DHW (gravity fed HW pumped CH system; Potterton floor standing (?1200) balanced flue boiler replaced 6 years ago) keeps boiling. I have had thermostats on both boiler and cylinder replaced, new programmer and 1 new mini valve but the hot water continues to boil - it is very frightening when it happens and I have to switch off immediately. Should there be another mini valve (can't find one). Could there be dirt in the system. Could there be something wrong with the feeder tanks? Plumber and electrician both mystified - can anyone help

Sheena Heaton, September 2008

Now know there is only one mini valve. Everything seems to work OK when the CH is on but when the water is on by itself it still boils - when the CH is on the water does not get very hot - although I have turned down boiler and cylinder thermostats. one Plumber has now suggested changing the printed circuit board in the boiler and another has said this would be no good and suggested a new boiler. As the exisiting boiler is only 6 years old I am reluctant to change it - given the cost. Any suggestions gratefully received

sheena heaton, September 2008

when the cylinder stat reaches the set temp of 60 c is it switching the boiler off ?
and shutting the motorised zone valve ?

hi-spec plumbing & heating (essex), September 2008