A27
Herz
Plate Heat Exchanger Blockage / DHW Overheat
Boiler shows A27, burner stops during DHW draw, temperature overshoots
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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 Herz plate heat exchanger, stuck DHW thermostatic mixing valve, restricted primary flow through plate, faulty DHW NTC
How to Fix / Troubleshooting
Safety: Hot water can cause scalding. Isolate power and water before opening DHW components.
- Check DHW temperature: Measure outlet temperature; if excessively high, reduce setpoint and test again.
- Descale plate heat exchanger: Remove the Herz plate heat exchanger and flush with an approved descaler using a pump kit until flow and temperature stabilise.
- Inspect mixing valve: If fitted, ensure the DHW mixing valve operates smoothly and is not stuck in a hot position.
- Replace plate if needed: Severely scaled or corroded plate heat exchangers should be replaced with a Herz original part.
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Repair Difficulty
Hard
4/5
Required Part
Herz plate heat exchanger
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