Combi boiler woes…

So I noticed my boiler has lost pressure today, and had thrown a low pressure warning code, and shut down.

Refilling it I noticed it was slow to build the pressure back up… no signs of internal leak, so I checked outside and I’ve got a waterfall coming out of the copper pipe in the pic..I’d assumed this pipe was defunct and the plastic pipe was the condensate drain pipe.. anyone know what the copper one is… expansion vessle burst? PRV Stuck open?

I’ll have to call the plumber but would like to understand what’s going on better!

Pressure relief valve safety blow off.

I am not a heating engineer but if you track it back to the combi between it goes to a AA battery sized bit of copper and a red knob with 3 bar stamped on it

Its the pressure release valve outlet from in the boiler. The part is pretty cheap, and could even be a generic one, rather than Manufacturer only. They do fail occasionally, the seal breaks down, and lets the water out.

If it’s that then you need to find out if it’s faulty or the system is overheating massively and blowing out of the prv.

Get an accomplice to put a finger on the end , then slowly crank up the pressure with the filling loop.
Get it to 20psi or 1.5bar. if it holds then it’s most likely a duff prv

Thanks I was hoping it would be that.. I can’t really check the rear of the boiler as it’s attached to the wall directly behind the picture above.

I can see the where the plastic (condensate) pipe goes through the wall as that’s just below the boiler but I can’t see where the copper pipe enters the house (looking from the inside). Even though from the outside both pipes are exiting right next to to each other…

Expansion vessel got any pressure in ?

Expansion vessel got any pressure in ?

Not sure… I figured I’ll have to call someone out anyway but I suppose I should probably try check that just so I know if they try to rip me off.. I guess an expansion vessel is more expensive to replace than a pressure release valve!

Means taking the front casing off the boiler I think.. I’ll double check the error code list in the manual but if I recall correctly the error I got simply means low pressure, as confirmed by the physical pressure guage.. I dunno if it would throw a more specific code for a pressure vessel issue…

No but blindly replacing a prv without checking you have pressure in the expansion vessel is foolish.

Thanks, I’ve located the schrader valve on the exapansion vessel, but guess what… it’s gonna be difficult to test it or even get a pressure guage on it without partially dismantling the lower part of the boiler/control pannel of the boiler, because, heaven forbid these thing’s should ever be user servicable!!!!!

My guess is that tank comes out to let it be checked

If no pressure. When the pump starts and system heats up. It trips the prv as the pressure rises in both occasions.

Lots of other reasons too but rarely a prv gives up without an external factor

My guess is that tank comes out to let it be checked

That looks to be the case, it looks like I can pull the expansion vessel out, by unscrewing a single bolt/screw at the top as I can rock it back and fourth a bit, and it’s connected by braided flexi-hoses… it’s in a bit of a tight space, so I can’t see directly but the top bolt feels like a standard 6mm or 8mm hex bolt…

It looks and feels like the whole expansion vessel is suspended by that one bolt…

You don’t need to get to the expansion vessel to diagnose it.

Top the system up to standard pressure and run it up. If it overpressures and blows off, it’s the expansion tank.

If it stays at ‘safe’ pressure and pisses out, it’s the valve.

You don’t need to remove/replace the existing vessel, an additional one can be fitted elsewhere if that works for your system. It was for my old boiler.

Thanks that’s useful.

Just waiting on a call back from plumber but it won’t even get close to over pressurizing… I can get it to about 1.5bar from the filler loop…but it takes a long time.. Longer than it should…

I t will start heating but Waters coming out the copper drain the whole time and it slowly losses pressure until it gets too low.

The boiler then throws a low pressure error code and switches to standby.

Bloody thing seems to be holding pressure now, although a bit on the low side, it’s heating the rads quite happily, although still a slight drip out of the copper drain pipe… so it might fail into low pressure mode in a bit…

It’s been off all summer, well, it’s been on but I’ve had the thrmostat set to 5c or something, so the CH never kicked its just provided hot tap water …, I just wonder if it needs a bit of exercise/cycling on/off?

Edit, spoke to soon, pressure has dropped to the edge of the red on the dial, so It’ll error shortly.

Did you try testing the Schrader valve? You should be able to get the short end of a small Allen key on the tip.

Did you try testing the Schrader valve? You should be able to get the short end of a small Allen key on the tip.

I cant realisticaly get to it without pullin the expansion tank out,  the gap is too small…amd it has what looks like a screw on vave cap on it.

