Ian - this is not a very clear description and any answer you may get will not be worth much - have another go...
State the make and model of the boiler (if it is actually a boiler)
State clearly what the problem is - what it is not doing that it should - or what it is doing that it should not be doing - and under what circumstances.
Break the clauses up with dashes or commas or full stops to aid clarity and understanding.
Good luck...
Peccavi
November 2009