Combi boiler loses pressure whenever radiators are on

27 Feb 2006
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United Kingdom
Ever since I removed and replaced a radiator (whilst decorating) my combi boiler has been experiencing a complete loss of pressure.

It happens like this: I pressurise the system to 1 bar (as per the manual) and the hot water works fine.

But as soon as I turn on the heating, the pressure (as read on the pressure gauge) goes right up to 3 bars and water starts leaking out of the boiler - presumably via the pressure release value.

When the radiators are switched off and the system allowed to cool down, the pressure drops to 0, and the hot water stops working until I repressurise it again.

Any suggestions? Thanks :)
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The radiator you have removed previously contained sufficient air at the top to cushion the extra volume (approx 4% of system volume) of expanded water when the heating came on.

Since refilling and bleeding you now have no air in that radiator.

This expansion water is normally accomodated by the expansion vessel within the boiler. The expansion vessel should normally be pressurised with air to abount 0.7 Bar (in a normal flat/house).

Check previous posts etc for more details.
GasGuru - I've got exactly the same problem, mines a Baxi 133 HE Cond Combi with 17 rads. Can you tell me what the expansion vessel pressure should be and how you work it out? What happens if its more or less than that.

Cheers, Andy
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