Combi boiler losing pressure after CH has been on

Hi there. I'm very new to this site so apologies if I'm not posting correctly!

We have a combi boiler which has been working fine for hot water. However we switched the central heating on last week and although everything fires up and the radiators work fine, as soon as the heating is switched off, the boiler loses pressure and we have to top it up back to 1 to get hot water. All is then fine until the heating comes back on again.

It seems to only lose pressure after the heating switches off.

Does this sound like a leak? Or a boiler issue?

Any suggestions will be most gratefully received!

Tell us the make and model.

Watch the pressure as the heating warms up and tell us!

Tony

Will do that tonight. Sorry, I can't think of the make and model off hand so will do some homework and let you know. Thank you for replying!

The boiler is an Excalibur. It was fitted by the previous owners and no manuals were given to us so I don't know anything else about it! It has a clock type timer dial on the front with lots of black switches on it. This says GRASSUN on it if that's any use...

Anyway, the pressure was at 1.2 before I switched the heating on. It went up to 3.5 whilst the heating went on. I only left it on for about 20 mins. The pressure then dropped to 0.8 after I switched the heating off. The hot water is still coming on. I haven't topped it up yet.

I also checked the thin bent pipe (copper?!) on the outside of the wall leading from the boiler which I think is the overflow pipe (sorry, very stupid about these things) and it was wet. I specifically checked it before I switched the heating on and it was bone dry.

Does this sound like the boiler is leaking out of this pipe?

Thanks so much in advance for any advice anyone might have.

no it sounds like you need a new expansion vessel or just it recharged if you're lucky
its not an over flow pipe btw its for the escape of excess pressure
matt

youve lost the pressure in expansion vessal at back of boiler. this takes up any expansion in system when on, especially heating, when the vessal loses its pressure there is no where for expansion to take place and the pressure builds up in system till the pressure relief valve opens and drips outside. this will need to be re pressurissed via a footpump. unless youve done it before call a pro it wont cost a fortune pretty easy job

Thanks so much to both of you. Super helpful.

The FAQ has a good section dedicated to that subject. If you've got a pump or a pressure guage it's fairly easy to recharge or check the expansion vessel.