99% hot water sensor failed, about 12 quid and a doddle to change and cheaper than most call outs. Way to tell is to look through boiler sight glass at flame. When you run a hot tap the flame will light onto full and a few seconds later as water starts to reach temp the flame should start to lower and lower to the min gas pressure, but on the baxi when sensor fails it just stays on full flame roaring til hot water then shuts off for couple seconds and repeats.
Thats the most common, other failures possible but thats the cheapest place to start and most common so why not, turn off cold water supply, isolate elecs, open low hot tap, slowly unscrew sensor with spanner (as sensor unscrews should get some water from tap) replace, nip up and turn back on supplies, observe for leaks.
x, September 2009
