The backup electric resistance may not activate even when the controller gives the correct command.
Possible causes
Burnt relay, contactor with worn contacts, fuse, or power supply problem.
How to isolate the fault
Check if there is a command at the output and if power is actually reaching the resistance.
Symptoms
The unit may fail to reach the setpoint at low temperatures or for DHW (Domestic Hot Water).
Safety
Power measurements must be performed by a qualified electrician/technician.
