Boiler displays E20 and will not start, often after wiring changes or thermostat replacement.

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

Incorrect room thermostat wiring, Open circuit on call-for-heat terminals, Faulty external controls, PCB input fault.

Safety first: Isolate power before accessing wiring terminals.

User checks:

Ensure the programmer is set to ON and the room thermostat is turned up above room temperature.
Check any external controls (e.g., wireless receiver) for batteries and status lights.

Engineer checks:

Verify correct wiring of the room thermostat and programmer to the boiler's low-voltage control terminals.
Link out the call-for-heat terminals at the boiler to test if it fires without external controls.
Check continuity of control cables and replace them if damaged.
Replace faulty external controls or, if necessary, investigate PCB input issues.

Correct wiring and control configuration are essential for safe boiler operation.

Medium 3/5

Room Thermostat / Control Wiring

Check Price on Amazon

We may earn a commission from links on this page.