blowing fuse

I have been helping a friend with a combi unit (Navien CH-240 ASME) issue and am in need of some advice... I maintain my own tankless system but only for DHW so I know enough to be dangerous! Hopefully this is the right forum for this topic. Short story: What started with a small leak at the...

My Gledhill boilermate OV stopped heating the water, blew the on board fuse and tripped one of the three breaker switches inside the boiler. The gas boiler was showing a low temperature but was ok. To fault find turned Gas boiler off and on to electric only. (Switch setting) replaced fuse...