Low Boiler Pressure when Off

25 Nov 2012
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United Kingdom
Hi, first post so be gentle :)

We've got a British Gas CDi Series boiler from circa 1999 in our new house. Recently we noticed that the pressure when the boiler is not in use is around 0.5bar or less. When fired up the pressure will rise to approx. 1.5bar which i'm led to believe is a good operating pressure.

Is it normal for the inactive pressue to be this low?

Also, we recently had a gas engineer in looking at another issue and he noticed that the pressure was too high apparently, so he let some out. I wasn't around at the time so im short on details as to what he did and what the pressure was.

I've read that if the boile rpressure drops below 0.5bar that some boilers wont fire up due to possible damage? I'm tempted to top up the pressure, but a bit concened that then the operating pressure will be too high.

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Top the pressure to about 1bar. Then put the heating on and keep an eye on the gauge to see what the pressure does
OK thanks, is there a maximum pressure I should be worried about? Presumably if i top up to 1bar when off then it should go to 2bar when on. Is this acceptable?

Thanks again :)
Right, so I topped up to slightly over 1bar because i wasn't quick enough with the know (ooh er!). Now when the heating is on the pressure was creeping bast 2.8bar, which i thought may be a bit too high now :eek:

I'm thinking I should let some back out? If that's the case, where is the PRV? Cant seem to find a link that shows me how to let water out :oops:
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As you would have read in the FAQ on this site, had you looked, you would see that your expansion vessel is failed or needs repressurising.

You might be able to do that but would need to read the FAQ to see how.

Or call your boiler engineer! But as he did not twigg the EXV was failed perhsps he is a muppett? Or did not want to do another "while you are here could you...."

Just bleed a radiator and the pressure will drop
Don't touch the PRV, once opened it may not seal properly,
The PRV will open at 3bar so 2.8 is a little high
Thanks again.

@Tony - I had read the FAQ to find out how to add pressure, but had overlooked the part about Pressure dropping and rising. I think i'm probably out of my depth with regards to that so will probably call an engineer to take a more thorough look.

@Mandate - I'll not touch the PRV then, thanks. May bleed a rad though to keep the pressure more in the norms.

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