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I got a new boiler 12 years ago, keeping the existing radiator circuit (moving 1 and adding 2 radiators). It's been needing topped up now and again (~quarterly?) for a few years and had leak sealant added at one point which helped.
After 6 months of no use, the pressure is now dropping withing hours.
I've looked at all the visible joins, nothing obvious. A couple of radiators (bathroom and kitchen) are quite rusty at their bottom edge - is this the most likely cause?
(Googling suggests so - but most advice seems to be on videos which I can't stand)
Knackered expansion vessel, check the overflow to see if the boiler is venting to relieve pressure after you top it up and it has warmed up.
Cheers, I guess that's an engineer job? (You seem confident this is the issue?)
Leak somewhere? We chased ours for weeks over the winter, isolating parts of the system (can you isolate HW and upstairs / downstairs circuits?) before finally finding under a concrete floor slab (after getting in a specialist leak detection company).
No visible leaking and I haven't taken up floorboards yet.
You'd prob see it if it was upstairs (stained ceilings etc) if you're losing that much. Downstairs would be my starting point (if you rule out expansion vessel)
ballsofcottonwool
Free MemberKnackered expansion vessel, check the overflow to see if the boiler is venting to relieve pressure after you top it up and it has warmed up.
Exactly what happened to us, after British Gas basically shrugged their shoulders. Ended up getting a leak detection company in as there was no obvious leaks anywhere else (to be fair British Gas then refunded me for that with no issue)
Knackered expansion vessel, check the overflow to see if the boiler is venting to relieve pressure after you top it up and it has warmed up.
Same here. You could see the overflow pipe was dripping all the time outside. Local guy fixed it for about £150. Fine ever since.
Top it up, fire up the central heating and watch the pressure gauge. If it spikes straight to 3 bar and then drops rapidly, it's the expansion vessel (as a temporary bodge you can just drain half the water out of a radiator).
If it doesn't then you'll want to start hunting under the floors and in the walls for the leak, because that's a lot of water to lose. Rusty valves on the radiators unlikely to be the cause.
If the expansion vessel is separate to the boiler and there's a valve on the connection, they're a relatively simple DIY fix, usually only needing a shock pump and a washing up bowl.
Thanks guys. Downstairs is a separate property and they've not said anything.
Rust on radiators is on the bottom - not the valve.
I guess the overflow will be going through the condensate pipe?
Two possible ways to firm up the diagnosis from ballsofcottonwool.
First, pressurise it but keep it turned off, if drops over time, it's a leak somewhere. Turn it on, get it hot, and look for water coming from the overflow, if so, and if it drops as it cools, it's the expansion vessel.
Second, as I understand it boilers have either a sealed casing, which means you must be a GasSafe engineer to open them, or just a wrap round screen, open at the top. If the latter, look for something with a Schrader value on the top, that's the expansion vessel. The vessel is full of water and the valve blows up a rubber bladder inside. If you release the valve fractionally, and get water, the bladder has split, needs a new vessel. If you get air, check the pressure. From memory, should be at least 15psi when cold. Pump it up and see if that fixes the issue.
Top it up, fire up the central heating and watch the pressure gauge. If it spikes straight to 3 bar and then drops rapidly, it’s the expansion vessel (as a temporary bodge you can just drain half the water out of a radiator).
Ooooh, good tip! My boiler has a knackered diaphragm (water coming out of Schrader, pressure dropping due to pressure valve having been triggered and now constantly slow dripping) but I'm having to nurse it though the next 4-5 weeks ideally as a new boiler is being fitted as part of building works that have just started.
Tie tissue around the rad tails to see if they're leaking.
had a similar issue. thought it was the expansion vessel knackered, but it turned out that it just needed repressurised. schraeder valve, 1 bar. heating engineer used a bike pump.
If it's a leak then it will be significant if you're losing pressure that quickly - not just a weeping rad tail.
Ok the condensate pipe is dripping happily which I guess means it's the expansion tank @ £100 plus fitting.
Thanks guys!
Or might just need pumping up.
See gray beard above who explained it perfectly:
look for something with a Schrader value on the top, that’s the expansion vessel. The vessel is full of water and the valve blows up a rubber bladder inside. If you release the valve fractionally, and get water, the bladder has split, needs a new vessel. If you get air, check the pressure. From memory, should be at least 15psi when cold. Pump it up and see if that fixes the issue.
Wouldn't recommend my strategy - but it worked.. (Strategy implies planning actually.. this is just what happened)
Similar symptoms got worse over a few months. Eventually got to the stage you could leave the valve open to fill it and water would flow out as fast as it went in.
At this stage, simply listen for the waterfall and then get a plumber to fix the leak.
We had a solder joint on a pipe under the floor that had been weeping, then leaking properly and had just split completely.
Ok the condensate pipe is dripping happily which I guess means it’s the expansion tank
Or the safety valve, a pressure relief valve which may or may not have a separate drain. Generally sitated at the mains water connection of the boiler.
