When it gets really cold the condense pipe freezes and the boiler stops working.
What idiot designed a system that causes the heating to shut down when it actually gets cold.
I am heading outside with a hot kettle to try and melt the frozen condense pipe.
That may have cleared it. But need to find the bit of lagging that has come off.
My son just asked why it needs to go outside. Excellent question. I looked. Running the pipe through wall to utility room and in to the drain there would have been a way simpler route and involved a hole in an interior rather than exterior wall. Damn plumber.