Oh the PRV is toast I don’t debate that. But something’s tripped the PRV and usually when that happens debris jams the PRV open. People blindly replace the PRV only for the same thing to happen. Often the expansion vessel has burst or depleted meaning a hot system is over pressure.

When was it last serviced.

The PRV should never be used to drain the boiler as crap can then get into the seal on the thing and stop it from sealing properly (maybe someone servicing the boiler has opened it?). you could try opening the PRV a few times to see if you can flush any crap out otherwise it’ll need replaced.

meaning a hot system is over pressure.

When was it last serviced.

Its never gone over pressure, to my knowledge..

Probably 18 months ago..

The PRV should never be used to drain the boiler

I’ve never used it to drain the boiler… not voluntarily anyway! this just started happening a few days ago.

Its never gone over pressure, to my knowledge..

You wouldn’t know till the PRV had emptied the pressure

heaven forbid these thing’s should ever be user servicable!!!!!

Well, the whole point of gas appliances is that they AREN’T user serviceable.

You may find that once the PRV triggers, it doesn’t necessarily return to the closed position and might need to be reset manually. The first question that needs to be answered is why it triggered in the first place – if you can’t get to and test the expansion vessel, then you can either add another vessel of equal or greater capacity in the heating loop somewhere, or as a short-term bodge, introduce some air into a radiator and lock both valves open.

Well, the whole point of gas appliances is that they AREN’T user serviceable.

Well, I wouldn’t be touching the gas part of the boiler… if the PRV and expansion vessle were at all accesible without removing a lot of the other gubbins i’d have no problem testing and replacing those parts as it looks like a doddle… but as it stands i’m gonna call someone out as it has potential to turn into a can of worms.

It’s just annoying paying labour and call out charges, and I’m just gonna have to take the guys word for it if he says both PRV and expension vessel are shot.

but as it stands i’m gonna call someone out as it has potential to turn into a can of worms

Good, so don’t keep running the system 🙂

It’s just annoying paying labour and call out charges, and I’m just gonna have to take the guys word for it if he says both PRV and expension vessel are shot.

Ring around Gas Safe people, their costs vary (obviously) and see what the manufacturer would charge too. Your chosen person should be able to show you why they’ve made replacement decisions

So..plot twist…

Got plumber out, he tested stuff and said what had actually happened is the mechanical guage is broken and under-reading… checking the pressure from the digital display is reading normal, so I’ve been over-pressurising it as ive just been reading the mechanical guage.

PRV and expansion vessel fine, but he recharged it as it was flat and did a standard boiler service at the same time, £155 all in.

He also said teh automatic air vent was leaking a bit,so he’s disabled/closed it till that can be replaced.

Price for new guage is £237  !?!?!, and with a new auto air vent comes to £335 all in…

So about £500 in total… ouch…

Surely you could leave the boiler gauge in situ and just ignore it? Just set the water pressure using a new (much cheaper) gauge fitted anywhere on the adjacent pipework. And in the meantime the plumber could tell you x bar on the boiler gauge equals y bar in reality.

Thanks yeah he did say I could leave the broken mechanical guage in situe and explained how the access the pressure reading through the digital display.. So that’s an option.

He did say the the automatic air vent does need replacing sooner rather than later though.

That would only be about 100 quid.

Have a look at your local plumb centre. I replaced the leaking pressure gauge on my Worcester combi back in 2018. Cost me £24 for the part. Easy to replace, even had a self sealing valve on the connector. Dunno what your Baxi is like but YouTube is your friend.

Thanks yeah I’ll have a Google.. I thought it sounds very expensive for a guage.. Unless it’s made by Snap On, lol!

But it looks like it’s kind of integral to the control panel/main controller PCB which is all mounted in a box that kinda flips down on hinges.

So. Maybe baxi don’t supply just the guage on its own?

The Bosch boiler was like that. The whole front panel just flipped down then there were a couple of tabbed tangs holding the gauge in place. Good luck.

The Bosch boiler was like that. The whole front panel just flipped down then there were a couple of tabbed tangs holding the gauge in place. Good luck.

Thanks…

on further inspection it looks like I could open up half the control pannel …but oddly the ‘mechanical’ pressure guage is now reading in the ball park of what it should be.

It looks like it would be just a wire and a cheap guage… it’s not directly reading the pressure.. it just has what look like a thin electrical wire feeding it.

Is it really a wire or actually a really small bore long siphon tube so the gauge is measuring cool liquid?… A quick Google suggests the gauge should be under £50.

I think you are right, on closer inspection, it’s not an electrical wire, it looks and feels more like a very small bore, nylon hose!!

Yep, if it’s anything like mine was, it will be a small bore hose.