A lesson was learned yesterday. My boiler decided it was no longer going to fire up, so I had a few rather cold evenings in the winter. Ordered a replacement controller on eBay, plugged it in, and it still did the same. I was rather confused.
Turns out, there's a fused switch next to the boiler. The fuse had gone, so there was no power at all to the controller. But the controller has a built-in battery, which continued to make it look like it was still working. Useful!
If your Randall Danfoss controller decides it's no longer going to start your boiler heating up, check the fuse first. Looks like it's supposed to be a 3A. Whoops. Still, I'm keeping the spare controller just in case of other winter trouble, until I replace the boiler with a proper combi!
Edit: Turns out it was a good idea to keep the spare controller. It just popped again.
