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ooof, that sounds like a nightmare - thankfully for us it was just the temperature sensors somewhere had failed, or were reading incorrectly, so it thought it was chucking out higher temps and therefore modulated down so all we got was lukewarm water.
This may be unfair, but my experience of talking to a lot of plumbers/vaillant/viessman about different control systems and weather compensation etc., is that there isn't a lot of proper understanding of how they actually should work. Dread to think what will happen when the UK eventually starts installing heat pumps everywhere
terlg
Lemme guess. CO / all gas sensor - error code F55?
Apparently they’re much more complicated to commission and set up properly. FWIW an engineer eventually worked out that the flue fan power wasn’t high enough so the condensed water was running back into the boiler and tripping the sensor. TBF we do have a slightly convoluted flue run, but it took months for someone to work this out, all the other guys just changed various parts at random.