I wonder if anyone can help. As it’s Bank Holiday I can’t call our plumber out until later in the week. We have a Warmflow UC120HE combi boiler (runs on oil) which is working fine for hot water but trips the circuit breaker if we try and switch on the central heating. With the case off it looks like this:

I’ve done two things that may shed some light onto it or they may just be coincidence.

1. There was a small leak which was dripping onto the pumps. I’ve tightened the nuts where the leak was and dried out the pumps in the area under their black cover where the wires connect up.

2. We’ve ripped out an old kitchen in a different room and had disconnected the earth wire where it was attached to the pipes under the sink. I don’t think we need it anymore as we’ve had a full rewire and the old kitchen was always going to be ripped out. However I have now re-attached the earth to the pipes.

All other electrical items in the house are working fine and as I’ve said above the boiler works fine for hot water.

Any thoughts or is this best left to a professional?

Next to my cistern is a box with a valve and switch to swap between water and heating. This should switch automatically but mine died. It is possible to flick it manually.

If you’re not confident, and it’s tripping electrics, better off waiting for the professional IMO.

Sounds like the diffuser(?), bit that switches between water/heating operation, especially with small leak. Had the same on a Vaillant combi recently, professional help needed. Replaced crap plastic part with a metal one at part cost of £130 but whole system needed draining to do it.

Try disconnecting the central heating pump and see if the fuse still pops (I am assuming as there are two pumps, one must be for the heating only????)

Diverter valve?

Diverter valve failure would mean no hot water, no?

I’ve disconnected the central heating pump. Nothing happens when I switch the heating on, the fuse doesn’t trip either. Hot water still works fine. I did notice quite a bit of dampness in the pump cap although it was bone dry inside so I’ve left it off for the time being to see if it just needs further drying out.

Thanks for all your replies. I’m guessing if this last attempt at drying it out doesn’t work I’ll call a plumber in the morning. The strange thing is that we had a major leak about 6 months ago and the pump kept working fine even though water was pouring over it.

Sounds like a short circuit in the pump then (armature burned out?).

Mid position/three way valve?, if that has failed the it will only give you which ever position it failed in HW, CH or both.
There is usually a manual override lever on the actuator to set it to give both functions.
Mine is in the airing cupboard down stream of the pump.

Pumps are notorious for tripping electrics. That looks like a Pump Plan so pump valves off check for water out of bleed screw. Change pump head pretty simple 4 Allen bolts. Done!

I had a similar issue a few weeks ago in someone’s house. The pump had jammed solid and the resulting over current was tripping the MCB.