There should be a filling loop connection on the central heating return pipe close to the boiler. Usually the filling loop is left connected between the cold water in and the CH return. If you see a small valve in the cold not connected to anything and an isolation valve going into the CH return with nothing connected to it, you will need to buy a filling loop to join them up before refilling (try B&Q).
If pressure is lost after filling up it (fill to approx 1 Bar) could be that you have a water leak or that your expansion vessel has an internal leak. You can check the latter easily. At the bottom of the boiler you will see a large red vessel. If you unscrew the black plastic cap (if fitted) from the Schrader valve (car tyre type) and momentarily press in the release pin in the middle of the valve. You should hear a small hiss. If water comes out the diaphragm is split and the vessel needs renewing. If you hear no hiss but no water comes out, pump it up to 0.5 Bar with a car/bicycle pump.
Sometimes the auto air vent on top of the boiler sticks and air is not expelled from the heat store. You can test this by very carefully pushing a thick piece of wire down the small hole in the top. If when you open the valve water comes out it is working. If air comes out it is not. Having freed it, it may then work OK or it may then leak water. If it is the latter you must replace it (try B&Q).
Kevin
October 2005
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