I've noticed recently the pressure on my 1.5 year old Worcester combi boiler has been going through the roof whenever the heating comes on. It goes to around 3.5 bar, having been set at 1.5 when cold. I reckon the expansion vessel needs recharging - is this something a competant DIYer can do? Is there a way of checking that it actually in need of charging?

I'd include more details of the boiler but I was late for work this morning when I noticed the rather alarming pressure...

James, May 2007

how do i know how much air to put in?

JD, February 2012

An hour of fun indeed,But now a perfect pressure and no more annoying top-ups,thnx all for your input....

paul, May 2007

Logically the only way it would loose pressure is through a leak, either directly to atmosphere or through a ruptured diaphragm to the water (and hence through the automatic air bleed to the atmosphere). Assuming the vessel has no leaks then the car tyre type valve (Schrader (sp?)) could be faulty, you could replace the core with a new one. There should be a decent valve cap on the valve with an 'o' ring to seal against the lip of the valve, so that if the core isnt perfect air still cant escape. Perhaps the cap is missing and dirt has got into the valve. FWIW I find it odd that such a young boiler has this problem, but am not an expert, I cant see how draining and refilling the system would make the expansion vessel loose pressure.

c, May 2007

Incidentaly - do you know why an expansion vessel would run out of pressure.... I have been just moved a radiator recently, so the system has been partially drained and refilled a few times.

J, May 2007

Exactly what I needed to hear - thanks so much for taking the time to help. I'll have a go soon and let you know if I need any more help.

Thanks very much

James, May 2007

Yes you can do it. I dont know your specific boiler but the expansion vessel is behind the boiler and has a car valve type connector at the top. This is where you can connect a car tyre type pump/guage.

With zero pressure in the water system (front panel guage - you can probably let the pressure off by manually lifting/twisting the over pressure relief valve, or opening a bleed point on a radiator (v messy)) you will need 12 or 15 psi in the vessel (check your manual).

Before the pressure starts to build in the vessel you will need to displace around 8 litres of water (vessel specific)from the vessel through the over pressure vent or the bleed point, only then will the pressure start climbing, it takes a lot of pumping.

I do not know how reliable the expansion vessels are in your make, check that the vessel isnt faulty first by pushing down the central pin in the valve, if damp air with a little moisture comes out it is OK, if water squirts out it needs replacing, not recharging.


After recharging refill the system up to the top of the working pressure range, (1.5 bar?) turn the CH on and check that all is well. Recharging the vessel may also cure any need for frequent small water top ups.

While the system is at zero pressure it may be a good time to inject some fernox system protector maintanance gunk with a caulking gun at a bleed point, the adaptor supplied can be usefull when letting out the initial system pressure too. If you have any small leaks you could try giving it a shot of fernox leak sealing concentrate stuff as well.

An hour of fun for a wet Sunday afternoon.

c, May 2007