Combi boiler

[Mike Lane]

Hello everyone!

My problem is that I have a Worcester condensing combi boiler about 3 years old that is behaving oddly. The pressure is fluctuating wildly.

When it is completely cold the pressure gauge falls to zero. The boiler still starts though, and when it reaches its maximum working temperature the pressure rises to nearly 3 bars, which is well above the green zone marked on the pressure gauge.

I am getting it serviced in a couple of days by the firm that installed it and they said they would 'look into' the pressure issue, but I wonder in the mean time if anyone could suggest what may be wrong. I rather fear that the firm I am dealing with may come up with some bullshit explanation - especially since the boiler seems to be working perfectly satisfactorily (except for the pressure readings).

[<me9]

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.

[Mike Lane]

On Sat, 31 Mar 2007 00:56:21 +0100, [e-mail] 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?

[John Rumm]

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:

a temporary fix you can drain some water from a radiator so that it is partially full of air. Don't bleed this out, and it will give some makeshift expansion room.

[<me9]

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.

[Mike Lane]

On Sat, 31 Mar 2007 19:04:59 +0100, [e-mail] 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.

[Ed Sirett]

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

[Mike Lane]

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.

[<me9]

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.

[Mike Lane]

On Sun, 1 Apr 2007 23:22:59 +0100, [e-mail] 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!