Hot-water systems can involve scalding water, mains electricity and pressurised cylinders
Do not remove boiler covers, immersion-heater covers or adjust unvented-cylinder safety devices unless qualified. Hot discharge, overheating, heavy tundish flow or cylinder leaks need urgent attention.
MOST LIKELY CAUSES
Start with these
Symptom variations Match the temperature, timing and system behaviour. ⌄
- Cold water only at every hot tap
- Hot water worked earlier then stopped
- Heating works but hot water does not
- Immersion switch is on but water stays cold
- Boiler shows a fault code
Quick checks Use user controls and visible observations only. ⌄
- Check whether all hot outlets are affected.
- Identify whether the system uses a boiler, cylinder, immersion heater or a combination.
- Review visible timer, thermostat and boiler status indications.
- Note whether the problem occurs during heating, after heavy use or continuously.
- Stop if water is dangerously hot, leaking or discharging through a safety pipe.
Step 1 — Check whether the fault affects all hot outlets Use visible controls only and stop if pressure or temperature is unsafe. ⌄
Process
Run more than one hot tap briefly.
Step 2 — Check visible controls and power Use visible controls only and stop if pressure or temperature is unsafe. ⌄
Process
Inspect boiler status, timers, thermostats and immersion switches without opening equipment.
Step 3 — Allow sufficient recovery time Use visible controls only and stop if pressure or temperature is unsafe. ⌄
Process
For stored systems, wait for a full heat-up cycle.
Common causes explained Why hot-water systems lose heat, overheat or discharge. ⌄
No hot water may be caused by a boiler fault, failed immersion heater, empty or cold cylinder, timer setting, thermostat fault, power loss or a supply issue.
Hot-water faults can involve the heat source, storage cylinder, controls, mixing valves, circulation pipework or safety devices. The exact system type determines the safe repair path.
Advanced diagnostics Plumber or heating-engineer checks. ⌄
- Test boiler hot-water demand and controls.
- Check immersion element, thermostat and safety cut-out.
- Inspect motorised valves and cylinder sensors.
- Confirm cylinder heating circuit flow.
- Diagnose electrical supply and control faults safely.
Parts that may need replacing Common components linked to this fault. ⌄
- Immersion heater
- Immersion thermostat
- Motorised valve
- Cylinder thermostat
- Boiler control component
When to call a plumber or heating engineer Conditions that should end basic troubleshooting. ⌄
- Water is dangerously hot or the system appears to be overheating.
- A tundish or warning pipe is discharging hot water.
- An unvented cylinder or safety valve is involved.
- The cylinder body, immersion heater or boiler is leaking.
- Electrical testing of immersion or boiler controls is required.
- The fault persists after checking normal user controls.
Frequently asked questions Common questions about hot-water faults. ⌄
Why does hot water run out faster than before?
The cylinder may not be heating fully, an immersion element may have failed, or demand may exceed the stored volume.
Why is a tundish dripping?
It shows that a safety valve is discharging, often because of expansion, pressure or overheating faults.
Can I turn up the thermostat to fix lukewarm water?
Only normal user settings should be adjusted, and never beyond safe manufacturer guidance. Persistent faults need testing.