A Worcester 31i boiler just a year old. Central heating working as expected. DHW supplying 2 sets of monoblock mixers + shower unit. Boiler set to default with DHW at 50C and preheat at 55C. Pipe runs less than 4 metres.
The problem is - when a tap or shower is on fully hot, all is fine, but turn it down just a fraction and the boiler switches off and the water goes cold. Switch it on again and I get about 5 seconds of warm water ( presumably from preheat ) then thirty seconds of cold before the hot water arrives. This means that in the shower the water is hot - then cold - then hot again - then cold ( and squeals from the missus.) The installer inspected the system and said that was a characteristic of Worcesters - which didn't help much.
Before this I had a Vaillant boiler for well over a decade and this didn't show the same problem.
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