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Last update to this saga: replaced the room thermostat (a Honeywell) for a Vaillant VRT 15 and all problems are solved, it was clear that the old thermostat stopped working as expected. Thanks everyone for the input and hopefully this story will help someone else out there.

Thanks everyone who have helped here, and to give some closure (or some extra information, if that helps anyone landing here from an internet search), I've just discovered that the boiler might have been working well all along, and the mechanical thermostat seems to be the problem. Basically, it...

Josh - I admit that I can't find the terminals marked RT24 in the installation manual, and don't recall seeing anything bridged. Having tested it a few days more, it appears that the boiler is behaving as expected, so it was [hopefully] a slight misunderstanding from my side that d.08=1 meant...

No, it hasn't always done this - I'll check the RT24 terminal though, just to be sure. A slight behaviour update though: yesterday I left it running (as I needed to heat up the house a bit) and eventually the CH behaved as it should be, with d.08=0. I thought that maybe I was being impatient...

Hi, all, I wonder if anybody can help with a problem that I have on a boiler that's about a decade old but in good condition. Basically, when I switch on the central heating, it will run regardless of the demand signal from the simple Honeywell thermostat that I have attached to...

Hi, Trev - have you managed to fix your issue? If so, what did you do? I have a very similar problem - I own the same boiler with a simple room thermostat connected to terminals 3 and 4. I tested that it works and that there is continuity in the cable; however, when the CH is ON, the boiler...