Do some fault finding and let me know more - is the pilot light ok? (probably), is it firing up? (prob not). It 'sounds' like a common problem with the 3 way diverter valve. This valve has a diaphragm in it that fills up & expands & presses a pin that presses a microswitch to tell the boiler to fire up. The diaphragm is prone to wear & may have developed a hole that got bigger & is no longer inflating. Fault find by first checking your pressure, it should be at 1bar. Turn on the hot tap a 1/4 turn and listern to the boiler to hear if it is switching - if it does its ok is prob another problem? If there 'is' a hole in the diaphragm, turning the tap on a bit won't help so try turning the hot tap on to full blast from closed, the burst of pressure might make it temporarily fill up & switch then drop away & turn off. Going off your description of the problem, more than likely it is the diapragm because of the way you said it slowly got worse. Check this link for a diagram of the boiler http://www.centralheatingonline.co.uk/images/tech.jpg on the 3 way valve check the micrsoswitch is ok, it 'should' just pull off. Check with your finger to see if the switch actually operates ok, there is usually a disc on the 3 way valve pin that pushes the microswitch - check that hasn't broken. If all of the above is ok then the problem is prob in the gas section of the boiler & you'll need a prof.
Lee, May 2006
