Worcester 280...when the hot water is on it gets very hot then goes cold for a minute or so then comes on, gets very hot, then off and so on. My plumber looked at it and tested the thermistor, the gas valve and changed the control board and it was still the same. He cannot think of anything else. Can anyone help? Thx, Simon

Simon, September 2006

Hi, sounds like a flow problem in the primary circuit. This can be caused by a blockage in the primary or secondary heat exchangers or the most likely cause is a lazy pump. What happens is the temp gets so high that it trips the overheat stat before the thermistor can turn the gas valve off. The boiler takes a while to cool until the overheat will let the burner ignite again.
Ive fixed this problem with new pumps in the past.

Dave. G20 Gas Services.co.uk, September 2006