Without trying to be funny, any clues to the make and model of this boiler? I find it helps sometimes when asking questions such as you have posed.
I repair boilers for a living, but with the best will in the world, I'd have to be psychic to begin to answer your question.
Carefree
July 2010
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.