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HOT WATER & CYLINDER FAULTS

Hot Water Temperature Fluctuates

Fluctuating hot water can be caused by unstable flow, boiler modulation problems, scaled heat exchangers, faulty mixer valves or changing demand from other outlets.

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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

Flow rate changing Other taps or appliances may alter demand.
Boiler modulation fault The burner may cycle too aggressively.
Mixer valve fault The blend may shift between hot and cold.
Scale or restriction Poor heat transfer can create unstable temperatures.
◫ Symptom variations Match the exact heat, recovery or discharge pattern. ⌄
  • Shower goes hot and cold
  • Temperature changes when another tap opens
  • Only one mixer affected
  • Boiler cycles rapidly
  • Problem worse at low flow
✓ 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 — Compare multiple outlets Allow hot-water components to cool and isolate electrical power before opening covers. ⌄

Check whether the issue is local to one tap or system-wide.

2 Step 2 — Test with stable flow Allow hot-water components to cool and isolate electrical power before opening covers. ⌄

Avoid opening other outlets during the test.

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

Note whether the burner cycles or shows an error.

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

Fluctuating hot water can be caused by unstable flow, boiler modulation problems, scaled heat exchangers, faulty mixer valves or changing demand from other outlets.

⌘ Advanced diagnostics Heating-engineer checks and repair work. ⌄
  • Check boiler modulation and flow sensor.
  • Inspect plate heat exchanger.
  • Test thermostatic mixer valve.
  • Measure flow rate.
  • Check inlet filters and scale.
▣ Parts that may need replacing Common components linked to this fault. ⌄
  • Flow sensor
  • Plate heat exchanger
  • Thermostatic mixer valve
  • Temperature sensor
  • Diverter valve
! 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.