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?
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
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