F16
Glow-worm Pump overrun / circulation fault

F16 displayed, furnace locks out with signs of rapid temperature rise

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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

Pump not running or seized, Airlock in primary circuit, Blocked system filter on Glow-worm return, Incorrect wiring of pump overrun

How to Fix / Troubleshooting

Safety: Isolate power and allow the appliance to cool.

Technician checks:

  • Confirm that the pump runs during and after burner operation (overrun period).
  • Bleed the pump and high points in the system to remove air.
  • Check and clean any magnetic or mesh filter on the return to the furnace.
  • Verify correct wiring of the pump to the Glow-worm PCB or pump overrun relay.
  • Replace the pump if it fails to start or has low output.
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Repair Difficulty

Hard 4/5

Required Part

Circulation Pump
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