Combi boiler pressure question

[john2k]

I have a baxi potterton Combi boiler. Just noticed that the pressure was low in the red and I can swear I last topped up a week or two ago. Before that I didn't top up for about a year it was fine. But now since my last top-up I noticed it went low again in the red. As soon as I turn central heating on the boiler pressure rises considerably. Few years back I had similar issue and baxi changed the pressure relieve valve. Wondering has this gone again? Is there a way to test it?

 

[denso13]

Possibly the pressure vessel needs checked and re-pressurised https://www.diynot.com/diy/threads/faqs.37170/#post-251090

 

[CBW]

Sounds like expansion vessel, however to access this on your boiler will need to be done by a suitably qualified engineer - gas safe registered as it involves access behind the combustion circuit and will need 26.9 checks to be carried out.

 

[ianmcd]

As others have said the expansion vessel needs checked/pressurising, this should be done during the annual service but very seldom is

 

[john2k]

Thank you. Booked in a service engineer to come and look at it. I requested a service too. What does a boiler service actually include?

 

[john2k]

So I've had the expansion vessel checked and re-pressurized. Apparently it's quite normal for the balloon like vessel to deflate and have these issues. Seems to be ok now. But when boiler is on and heating has been on for a while the pressure does increase a bit compared to what the pressure is when the boiler is idle. Is that normal? When boiler is off it's at about 1.5 and when heating turns on it goes to about 2 and when it cools down the back to original place.

 

[CBW]

Yes water expands as it heats, so it’s normal.

 

[fixitflav]

Yes water heats as it’s expanded, so it’s normal.

Or vice versa!

 

[Madrab]

Yes water heats as it’s expanded, so it’s normal.

OOOOOOO .... we have a new law of physics ....

 

[CBW]

OOOOOOO .... we have a new law of physics ....

Edited post, just re-read it