No hot water? How to work out what has failed
Loss of hot water usually comes down to one of four things: no gas or power to the appliance, a control or timer fault, a failed diverter valve on a combi boiler, or a scaled or failed element or coil on a stored hot water system. Checking whether the heating still works, and whether the fault affects every tap, narrows it down quickly.
Key facts
- Heating works, hot water does not
- Usually a diverter valve, cylinder coil or timer fault
- Neither works
- Usually the boiler, its power supply or gas supply
- Hot water goes cold quickly
- Scaled coil, failed thermostat or an undersized cylinder
- Standard hours attendance
- £99 including the first hour
- Gas work
- Carried out by Gas Safe registered engineers only
Symptom to likely cause
| Symptom | Most likely cause |
|---|---|
| No hot water, radiators fine | Diverter valve, cylinder coil, motorised valve or programmer. |
| No hot water and no heating | Boiler lockout, low pressure, power or gas supply. |
| Hot then cold within a minute | Scaled heat exchanger or a failing flow sensor on a combi. |
| Lukewarm only | Thermostat set low, failed thermostat, or a scaled immersion element. |
| Hot water only on one tap | A scaled or failed tap cartridge, not a system fault. |
Safe checks before you call
- 1Check the programmerConfirm hot water is scheduled on and not left in a holiday or off setting.
- 2Check boiler pressureOn a sealed system, look for roughly 1–1.5 bar cold. Very low pressure often means a lockout.
- 3Check for a fault codeNote any code on the boiler display — it saves diagnosis time on site.
- 4Check the immersionIf you have a cylinder with an immersion heater, check its switch and any tripped breaker.
- 5Stop and callDo not open the boiler casing or touch gas components. Call and quote the fault code.
First, narrow it down
Two questions do most of the diagnostic work. Does the central heating still run, and is the problem at every hot tap or only one?
- • Heating works, no hot water at any tap — on a combi this often points at the diverter valve; on a cylinder system, at the coil, motorised valve or immersion.
- • Neither heating nor hot water — look at gas supply, power, pressure and any lockout code on the boiler first.
- • Only one tap is cold — that is a tap or local isolation issue, not a heating fault.
- • Water is warm but never hot — usually a thermostat setting, a scaled element or a cylinder that is not recovering.
Safe checks you can do yourself
None of these require tools or touching gas work. Do them before you call and you will get a better answer on the phone.
- • Check the boiler is powered and not showing a fault code, and note the code if it is.
- • Check the programmer or timer has not reset after a power cut, and that hot water is actually scheduled on.
- • On a combi, check the system pressure gauge — many will not fire below about one bar.
- • On a cylinder, check the immersion switch and its fuse or breaker.
- • Check other gas appliances work, which tells you whether the gas supply itself is the problem.
Why hard water shortens the life of a hot water system
Across Surrey, south London and the surrounding counties the mains supply is hard. Scale coats immersion elements and cylinder coils, insulating them so they heat water more slowly and eventually fail. A cylinder that used to recover in half an hour and now takes two is usually telling you about scale rather than about the boiler.
When to stop and call
Stop immediately and call if you smell gas, see water leaking from the boiler or cylinder, or the appliance keeps locking out after resets. Gas work is carried out by Gas Safe registered engineers, and our registration number is available on request.
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