Central heating pressure gauge...sudden drop when pump stops

25 Jan 2008
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United Kingdom
Hi there,

I was wondering whether anyone could explain the following:
I have a sealed system, with GlowWorm 18sxi boiler. Now, I've noticed (whilst decorating - hence sat in front of the pressure gauge in the airing cupboard for prolonged periods of time!) that when the heating comes on, the pressure gauge slowly rises.....gradually up to 2bar (good sign, I guess, that the pressure vessel is doing its job). Then, when you hear the pump stop the pressure gauge instantly drops to 1.5bar - literally, as soon as you hear the pump stop the gauge plummets 0.5 bar. It then gradually drops to 1 bar as it cools down. Is this sudden pump-related drop "normal"?

I'm just interested to know if this is how you'd expect the system/pressure to work.....

Thanks for your time,

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