Thermona Boiler Fails to Shut Down on Demand

Boiler runs continuously and overheats the property, even when thermostat is turned down.

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.

Possible Causes

Stuck relay on Thermona PCB, Room thermostat contacts welded closed, Incorrect wiring bypassing controls, Faulty 3-way valve allowing unwanted flow

Safety: Overheating can cause discomfort and high gas usage. Turn off the boiler if it will not respond to controls.

Check thermostat: Turn the room thermostat to minimum or OFF. If the boiler continues to run, the fault is likely in wiring or PCB.
Isolate heating: Switch the boiler off at the main switch to stop heating.
Engineer diagnosis: A technician should test the thermostat circuit, check for incorrect links at the Thermona control terminals, and inspect the PCB for a stuck relay. Replace the thermostat or PCB as required.

Hard 4/5

Main Control PCB

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