F20
Protherm
Primary Heat Exchanger Overheat
F20 displayed, furnace shuts down with overheat indication
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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
Severely restricted water flow through Protherm heat exchanger, Airlock in hydronic circuit, Failed circulation pump, Scaled or partially blocked heat exchanger
How to Fix / Troubleshooting
Safety first: Turn off power and allow the unit to cool fully. Do not repeatedly reset an overheating furnace.
- Step 1 – Check system pressure and air: Verify system pressure is within range (around 1–1.5 bar). Bleed air from radiators and high points to restore circulation.
- Step 2 – Pump operation: Listen and feel for the Protherm circulation pump running when there is a heat demand. If silent or very hot to the touch, it may be seized.
- Step 3 – Clean strainers: If fitted, clean any system strainers or filters on the return line to the furnace.
- Step 4 – Descale heat exchanger (technician): A technician can chemically flush the primary heat exchanger if scale or sludge is suspected.
- Step 5 – Replace pump if needed: If the pump does not run or has low output, replace it with the correct Protherm pump assembly.
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Repair Difficulty
Hard
4/5
Required Part
Circulation Pump
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