Quick Answer
If your hot water has stopped working, the cause depends on your boiler type. For combis, check for a boiler lockout or low pressure. For system boilers, check if the hot water cylinder has power. For immersion heaters, check the fuse and timer. Most causes can be diagnosed in minutes before calling an engineer.
No hot water is one of the most disruptive problems in a UK home, particularly in winter. The diagnosis varies significantly depending on what type of hot water system you have — and in 2026, UK homes still use a wide range of systems, from modern combination boilers to older hot water cylinders and immersion heaters. Getting the diagnosis right first saves time and often avoids an unnecessary call-out charge.
This guide walks you through diagnosing the most common causes by boiler type, explains the quick fixes you can try yourself, and tells you when a Gas Safe engineer or electrician is the right call.
In this guide
First Step: Identify Your System Type
The right diagnostic path depends on your system:
- Combi boiler:
Heats water directly from the mains — no storage cylinder. Instant hot water but flow rate limited. The boiler itself does everything.
- System boiler with cylinder:
Boiler heats water and stores it in a hot water cylinder (usually in an airing cupboard). Hot water is drawn from the cylinder. Runs out if demand is high.
- Heat-only / regular boiler with cylinder + tank:
Older systems with a cold water feed tank (usually in loft) feeding the cylinder. Often found in pre-1990s properties.
- Immersion heater:
An electric heating element in a hot water cylinder. Often used as a backup or as the sole heat source in properties without a gas supply.
Combi Boiler Hot Water Problems
With a combi boiler, if hot water has stopped completely, the boiler itself is the likely culprit. Check:
1. Check the boiler display for error codes
Most modern combis show a fault code when they lock out. Look up the code in your manual or Google your boiler model + the code. Common causes: low pressure, ignition failure, sensor fault.
2. Check the boiler pressure
Normal cold pressure is 1–1.5 bar. Below 1 bar, most combis will lock out and stop producing hot water. Repressurise via the filling loop. See our guide on why your boiler is losing pressure.
3. Try the reset button
Most combis have a reset button (often a flame or reset symbol). Hold it for 3–10 seconds. This clears a lockout and attempts a restart. If the boiler fails to start again after 2–3 resets, stop and call an engineer.
4. Check the gas supply
Turn on a gas hob ring. If it lights, gas supply is not the issue. If it doesn't light, call your gas supplier's emergency line.
5. Check the programmer/thermostat
Ensure the hot water is scheduled to be 'on' in your heating programmer. Some systems have separate controls for hot water and heating.
System Boiler and Hot Water Cylinder Problems
With a system boiler and cylinder, hot water and central heating can fail independently. If you have heating but no hot water:
- → The hot water zone valve may have failed — the motorised valve that opens to allow hot water to flow into the cylinder. This is a plumber's repair.
- → The cylinder thermostat may be set too low or may have failed. Check and reset (should be 60–65°C).
- → The programmer may have hot water scheduled as "off" or the timer may have reset after a power cut.
If neither heating nor hot water is working, the boiler itself is the likely cause — follow the combi boiler steps above.
Immersion Heater Problems
For an immersion-heated cylinder, hot water failure is usually electrical. Check:
- → The immersion heater's fuse at the consumer unit (fuse board) — look for a dedicated immersion heater circuit
- → The timer or programmer for the immersion heater — it may have lost its schedule after a power cut
- → The thermostat on the immersion heater itself — it may have tripped; there is often a red reset button on the immersion stat
- → Whether the immersion element has failed entirely — if the above checks pass, the element itself may need replacing (a plumber's job)
For a full comparison of immersion heaters vs combi boilers including running costs, see our article on immersion heater vs combi boiler: which is cheaper to run.
Diagnostic Table: Hot Water Problem by Symptom
| Boiler Type | Symptom | Likely Cause |
|---|---|---|
| Combi | No hot water and no heating | Boiler lockout — check error code and pressure |
| Combi | Heating works, no hot water | Diverter valve fault — call Gas Safe engineer |
| Combi | Hot water sometimes works | Diverter valve sticking — call engineer |
| System boiler | No hot water, heating works | Zone valve or cylinder stat fault — call plumber |
| System boiler | Neither work | Boiler fault — check error codes, call engineer |
| System/regular boiler | Hot water runs out quickly | Cylinder too small or stat set low |
| Immersion heater | No hot water at all | Check fuse, timer, and stat — possibly failed element |
| Any system | Lukewarm but not hot | Thermostat set too low, or part blockage |
Typical Repair Costs (2026)
If a repair is needed, here are typical costs. For full boiler replacement costs, see our guide on how much a new boiler costs in the UK. For signs it might be time to replace rather than repair, see signs your boiler needs replacing.
| Repair | Typical Cost (2026) | Notes |
|---|---|---|
| Diverter valve replacement | £200–£400 | Gas Safe engineer required |
| Zone valve replacement | £150–£300 | Plumber or Gas Safe engineer |
| Cylinder thermostat replacement | £60–£150 | Including labour |
| Immersion heater element | £120–£200 | Plumber or electrician |
| Boiler printed circuit board (PCB) | £300–£600+ | Often close to cost of new boiler on older models |
| Annual boiler service (preventative) | £70–£120 | Catches faults early |
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Frequently Asked Questions
Why does my combi boiler heat the radiators but not the hot water?+
The most common cause is a faulty diverter valve. In a combi boiler, the diverter valve directs hot water either to the radiators or to the hot water taps. If it sticks or fails in the heating position, you get heating but no hot water. This requires a Gas Safe engineer to diagnose and fix.
Why does my hot water run cold after a few seconds?+
This is called a "cold water sandwich" in combi boilers. It happens when hot water stored in the pipe from the last use is followed by cold water before the boiler fires, then hot water again. Some combis have a hot water pre-heat function that reduces this. If it is getting worse, there may be a fault with the hot water heat exchanger.
Can I get hot water without my boiler working?+
If you have a hot water cylinder with an immersion heater, you can switch to electric immersion heating while your boiler is repaired. This is more expensive to run but provides a usable backup. If you have only a combi boiler, you will need to use kettles or book emergency repair.
How quickly will a Gas Safe engineer fix a hot water problem?+
Many common faults (diverter valves, zone valves, cylinder stats) can be fixed within the same visit if the engineer carries the part. More complex faults requiring specific PCBs or heat exchangers may require a second visit once parts are ordered, typically 2–5 days.
Is it worth repairing an old boiler or should I replace it?+
If the repair cost is less than 30% of the cost of a new boiler, repair is usually the right call. If the boiler is over 12 years old and repairs are becoming frequent, replacement makes more sense economically. A new A-rated boiler can also reduce your gas bill by 20–30% compared to an old G-rated model. See our guide on signs your boiler needs replacing for a full assessment.
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