Combi boiler at 3 when on after plumber bled radiators - help!

[FloatingGlasshouse]

WB combi boiler - inherited with our flat purchase 2 years ago. Never any problems.
Pressure below 1 bar noticed August - repressurised and lasted for 4 weeks then had to do it again as had dropped to 0.5.
Called registered plumber (done good work for us before) this morning to find cause of persistent slow drop pressure, due to exchange with our buyer tomorrow.
He advised radiators needed bleeding, which he did, and then repressurised to 1.5bar.
Now noticed pressure up to 3 when boiler on! Never did this before. Called plumber who said pressure release valve must need replacing or something. Didn’t really understand.

Any help appreciated - could he have caused the problem when he put the pressure up? Previously I’d only done it to 1 bar. What rough cost are we looking at?

You just have to activate the valve manually buy turning a knob or pushing a lever depending on model to release the pressure. This is something you can do yourself in 10 seconds.

It may be that your expansion vessel is on the way out, or needs pumping up. If you can bear the cold, turn the boiler off and let the radiators all go cold. Note the pressure. Now turn it on and let it come up to normal working temp. Note the pressure.

What are the two figures?

Thank you both for your replies - the pressure when radiators are cold is 1.5 and goes up to 3ish when I turned it back on. Have turned them off again for now. Asssuming it is the expansion vessel, what’s the rough cost I should expect?

Bit worried as I’ve read can be several hundred if boiler needs to come off wall (only 20cm space above it to ceiling) but may be getting myself into a panic!

it can be pumped up (like a car tyre) at modest cost. If it needs to be changed you are entering the low hundreds. A new or additional expansion vessel can befitted anywhere on the system if that's more convenient, for example beside the boiler or in the airing cupboard. The manufacturer's original vessel is most often inside the boiler case, but sometimes it's awkward to winkle it out and get a new one in.

Excessive air in radiators can be compressed under pressure to take up expansion, which may be why your hot pressure increased dramatically after they had been bled. It's expansion between hot and cold that is the clue. If cold pressure is not excessive, then you won't cure the problem by releasing water.

Thank you again - that makes much more sense. I had contacted WB who can send an engineer to diagnose and fix with “non-significant” part (think this includes the expansion vessel) at fixed price of £275. I suppose it would be better to get someone to come and quote, in case it can just be pumped up.

We don’t really need the heating on but is it safe to use the hot water for showers? Pressure didn’t seem to go up when I did the washing up.

yes, very little water is heated when a combi is supplying hot taps, so there will not be much volume of expansion.

A houseful of radiators may contain in the region of 100 litres, so when heated it expands quite a bit..

Many thanks - as you can probably tell, I know next to nothing about plumbing!