Hi Guys, just wonder if someone could take pity on me who knows a little about boilers.

I came back this evening and topped up the pressure in my combi boiler, not much ive done it loads of times. Boiler came on i thought nothing of it later i go to fetch something from the kitchen and notice a pool forming under the boiler so i open up the hatch to see that the pressure has sky rocketed i mean its not even on the gauge any more so i immediately turn it off (at the same time the leaking stops) and ring british gas, they say no problem engineer tomorrow. Now me i cant sleep with this huge pressurised thing sitting there like a ticking time bomb so i think how can i relieve the pressure so i let some water out of the radiators and low and behold the pressure gauge drops and eventually i get it back to normal so i switch the boiler back on and its working just fine.

So if you have made it this far here is the question, have i done something really silly like over-filling? Should i cancel the engineer for tomorrow?

Did you fully close the filling loop?

Maybe your expansion vessel has a problem and you’ve filled it with water so no room for air to absorb the expansion so the pressure rockets?

Well i thought i closed the filling loop but i did check it and close it tighter when the issue happened.

If there was a problem with the expansion vessel would it keep occurring once i turned it all on again?

I came back this evening and topped up the pressure in my combi boiler, not much ive done it loads of times.

Suggests to me that something isn’t quite right. I’d let the engineer have a look.

No idea, but I’d expect so until it was recharged. They normally have a schrader valve but I’d only do it if I knew what I was doing.

The pressure vessel has to be tested when there is no water in it on the other side of the diaphragm to the air. Probably not a good idea to mess with it unless it’s diconnected. Does sound suspiciously like there’s not pressure on one side. A ruptured diaphragm or leak from the schrader valve.

The leaking water is probably coming from a relief vent when the boiler gets up to too high a pressure.

There is usually a combined water dump / pressure relief valve. You rotate it 1/4 turn to release water (and hence pressure). The same valve also blows off automatically at around 3 bar. Google images “combi boiler pressure relief valve” – you rotate the red bit. Might first be worth checking that the original installer attached it to a pipe that goes outside or to a drain…. If he didn’t then maybe that is the source of the puddle 🙂

The schrader valve is for pressurising the expansion vessel (with air not water).

Have you not got an owners manual for it? The dump valve should be clearly explained. Otherwise google for a manual.

Possibly sounds like some combination of one or all of over fill / pressure valve blow off / expansion vessel failure None of which should be very expensive to repair (if expansion vessel is failed but not leaking then a cheaper option is sometimes to have an external one fitted if space is available).

Interesting, i mean it seems to be working just fine now but probably best if i have someone in the know look at it anyway.

It was serviced last week, i would suppose any major faults should have been picked up then?

Okay so here is the update, the boiler now continues to leak and even when turned off but at a slower rate, british gas have let me down with a visit today after waiting in all day in the cold so hopefully going to see them tomorrow morning

Now me i cant sleep with this huge pressurised thing sitting there like a ticking time bomb

Boilers have loads of fail safe mechanisms in them, it’s very, very hard to get one to blow up on you without modifying it.

There was a brand new looking boiler expansion tank lying abandoned in the car park of the Manchester Velodrome last night. Weird!
Wish I’d have picked it up, would have made a lovely tubeless tyre ghetto inflator.

Brother Will – it has probably blown a seal somewhere if the pressure went that high.

my boiler was doing exactly this…..expansion vessel replaced this afternoon!

The pressure relief valve I mentioned can leak if it has “gone off” due to over pressure. I had one go immediately after manually dumping the system pressure to add a radiator (I think the seat in the valve can get damaged when it is operated). It was about £15 for a new valve. Have you found where the pipe goes from the valve (mine is a small pipe that runs outside and there was a continuous small trickle of water when it failed).

Or the high pressure could have started a leak somewhere else……

This still leaves the issue of why the over pressure happened in the first place (which could very well be the expansion vessel).

i would have absolutely no fear of using the boiler. clearly the pressure in yours rose but it sounds as though you are regularly topping it up so i suspect you have a persistant leak or your pressure vessel is bust.. i’d only fill to three quarter bar and knock the temp on the boiler to about a third. this will allow the thing to heat up without expanding the water too much.. get BG to sort if they are your insurance company of choice.. be warm

As Mick r said, If you have to top up often the pressure relief valve is probably broken it is worth changing out as it’s not expensive. You can check the expansion vessel pressure quite easily. We had a couple of engineers look at our boiler and they did not recognize the PRV was leaking I bought a one for the third engineer and asked him to replace it. I seemed to do the trick.

I am no expert just someone who lived with the problem too long.

Okay update, British Gas came today and had the boiler in bits said it was a blown gasket so another visit tomorrow to resolve, in the mean time told me the boiler was safe to use if needed.

My boiler decided to do this today.,..

Expansion vessel was no longer at 1 bar and full of water- but not coming out of the schrader valve. So no longer an expansion vessel , more ust a another rad full of water

Quick drain of th vessel and recharge with the bike pump/ checked with the shock pump to 1 bar and repressure the system

Not before a wee panic though. Neveruse the heating , just turned it on to help dry out some plaster work i was doing

Ours had a similar issue…
Pressure had dropped so put 1.6 bar back in.
Next thing there’s a “woosh” & the airing cupboard has had a nice shower of dirty rad water. 🙁
Low pressure relief valve is now slowly dripping water.
I figured the valve had operated, & a bit of muck had got in the seat & was allowing a slight leak.
So I swap out the valve, but notice that despite draining down the boiler, the pressure was still showing 1 bar.
Boiler fires up but alarms out with an over-temp fault.
Thinking a gummed up heat exchanger so I whip that out for a rinse out but its spotless.
Turned out to be the pressure sensor, gummed up with sediment making the pressure read constant 1 bar even when there’s no water in it, but the over temp fault had cooked a gasket in the fire box (my terminology 🙂 ) also.
Took a Gloworm Engineer to fix it though, independent plumber gave me 30 mins (for free) but said best to call them as they have every spare on the van.

Okay so update, when the engineer came out on monday and tried to replace the gasket the part the gasket was attached to sheered its bolt in the heat exchanger. So the engineer orders a new heat exchanger and im told it will be here friday and because the part cannot be re attached i now have no heating and no hot water. Ive been away for a few days so with the exception of scrounging up a oil fired radiator it hasn’t been too much of an issue. I even come back early for this morning, engineer arrives and tells me the part was damaged and so now i have to wait until monday and now my patience is wearing thin!

Probably too late for you Will.
But as a temporary measure if your expansion vessel goes and you need heating/hot water over week end etc.
Drain a radiator, then don’t vent the air out on re-fill. It makes a very good temporary means of expansion.

Simple but works and is the difference between no heating and nice and toasty until Permanent fix.

Boiler is now fixed, thank you everyone for their input the singletrack hivemind has taught me loads about boilers over the past 10 days.

Lucky you . Ive found out my expansion vessel is just dandy after a propper drain and recharge but my prv is supposed to pop at 2.5bar – because the expansion vessel wasnt really working as such it blew the prv a couple of times – as per previous experiance they never really seal again so now it pops off at 1.2k so although we have heating its a bit hit and miss – good news is prv is 22quid plus vat and a cinch to fit – esp as im draining down to fit a magnaclean