Issue with Worcester 280?

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