BoilerTroubleshooting2026-01-22
Boiler No Hot Water: Common Causes
Heating works but taps run cold? Suspect the diverter valve, flow sensor or plate heat exchanger.
On a combi boiler, heat working but no hot water points to the hot-water side of the unit. Here are the usual causes.
Diverter valve and flow sensor
A stuck diverter valve keeps the boiler in heating mode and won't send heat to your taps. A faulty flow sensor may not detect when you open a tap, so the boiler never fires for hot water.
Plate heat exchanger
Scale build-up in the plate heat exchanger (common in hard-water areas) reduces hot-water performance, causing lukewarm or fluctuating temperatures.
Key takeaways
- Heat works but no hot water = hot-water-side fault
- A stuck diverter valve is a frequent cause
- Descaling restores plate-heat-exchanger performance
- Fluctuating temperatures often mean scale
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