Vaillant
Unstable Domestic Hot Water Temperature
Hot water temperature fluctuates or runs hot then cold on a Vaillant combi boiler
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Partially blocked plate heat exchanger, Faulty DHW temperature sensor, Sticking diverter valve, Limescale in DHW side
How to Fix / Troubleshooting
Safety: Water may suddenly become very hot; be cautious when testing taps or showers.
- Check flow rate: Open the hot tap fully. Very low flow can cause temperature swings; clean tap/shower filters and check mains pressure.
- Test multiple outlets: Compare behaviour at kitchen tap and bathroom. If only one outlet is affected, the problem may be local to that mixer or tap.
- Run boiler at moderate setpoint: Set DHW temperature on the boiler to around 50–55°C to reduce extremes.
- Engineer service: A technician can clean or replace the plate heat exchanger, check DHW NTC sensor, and service the diverter valve.
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Repair Difficulty
Medium
3/5
Required Part
Plate Heat Exchanger
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