Combi boiler

See the sealed system FAqs (someone will be around shortly with the URL).

The symptoms are exactly what you get with loss of pre-charge pressure in your expansion vessel.

On Sat, 31 Mar 2007 00:56:21 +0100, snipped-for-privacy@privacy.net wrote (in article ):

Well I'd be grateful for that

Eek, yes it sounds expensive! I wonder if it's normal in a relatively new system? Does it mean replacing the expansion vessel?

Classic case of loss of expansion space. The boiler contains an expansion vessel that is charged with an air bubble. As the system heats up and the water expands this is designed to allow the expansion y the bubble compressing. If the air charge is lost, or the rubber diaphragm that separates the air from the water fails, then you lose this capability. The result is the pressure shoots right up until the pressure relief valve opens and discharges some water (usually outside). Once the system cools you end up with no pressure since you have lost water.

For details on expansion vessels see the sealed system FAQ:

google is your friend.

If the diaphram has failed then a new pressure vessel is needed, usually cheaper to fit a second one near the boiler.

If it has just lost the charge, then it's just a matter of pumping it (with the water side depressurised) up via the schraeder valve with a suitable foot pump (difficult at high level) or bicycle inflator. They usually run at about 10psi. (0.7 bar), checkable with a tyre pressure guage.

On Sat, 31 Mar 2007 19:04:59 +0100, snipped-for-privacy@privacy.net wrote (in article ):

Well, my local firm sent an engineer round today who did just that - pumped it up with a foot pump and let some water out, and everything seems to have returned to normal.

So, unless the pressure does not hold, it looks as though that has cured it. Thanks for your advice.

IME that is not very likely to be a permanent solution. There is a reason which the vessel lost it's precharge.

On Sun, 1 Apr 2007 20:07:09 +0100, Ed Sirett wrote (in article ):

Yes that thought occurred to me as well, but I'll just have to wait and see. It doesn't seem to be losing pressure immediately so I'm hoping that whatever is leaking is doing it very slowly.

Mine needed recharging 3 years ago (after 10 years) it hasn't significantly dropped since, might last another 7 years.

If it's dropped in a short time that may be of greater concern. If it does it again it will need further investigation.

On Sun, 1 Apr 2007 23:22:59 +0100, snipped-for-privacy@privacy.net wrote (in article ):

Well it was installed new just over 4 years ago, so it's a bit soon to be giving trouble it seems. Still I'm keeping my fingers crossed at the moment!