Well, I thought it had a problem....until I fixed it
Problem has been going for a good few weeks now....
Turn a cold water tap on and at turning it off the boiler goes into H/W mode for about 1 sec then leaves a pump running for about 20secs before either shutting down or going back to C/H mode. Same thing happens if you flush the loo or if the washing machine / dishwasher is taking in cold water.
Decided to do a google on the problem and a couple of sites suggested there was air trapped in the cold water pipe somewhere and suggested running all the cold water taps to try and get rid of the air lock.
The local water board did turn off the water about a month ago for a few hours to fix a leak a few roads away.
All the cold taps run water fine......then it suddenly dawned on me that it might be the shower, as the shower tap is always set to max hot as the H/W temp is controlled by the boiler (45c). Turned the shower to max cold and turned it on, it spat and hissed for about 10 secs before settling down to constant flow of cold.
Boiler problem has gone away and running a cold tap on max and shutting off doesn't affect the boiler now
To think I was ready to callout the boiler manu as I thought it was a boiler problem and all it was, was trapped air
Job jobbed