is not the fault, and the same indicator can have several causes.
| Indicator | What it usually points to |
|---|---|
| Poor flow rate through a mixer or shower rose | Blockage or scale in the rose, the mixer cartridge, or a strainer. On a gravity system it may simply be insufficient head |
| Unbalanced hot and cold pressures | One supply fed from the mains and the other from a cistern — the mixer delivers whichever side is stronger |
| Valves passing water | A worn seat or seal letting water through a valve that should be shut. On an unvented cylinder, a relief valve passing is telling you something specific |
| Water leak | A failed joint, a corroded cylinder, or a relief valve discharging as designed and being mistaken for a leak |
| Expansion noise | Ticking or creaking as pipework heats and lengthens against a joist or clip that grips it too tightly |
The last one is worth understanding rather than memorising. Expansion noise is not a fault in the water system at all — it is a fixing problem. Copper expands as it heats, and if it cannot slide, it moves in jerks against whatever is holding it. That is the tick a customer hears when the heating comes on and stops hearing an hour later.
The ten common faults
The specification names ten. They fall into groups, which makes them far easier to hold than a flat list.
The heat isn't being made
- Thermostat failure. Stuck open and the water never heats; stuck closed and it never stops. A cylinder thermostat that has failed closed is the usual cause of water boiling.
- Immersion element failure. The element burns out — often because scale has insulated it until it overheats. No hot water on the electric side, while the boiler side still works.
- Failure of the heat source. The boiler itself: no gas, lockout, pump failure, no electrical supply.
The heat isn't getting into the water
- Failure of the coil. The indirect coil corrodes or splits. The tell is distinctive: primary water mixes with the stored secondary water, so the hot taps run discoloured and the F&E cistern level rises or falls unexpectedly.
- Loss of the air bubble in a single feed cylinder. A single feed (Primatic) cylinder separates primary from secondary with a trapped air bubble instead of a physical barrier. Lose the bubble — through overheating, or by draining and refilling it wrongly — and primary and secondary water mix. Same symptom as a failed coil, different cause.
- Limescale. Insulates the element or the coil, so less heat reaches the water and the surface runs hotter, scaling faster still.
The heat is going somewhere it shouldn't
- Water boiling. Control thermostat failed, and on a vented system you get steam and violent noise from the cylinder. On an unvented one the safety chain should catch it.
- Warming to cold pipes. Heat migrating into the cold distribution, usually by parasitic circulation or by hot and cold pipes clipped together. It raises stored cold water above 20 °C, which is a Legionella risk rather than just an annoyance.
The water isn't moving
- Air locks. A pocket of trapped air in a low-pressure gravity run. There isn't enough head to push it through, so flow stops or comes in bursts. Common on open vented systems where pipework doesn't slope consistently.
- Blockages. Scale, debris or corrosion products in a strainer, a shower rose, a mixer cartridge or a valve.
Connecting an indicator to a fault
This is what the exam actually tests, so it is worth practising the reasoning rather than the lists.
No hot water at all. Work back along the chain: is the heat being made (boiler, element, gas, electricity), is it reaching the water (coil, thermostat), and can the water reach the tap (air lock, blockage)?
Hot water at some taps but not others. The heat source is fine. That points at the distribution — an air lock, a blockage, or a partly closed valve on one branch.
Water discoloured at the hot taps. Suspect a failed coil on a double feed cylinder, or a lost air bubble on a single feed one. Both let primary water into the stored water.
Water boiling or steam from the vent. Control thermostat failure. Isolate the heat source before anything else.
Cold taps running warm. Heat is migrating — parasitic circulation, or cold pipework clipped against hot. Check the temperature of the stored cold water, because this is a Legionella question as much as a comfort one.
Shower fine at first then goes cold. The cylinder has been drawn down and the recovery rate isn't keeping up. Not a fault at all if the cylinder is simply undersized for the household.
What you do, and where you stop
The competence boundary matters here as much as it does in the electrical unit.
- Unvented cylinders need G3. If you don't hold it, you don't service, commission or work on the safety devices — full stop.
- Gas appliances need Gas Safe registration. A boiler fault is not a Level 2 plumbing repair.
- Electrical work beyond isolating and reporting is outside your competence, which is exactly the boundary set out in the electrical faults post.
What is squarely yours: re-washering taps and mixers, clearing blockages and air locks, replacing an immersion heater on a vented cylinder if you are competent to do the electrical connection, descaling, and identifying the fault clearly enough that the right specialist arrives with the right part.
Common exam traps
Trap 1: an indicator is not a fault. "Poor flow through the shower rose" is what the customer noticed; the fault is scale, a blocked cartridge, or insufficient head.
