Why does my Sabre 25HE pcb fuse keeps blowing?

have you noticed any leaks inside the boiler?
any signs of rust or corrosion under the boiler?

Q
April 2015

Looking at the circuit diagram in the installation manual ( http://cdn0.vokera.co.uk/wp-content/uploads/2009/04/compact-se-installation-and-servicing-manual.pdf ).

The live supply feeds 4 things:

PCB transformer (All the time) (Powers the electronics on the PCB)
Burner Fan (via a Via a relay, so only on when needed)
Circulating Pump (Via a relay, so only on when needed)
Ignition Board (Via a relay, so only on when needed)-The ignition board also manages the gas valve solenoids.

If one of these loads is faulty (Short? Failing and going low resistance, may be when hot? or it is a load that is only switched on after the boiler has been running for a while, that could be a problem.

You MAY have some luck disconnecting the fan, pump etc and checking the resistance as soon after the fuse has failed as possible (a minute or two). Blowing a 2A fuse at 240V means the resistance is less than 120 Ohms

-------------DON'T FORGET TO ISOLATE THE MAINS FIRST!!!!!!!!

Good luck
March 2015