The two amp mains fuse on the 350 combi board near the mains input connector has blown. Replacing same all ok until domestic hot water or central heating demand. Then blows instantly. All boards appear both clean and dry. Apart from the fuse there's no visible sign of distress. I do have electronics experience and probably could replace a defective component on a board, if necessary

HowardA, August 2008

Most common culprit on this one is the fan resistor although anything on that board that's mains powered can do it.

Easy way to pin it down - disconnect everything (external controls if fitted, pump, fan, gas valve etc) off the board, add new fuse, connect things one at a time, see which one makes it go pop.

BunSlinger, September 2008

Thanks for this suggestion. The plot now thickens. Inserting four one hundred watt light bulbs connected in parallel to make a total of four hundred watts, and then wiring them across the fuse holder, with no fuse installed, and powering up the boiler either dhw or central heating, the boiler works
fine. The glow from the bulbs varies as various relays operate, but never bright enough to equal the 480 watts
needed to blow the fuse. So either the fault has cleared,
or it's an intermittent fault. The voltage across the bulbs is less than 40, meaning the boiler unit is getting about 200 rather than 240 volts. Oh, the dc resistance of both pumps, and both gas valves tests ok. Will see what happens over the next few days. Thanks again

HowardA, August 2008

wouyld think its either a gas valve solenoid, fan or pump thats blowing the fuse, not the board itself

T, August 2008