I have a SIME Super 102 combi boiler. When any DHW tap is opened the boiler starts up ok and pilot light comes on followed by main burner. However after approx 1 minuite the main burner starts to "hunt" i.e the burner jets appear to pulse rapidly. If I open the DHW taps fully, the pulsing continues. I have fitted a new fan as I thought it was air starvation. Still the same issues. The flue is clear without any obstructions.Could it be the main gas valve modulating unit and what is the cure?

Thanks. MD

Mike Davies, September 2008

I had this problem in the past and the fix is fairly easy . The problem lies in the pipes which report air pressure in the flue . To the right of the fan at the top of your boiler compartment is a diaphram component which is connected via 2 shorts rubber tubes to the flue itself . These 2 brass pipes are joined inside the flue by a vertical brass tube approx 15mm in diameter . If one or both of these pipe become calsified (chalked up ) , then the boiler suspects that the flue is blocked and shutsdown after about a minute . If you are electrically minded , then you can test this out which a test meter .

The solution is a torch and a very fine drill to clear the pipe .

Stuart Kirk, September 2011