I have recently repaced my mercury vapour switch.Second time in 8 yrs. The pilot ignites, but the boiler will not fire up. The fan works O.K. Could it it be the gas control valve that has gone. If so, how much are they?

Mike Cox., May 2006

if you have fitted a new mercury expansion switch then the new point of call has to be the gas valve as thats the next thing to operate. If you are handy with a volt meter check for voltage on the solanoid on the gas valve or safer still check the resistance of the solanoid with the power off. (high readings or open circuit and its knackered)

Paul the gas man, May 2006