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HOT WATER & CYLINDER FAULTS

Hot Water Cylinder Not Heating

A cylinder may stay cold because the thermostat is not calling, the zone valve is stuck, the boiler demand is missing, circulation is poor or the immersion heater has failed.

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Hot-water cylinders can contain high temperatures, mains electricity and stored pressure

Do not dismantle immersion heaters, safety valves or unvented-cylinder controls unless properly qualified. Stop if water is scalding, the tundish runs continuously or the electrics trip.

MOST LIKELY CAUSES

Start with these

Cylinder thermostat fault No heat demand may be sent.
Zone valve stuck Primary water may not circulate through the coil.
Boiler or control fault The heat source may not respond.
Immersion backup failed The electric element may not heat.
◫ Symptom variations Match the exact heat, recovery or discharge pattern. ⌄
  • Cylinder remains cold
  • Boiler runs for heating only
  • Hot-water valve does not move
  • Immersion switch has no effect
  • Programmer shows hot water on
✓ Quick checks Simple checks before removing covers. ⌄
  1. Check the programmer and thermostat settings.
  2. Confirm the boiler or immersion has power.
  3. Compare hot-water performance at several outlets.
  4. Inspect visible valves and discharge points.
  5. Do not block tundishes or relief pipes.
1 Step 1 — Create a hot-water demand Allow hot-water components to cool and isolate electrical power before opening covers. ⌄

Turn the programme on and raise the cylinder thermostat.

2 Step 2 — Check boiler response Allow hot-water components to cool and isolate electrical power before opening covers. ⌄

See whether the boiler starts or shows a fault.

3 Step 3 — Check visible valve movement Allow hot-water components to cool and isolate electrical power before opening covers. ⌄

Observe the hot-water zone valve without removing covers.

⌁ Common causes explained Why hot-water and cylinder faults develop. ⌄

A cylinder may stay cold because the thermostat is not calling, the zone valve is stuck, the boiler demand is missing, circulation is poor or the immersion heater has failed.

⌘ Advanced diagnostics Heating-engineer checks and repair work. ⌄
  • Test cylinder thermostat.
  • Check motorised valve and end switch.
  • Verify boiler hot-water demand.
  • Inspect coil circulation.
  • Test immersion heater circuit.
▣ Parts that may need replacing Common components linked to this fault. ⌄
  • Cylinder thermostat
  • Motorised valve
  • Immersion element
  • Programmer
  • Circulation pump
! When to call a heating engineer Conditions that should end basic troubleshooting. ⌄
  • The cylinder is unvented.
  • The tundish or relief pipe runs continuously.
  • Water is dangerously hot.
  • The immersion trips the electrics.
  • Internal electrical or pressure testing is required.
? Frequently asked questions Common hot-water questions. ⌄

Can I block a dripping tundish?

No. It is part of the safety discharge system.

Why does hot water run out quickly?

The cylinder may be undersized, only partly heating or recovering too slowly.