We had a problem with our combi boiler for a wee while- the outside overflow pipe was dribbling a trickle of water, and called in our plumber.

He diagnosed a duff fill loop- apparently it was permanently just a shade open, with the result that the expansion vessel was over-pressured, and thus was dumping via the overflow.

New fill-loop and expansion vessel installed by him.

But now, when we run our shower (heated by the boiler), we get intermittent cold water. Its maybe a minute of warm, then a drop in temp to cold over about 5 seconds, it stays that way for about another 5 to 10 secs, then warms up again, and continues until the shower is shut off.

I’m worried. Has the over-pressuring of the expansion vessel caused an issue in the main part of the boiler itself?

I know very little about boilers and plumbing incidentally (IANAP).

thanks!

When we had this with our combi boiler and shower, it was a clogged heat exchanger. Cost of part was about £60, and took a couple of hours to fit (though on our contract we pay for parts, and the fitter’s time is covered by the contract, so I really didn’t pay attention).

Now toasty warm.

Thanks CC, that’s reassuring. I’ll look into that.

IF all it was doing before was dribbling overflow – whatever he’s done is the fault.
I’m assuming the shower was fine? No temp issues?
Get him back and put it right – not down to you to pay for his mistake.

Yep, the shower was fine beforehand hammy- I’ll draw him back in, was just looking for ideas on what it could be.

It does sound like the heat exchanger on its own.
BUT with him having replaced the pressure vessel the symptoms point elsewhere.
I’d be surprised if the vessel even needed changing – the bleed off was working fine to maintain safe pressure. Unless the bladder splits and it can’t maintain pressure they require very little of anything.
IME the OPV on the pump usually goes first which depending on setup exhibits the same symptoms.
Might be worth checking if there is a pressure regulator in the system too – it’ll be on the incoming cold. The boiler may have one built in.
Most come factory set to 3bar and I’ve lost count of the number I’ve seen that haven’t been reset to the installation. They need to read 3bar when the furthermost tap is fully open – causes poor flow in the hot (as the pressure drops) leading to hot/cold issues because the capsule can’t compensate fast enough.

IIRC one of the guys on here is a full time heating engineer so I’m sure he’ll pop in.

Thanks Hammy.

But now, when we run our shower (heated by the boiler), we get intermittent cold water. Its maybe a minute of warm, then a drop in temp to cold over about 5 seconds, it stays that way for about another 5 to 10 secs, then warms up again, and continues until the shower is shut off.

It’s not necessarily related. We had a similar problem with our old boiler, and it turned out to be a knackered circuit board – it was making and losing contact, giving the intermittent problem you describe. The idiots from British Gas followed a policy of replacing random parts (new heat exchanger, pressure vessel, pump)until this was about the last thing left. You could see it arcing.

We had something similar recently with the boiler losing pressure & requiring topped up every day & it turned out to be a leaking heat exchanger which was losing water into the condensate trap & subsequently the overflow, when the shower / hot water was on it was running hot & cold as the pressure fluctuated. We thought the expansion vessel was shot at first & the pressure was hitting 3 bar then dumping water via the overflow pipe but a friend fitted an external expansion vessel for us & it didn’t make any difference. The pressure never got above the set point when the boiler was running (that was an exciting hour) Only when we checked the condensate overflow it became apparent it was the HE instead.

I don’t know why he’s changed the EV if the system was just over pressurized – surely bleeding the system & re-setting it to the correct pressure would have had the same effect?