Somebody has asked me about a problem with the hot water from their combi - a 4 year old Worcester Bosch 24Cdi - which alternates hot & cold, with the boiler switching on for about 20 secs then off for about the same time. The initial problem they asked me about was with the shower but they say it does this with demand from other hot taps. From the Cdis I've seen I thought this was how they normally worked :-) but they say it used not to behave like this.
Any clues what's amiss? Obviously the diverter valve is a potential suspect but when I googled somebody (Ed I think) said these machines also have a separate flow sensor. Not sure how that works, and I can't find the paper copy of the manual I thought I had and Woosh Boster's web site seems to be on strike atm. :-(
Another previous thread suggested a scaled up DHW heat exchanger or dodgy temperature sensing thermistor might account for it. I've asked them about the flow and temperature but I won't hear for a day or two...