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. ⌄
- Tundish drips while cylinder heats
- Discharge is continuous
- Water in tundish is very hot
- Pressure rises during heating
- Dripping began after servicing
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 — Do not block the tundish or discharge pipe Use visible controls only and stop if pressure or temperature is unsafe. ⌄
Process
Leave the safety route open.
Step 2 — Note when the drip occurs Use visible controls only and stop if pressure or temperature is unsafe. ⌄
Process
Observe whether it happens only during heating or all the time.
Step 3 — Turn off heat if discharge is hot or heavy Use visible controls only and stop if pressure or temperature is unsafe. ⌄
Process
Switch off the normal heat source without opening equipment.
Common causes explained Why hot-water systems lose heat, overheat or discharge. ⌄
A dripping tundish shows that a safety or relief valve is discharging. It may indicate expansion-vessel failure, excessive pressure, overheating or a faulty relief valve.
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. ⌄
- Check expansion vessel charge and diaphragm.
- Test pressure-reducing valve operation.
- Inspect temperature and pressure relief valves.
- Verify thermostat and safety cut-out.
- Confirm discharge pipework complies and is unobstructed.
Parts that may need replacing Common components linked to this fault. ⌄
- Expansion vessel
- Pressure-reducing valve
- Temperature and pressure relief valve
- Expansion relief valve
- Thermostat
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.