Safety: Turn off power and allow the furnace to cool. Do not continue to reset an overheating unit.

Steps:

Check system pressure and gauges: On hydronic Buderus furnaces/boilers, verify that system pressure is within the normal range indicated on the gauge. Very low pressure can cause overheating.
Verify pump operation: When calling for heat, listen and feel for the Buderus circulation pump running. A silent or very hot pump body may indicate failure.
Bleed air: Air trapped in the heat exchanger or radiators can stop flow. Bleed air from high points and radiators using bleed valves, topping up system pressure as needed.
Check valves: Ensure isolation valves around the furnace and main heating loops are open.
Reset high limit: Some models have a manual reset high-limit switch on the heat exchanger. After cooling, press the reset button if present.

If E9 recurs, a technician should test the pump, expansion tank, and high-limit sensor and inspect for internal blockage.

Error E9, furnace shuts down on overheat, may be very hot to the touch

Possible Causes