Glow-worm
Unstable Domestic Hot Water Temperature
Hot water temperature fluctuates or runs hot then cold on a 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
Scaled or partially blocked plate heat exchanger, Faulty DHW NTC sensor, Inadequate flow rate or mixer tap issues, Modulation or gas valve problem
How to Fix / Troubleshooting
Safety: Risk of scalding from sudden hot water; be cautious when testing taps or showers.
- Check flow rate: Ensure hot taps are opened sufficiently. Very low flow can cause cycling and temperature swings.
- Test multiple outlets: Compare behaviour at different taps and showers to rule out local mixer valve faults.
- Check cold inlet filter: Some Glow-worm combis have a cold water inlet filter. If accessible, isolate water and clean the filter to restore proper flow.
- Engineer tasks:
- Descale or replace the plate heat exchanger.
- Test and replace the DHW NTC sensor if readings are unstable.
- Check gas valve modulation and combustion settings.
In hard water areas, fitting a scale reducer can help prevent recurrence.
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Repair Difficulty
Hard
4/5
Required Part
Plate Heat Exchanger
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