I agree with Edukator. The safety valve might also be at the back of the boiler (looking at you, shit engineers at Worcestor Bosch) and you may if you're really lucky, fix it by turning it to the test position and letting it snap back.
But it likely opened due to the system hitting a pressure limit, so I'd still investigate the expansion vessel.
Expansion vessel. Need to isolate the boiler, drain it and then pump up the expansion vessel, use a normal pump, not a shock pump, only needs 1.5 bar. Refill. If it's the bladder that has failed it'll be back to pressure yo yo, if not it will be fine for a while.
Similar problems with mine, losing pressure, then spiking to 3 bar when turned on.
I watched a few YouTube videos, isolated the boiler, drained it, re-pressurised the expansion chamber (trackpump to 1bar) and it's been right as rain for a month now 👍
Update... pressure dropped 1-0.5 over 12hrs switched off.
Back up to 1, switch on, goes up to 1.5-2, drops to 0.2 after switching off.
I've removed the cover and the panel behind, but there's all sorts of gubbins (and gas connections I think) in front of the expansion tank so it looks like I have to get a pro in grrrrr.
It's unlikely the safety valve is set to 2bar, French ones are 6 IIRC. That and the fact it's going down when not working says there's a leak somewhere not just water going through the safety valve at high pressure because the expansion vessel is not working. Yes it could be a leaky expansion vessel but it could be a leak anywhere else, and if you can't find the leaked water then the most likely is a leaking safety valve because that will have a drain to the sewers in theory.
UK won't be anywhere near 6 bar, probably around 3 bar. The pressure swing above is classic expansion vessel.
Stop calling it a safety valve.
It's a pressure relief valve and it'll be set at 2.5 bar in the UK. And given the boilers still dropping when it's off it's unlikely that
Play parts darts if you want but probably be as well getting it repaired by someone who knows what they are looking at. Everyone here's simply guessing based on the data you have given so far.
Please be so kind as to stop ordering me around and use polite forms.
It seemed a pretty good translation of "le groupe de sécurité" which is fitted to hot water tanks and heating systems and prevents the pressure going to high as the water in them expands on heating.
If it's set at 2.5bar then " pressure dropped 1-0.5 over 12hrs switched off. " could be explained by it not closing properly, or by a leak elsewhere. But if the leak is elsewhere you'd expect to find traces of water leaking. "It" being the safety valve obviously.
Cynic-all hasn't actually stated how quickly the pressure drops after the boiler goes off, if it's within an hour or so it's consistent with the pressure vessel issue, pressure drops as the water in the system cools. What Ed is refering to does sound like a safety valve, UK domestic systems generally don't have them, but closed loop systems do have a pressure relief valve that operates in normal operation. A lot of safety valves are single use, once they blow they stay open.
pressure relief valve that operates in normal operation
Say what ?
It's only there to stop pressure going above 2.5 which it should never do if the expansion vessel is working sufficiently.
If it's triggered the fault rarely if ever lies with the prv but an external cause that triggered it.
Once triggered it may continue to weap if debris jammed in it but replacing it without replacing the source of the issue is fools folly.
To avoid any confusion in terminology it's the left hand part in the pic below. You can test it by turning the little wheel on the left. If it weeps into the drain pipe which drops out to the left in the pic then opening and allowing it to spin/spring back sometimes stops it weeping.
The variations in pressure Cynic-Al is seeing suggest a problem with the expansion vessel. The pressure guage on my system doesn't visibly budge as temperature rises, turning the pump on and off produces more pressure change, about 0.1bar. When the expansion vessel needed inflating the pressure went up a bar or so as the system warmed up but there was no water loss as the pressure remained below the point the safety valve/pressure relief valve activates.
My system is heated by a wood stove but all the gubbins was pulled out of an old central heating boiler. Perhaps I should post it on the repurposing thread.
got a new boiler 12 years ago
I had a new boiler fitted and within a short time started to lose pressure rapidly.
The Baxi engineer found the feed pipe to the expansion vessel was blocked causing the pressure to increase enough for the relief valve to be activated and hence lowering the pressure and shutting the boiler down .
Edukator, looks like an old Chaffoteaux boiler you’ve used there! Don’t see them around anymore.
To answer your question Cynic-al I’d want to know the make and model of your boiler?
Just on something you said about the condensate dripping happily I’d hazard a guess that your boiler is an Ideal Logic and your main heat exchanger is leaking internally and this is running down the condensate. If it was your expansion vessel it would only do it with your central heating on and you would see the rise on your pressure gauge and water coming out of the pressure relief pipe.
Yep, if the condensate pipe is dripping when the boiler isn't fired up then it'll be the heat exchanger. New boiler time if that's the case. 12 years is around the life span of modern boilers, consider anything over that as a bonus. I would advise a boiler of that age not worth repairing if it's something costly like a heat exchanger.
Our boiler is 35years old! It needs my attention every few years but I can still get parts for it.
Another leak path is the diverter valve stem seals. Every time you open a hot water tap the diverter valve operates, eventually the seals wear. I've done mine a few times. It does dribble out of the casing but there isn't much to stop it.