F.23
Vaillant
Flow/Return Temperature Difference Too High (Low Circulation)
F.23 error; unit may start then quickly shut down, often with noisy operation
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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
Blocked plate heat exchanger, Partially closed valves, Failing circulation pump (on primary systems), Air in system, Scaled internal waterways
How to Fix / Troubleshooting
Safety: Isolate power and gas. Allow the appliance to cool.
- Check valves: Confirm all service valves on the heater’s primary and DHW circuits are fully open.
- Bleed air: Bleed the pump and high points. Air can cause poor circulation and large temperature differentials.
- Inspect pump operation (if fitted): With power restored briefly, listen and feel for pump running. If noisy, seized, or not spinning, the pump may need replacement.
- Clean/replace plate heat exchanger: Remove the DHW plate heat exchanger and inspect for scale or sludge. Descale using an appropriate descaling solution or replace if heavily blocked.
- Check NTCs: Compare flow and return NTC readings. A faulty sensor can falsely indicate a large temperature difference; replace any suspect NTC.
- Reset and test: After restoring circulation, reset the fault and run the heater under load to confirm stable operation.
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Repair Difficulty
Hard
4/5
Required Part
DHW Plate Heat Exchanger
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