Combi boiler loses pressure when idle but regains it when working

Fubar1953

Having a weird situation with my Worcester oil fired combi boiler. It is losing nearly all its pressure when not in use - e.g. overnight - but when it starts up again on the timer the pressure builds back up again on its own. I don't have to top it up using the filling loop. I've checked all the rads and there are no leaks. I've never come across this before and have always had to use the filling loop after a pressure drop. The boiler is now 20 years old and I am planning on replacing it next year but hoping to get one more winter out of it first.

FreeBear

Have you bled the radiators to make sure there is no air in the system ?

Fubar1953

FreeBear said:

Have you bled the radiators to make sure there is no air in the system ?

Yes thanks, that was the first thing I did.

Bendy_House

That is typical behaviour of a sealed system where its expansion vessel isn't working properly.

Water expands when heated, so it either has to 'go' somewhere when heated, and then come back again when cooled, or else you can expect large pressure fluctuations like you are doing. (And if the pressure reaches ~3bar, it'll open a safety valve and discharge the excess safely outside).

So, I presume that somewhere on your system is a - probably red - EV, but it could be built in to the boiler casing, or fitted elsewhere, say near a hot water cylinder?

Make and model of boiler, please?

The cure will likely involve that EV. Hopefully it's just lost air pressure and needs recharging, but possibly the rubber diaphragm inside has leaked, so it's fubared and needs replacing. (See what I di... ohnevermind)

Anyhoo - let's find that EV :-)

Fubar1953

Bendy_House - many thanks for the answer. The boiler is a Worcester Heatslave and I have posted a pic below. Presumably that red tank is the EV?

shiraz99

Yes the red tank is the EV. You need to get someone to check the pressure with the boiler isolated and if it's low the boiler needs to be drained so you can then try and pump the EV back up to 1 bar from the valve located at the top of the tank. If it's failed completely then this won't hold pressure and needs to be replaced. All done by someone suitably qualifie3d to do so, of course.

Bendy_House

As the wine merchant with the flake says

GDB2222

If the EV just needs pumping up, that’s a simple job. There’s usually a valve that looks like the valve on a car or bike tyre. It’s possible that this is different, though. 

I just checked a video, and there’s a standard valve on the top of the EV, accessed by removing the lid of the boiler.

 It’s really just a diy job, but at least if you call someone in you can be pretty sure that it’s a short job. That’s provided the membrane inside the EV hasn’t failed. If it has, a new EV is £100 plus the system probably needs to be drained completely to fit it.  

macman

I agree, it can only be the EV. The other reasons for pressure loss on a combi are a leak on the CH circuit or a non-seating PRV, but in neither case would the pressure rise again when the system warms up.

Bendy_House

The only issue regarding DIY repressurising, is that the system side should be at zero or else you won't get a true reading on the air side - it'll be being pressurised against an unmoveable object, the water. So, ideally, a rad bleed screw should be opened until no more water comes out = zero pressure - and kept open with someone catching subsequent ejaculations as the air side is given a blow.

Having said that, if the EV has just lost pressure and isn't leaking, then fitting a car or bike pump and giving it 6-12 pumps will very likely get it good enough; just monitor how the pressure fluctuates betwixt hot and cold. As you pump, and the pressure gauge rises, keep 'bleeding' it when it hits, say, 1.5+ bar, and bring it back down to 1.

Before pumping, press the centre pin of the Shrader and see if it hisses. And if water comes out, don't bother pumping...

shiraz99

GDB2222 said:

If the EV just needs pumping up, that’s a simple job. There’s usually a valve that looks like the valve on a car or bike tyre. It’s possible that this is different, though. 

I just checked a video, and there’s a standard valve on the top of the EV, accessed by removing the lid of the boiler.

 It’s really just a diy job, but at least if you call someone in you can be pretty sure that it’s a short job. That’s provided the membrane inside the EV hasn’t failed. If it has, a new EV is £100 plus the system probably needs to be drained completely to fit it.  

It's a job you shouldn't be doing diy though, in reality the OP shouldn't even be opening up the front panel. Not as "simple" as just pumping it up though. As I said, you first have to isolate the boiler to check it, ie, shut off the flow and returns, then you'll need to drain the boiler down completely before pumping it up, then hope it all works after.