Trap 2: discoloured hot water has two possible causes. A failed coil on a double feed cylinder, or a lost air bubble on a single feed one. If a question specifies the cylinder type, that tells you which.
Trap 3: expansion noise is a fixing problem. The pipe is gripped and cannot slide as it lengthens. Nothing in the water is wrong.
Trap 4: warming to cold pipes is a Legionella issue. Not just a comfort complaint — stored cold water above 20 °C is in the growth band.
Trap 5: a burnt-out immersion element usually has limescale behind it. Replacing the element without addressing the scale means doing it again.
Trap 6: unvented means G3. Any answer that has an unqualified person working on unvented safety devices is wrong.
Quick revision summary
Before the mock test, six things you need to be able to produce from memory:
- Five indicators: poor flow rate through mixer and shower rose, unbalanced hot and cold pressures, valves passing water, water leak, expansion noise
- Ten faults: thermostat failure, immersion element failure, failure of heat source, failure of coil, loss of air bubble in a single feed cylinder, water boiling, warming to cold pipes, air locks, blockages, limescale
- Discoloured hot water = failed coil (double feed) or lost air bubble (single feed)
- Warming to cold pipes = parasitic circulation or hot and cold clipped together, and a Legionella risk above 20 °C
- Expansion noise = pipework gripped so it cannot slide as it heats
- Competence: unvented needs G3, gas needs Gas Safe — identify and report rather than repair
📝 16-Question Mock Test
Click an option to see whether you got it right. Explanations appear instantly — no submitting at the end.
Expansion noise. The five named indicators are poor flow rate through a mixer or shower rose, unbalanced hot and cold pressures, valves passing water, a water leak, and expansion noise.
Copper lengthens as it heats. If a clip or a joist notch grips it too tightly, it moves in jerks instead of sliding, and each movement is a tick. It is a fixing problem, not a fault in the water system at all.
A blockage — scale in the rose, or debris in the cartridge or strainer. If only one outlet is affected, the fault is at that outlet. A thermostat, element or coil fault would affect all the hot water, not just the shower.
The indicator is what somebody notices and phones about; the fault is what is actually wrong. One indicator can have several possible faults behind it, which is exactly what the exam asks you to work through.
Ten: thermostat failure, immersion element failure, failure of heat source, failure of coil, loss of the air bubble in a single feed cylinder, water boiling, warming to cold pipes, air locks, blockages and limescale.
Failure of the coil. A corroded or split coil lets primary water into the stored secondary water, so the hot taps run discoloured. On a single feed cylinder the same symptom comes from losing the air bubble — so the cylinder type tells you which.
A trapped air bubble. It is the defining feature of the single feed (Primatic) design — and its weakness, because overheating or an incorrect refill can lose the bubble and let the two waters mix.
Legionella. Heat migrating into the cold distribution — by parasitic circulation, or from hot and cold pipes clipped together — lifts stored cold water into the 20–45 °C band. It is a health issue, not just a comfort complaint.
Limescale. Scale insulates the element, so it runs hotter to deliver the same heat and eventually burns out. Fit a new element without addressing the scale and you will be back doing it again.
Control thermostat failure. If it sticks closed, the heat source never switches off. On a vented system that produces steam and violent noise at the vent; isolate the heat source before doing anything else.
Not enough head. A gravity system may have a metre or two behind the water, which cannot shift a trapped pocket. A mains-fed outlet with two or three bar pushes it straight out. It is also why pipework should slope consistently towards the cistern.
The heat source and cylinder are clearly working, or the bath would be cold too. One outlet affected means the fault is local to that branch — an air lock, a blockage, or a partly closed service valve.
The cylinder is being emptied faster than it reheats. If it happens consistently and everything else works, it may not be a fault at all — the cylinder is simply undersized, or its recovery rate is too low, for the household.
G3. Installing, servicing or commissioning an unvented cylinder requires the qualification, and its safety devices are precisely what that qualification exists to cover. Any answer that has an unqualified person adjusting them is wrong.
Clearing an air lock. Gas work needs Gas Safe registration, unvented work needs G3, and consumer units are electrical work. Knowing where the boundary sits is tested as hard as the faults themselves.
The band is set from both ends. 60 °C minimum to control Legionella, and 65 °C maximum because scale deposits rapidly above it. Scalding protection at the outlet is then handled separately by a blending valve.
How PlumbMate puts this into practice
Two named lists and the reasoning that connects them — ideal spaced retrieval material, and the cheapest marks in the unit once they stick.
- Flashcards, not essays. One prompt, one answer.
- Wrong answers are logged and resurface more frequently in later sessions.
- The 3× rule. Three correct answers before a question clears.
- Explanations on every question, like the ones above.