Steve, check the air pressure in the expansion vessel. It is a flat cylindrical drum on the back of the boiler with a valve on top covered by a screw on dust cap (just like a car tyre). Pressure should be between 0.8 and 1 bar. If water comes out of the valve when you depress the nozzle with a pin then you have a torn diaphragm and the expansion vessel will need renewing - this is an easy job. If you recharge it then it may hold for a bit but if you have to keep doing it then replace the vessel. What happens is that this vessel acts as a pressure reservoir and it counterbalances the operating pressure of the CH heating system when it is running. If it fails then the pressure in the system can rise over 3 bar and relieve itself to the drain via the pressure relief valve (this is supposed to happen as it is a safety feature). The knock on effect is that when the system cools down then the pressure drops proportionately and ends up falling below that which is required to operate the system and you have to recharge it again. The normal operating pressure of the system required for start up is no more than 1 bar. When running it should always be less than 3 bar. Hope that this helps
Phil the fluter, January 2010
