Question about expansion vessel on heating system - I think the gas engineer broke it

[NigellaAwesome]

Is anyone knowledgeable about central heating systems?

I got my gas boiler serviced last Monday. The system had been losing a bit of pressure recently - about 1 bar over three weeks. I told the engineer this, and he said I needed to get a new part inside the boiler - something had a very small leak.

He came back on Thursday to fit the new part. He said he had to drain down the boiler to fit it (but not the whole system). He reset the pressure to about 1.8 bar.

The next day I was putting something in the hot press and noticed that the pressure was up to 3.5 bars. I phoned the engineer and he said it was the expansion vessel had gone.

I’ve phoned again today to get him to come out to fix it, and I have been quoted £150 to replace it. I had already paid £60 for the service, and £100 for replacing the part.

It seems like too much of a coincidence that the expansion vessel failed at the same time as him replacing the part, but the company say it’s not connected. I don’t know if the vessel has actually failed and needs to be replaced, or if it just hasn’t been depressurised correctly after the boiler was drained.

Can anyone advise? I don’t really want to shell out another £150 on top of the £160 I have already paid.

Usually the expansion vessel can be re-charged when the service is done, not always replaced. Ask him if it was recharged (it won't have been I bet) and see what he has to say.

What was the part that was replaced?

This is one for @PigletJohn who knows much more than I do about boilers.

BTW, if you let some water out of a radiator, gradually (bleed one of them with a radiator key) it might take the pressure down, he may just have topped it up too much. Do it in stages and wait until the correct place is reached on the dial.

If you go to far you should be able to top it back up again but depends on your boiler.

It may not be the expansion vessel, just topped up too much.

Thanks Shitley.

I don't think it was topped up too much, he did it to 1.8 bar, I saw it myself. But it went up to 3.5 the next day (and could have been the day before too).

It's been moving between 1.8 and 3.5 since.

I'll asked them to come out again to look at it.

Hi Nigella,
I does sound like your engineer is right. The expansion vessel is basically a bladder that takes up when the hot water expands. Over time this bladder will need recharging as it won't be able to take up expansion. You will notice this with a wet patch outside the house from the 15mm pipe. You can sometimes represurise this using a foot pump. But if the bladder is shot. It won't have any effect and will need replacing. He won't know this until he tries the first port of call which he has. He did try and save you money but it hasn't worked. So now it's option 2