Hot water temperature fluctuates or runs hot then cold on a combi boiler

Safety: Water may become very hot unexpectedly; take care at taps and showers.

• Run a single hot tap: Test with one tap fully open. If temperature is more stable than when multiple outlets are used, the boiler may be undersized or restricted.
• Clean inlet filter: Isolate cold inlet and clean the Main combi boiler’s inlet filter to ensure adequate flow.
• Check shower mixers: Thermostatic mixer valves can fail and cause fluctuations; test at a simple basin tap for comparison.
• Engineer actions:
  • Descale or replace the Main plate heat exchanger if partially blocked.
  • Test and replace the DHW NTC sensor if readings are unstable.
  • Inspect and service the diverter valve for smooth operation.