Look on the uk.diy newsgroup combi boiler faq for more information, or google 'combi expansion vessel' etc.
If you don't have the installation and servicing instructions you can download them (pdf) from the Vokera website.
The expansion vessel sits behind the boiler, it has a capacity of 8 litres or so and comes supplied with 0.8 bar(12 psi) air pressure (measured with no pressure in the boiler, ie zero on the front panel pressure guage), there is a car tyre type valve on the top right of the vessel for attaching a tyre pump and for measuring the air pressure.
Switch the boiler off, release system pressure down to zero either by lifting the over pressure valve, or by opening a radiator bleed point (messy). Connect an air pump to the valve on top of the vessel and start pumping. Stop when you get to 12psi. Around 8 litres of water will come from the radiator bleed point or out the overpressure outlet. It takes a while to displace all the water in the vessel before the air pressure can start to build. It is also possible that the diaphragm in the vessel has failed, if this is the case when you press down the inner pin in the tyre valve on the vessel a trickle of water will flow. After recharging the vessel close the bleed point and refill the system to the top of the operating pressure range (1.5bar), your big pressure fluctuations should be gone.
Or ... Phone Vokera (www.vokera.co.uk) and get the name of a local approved service man. Get the boiler serviced; checking the expansion vessel is one of the standard bits of maintenance, after a proper service you should be able to forget about your boiler for another couple of years. It is worth getting it done by someone who knows the model well. It may save you much hastle, inconvenience and expense.
cd, April 2007
