Central heating combi boiler pressure loss

[Elephant14]

A voice from the past here, but on an old topic. I see names from the old MF, offering competent advice, so am hopeful

We had a new Vaillant EcoTEC 825 boiler put in 18 months ago, together with a new central heating system, 15 and 22mm copper pipes, soldered joints, all as standard. The house is a small two up, two down, suspended timber floors, all traditional build. We have 3 rads first floor, 4 rads ground floor. The house is being renovated, so the system has had little use

Two months ago the boiler pressure dropped. 1.4 bar to 0.6 bar in three days. I checked the radiator circuit thoroughly, all compression connections at the rads, lifted the ground floor floorboards so I could see the oversite concrete to check for damp patches, checked ground floor ceilings under the lines of the CH pipe runs. No sign of damp staining or damp patches at all.
So I thought it must be the boiler. We called Vaillant as the boiler is still under guarantee. The engineer came round (within three days, good service) spent about 2 hours running tests and checking everything. He said the boiler is fine, it must be in the rad circuit. I told him I’d checked everything, but he said it’s not the boiler.

However, the pressure drop after his visit was much less, so he clearly did something. I suspect he found a leak just at the point where the heating flow pipe connects to the boiler. The nut shows green staining, which I think is recent. He probably didn’t mention it because he didn’t (as a Vaillant representative) want to take responsibility for anything other than the boiler. Anyway, I went around the rad circuit again (5th time at least), checking everything, and as before found no sign of leaks. I also put a plastic bag under the pressure relief outlet pipe and ran the CH for 30 minutes, with pressure readings going up to 2.3 bars – no water collected.

Following diagnostic advice found trawling another forum (Screwfix), I increased boiler pressure to 2 bar, then shut off the rad circuit flow and return valves. Four days later boiler pressure had dropped to 1.8 bar, two days after that to 1.7 bar. Must be the boiler, I thought. I then opened the flow and return valve. The pressure in the rad circuit should have been 2 bar, so on opening the valves the boiler pressure reading should have increased to somewhere between 2 and 1.7 bar. It didn’t, remaining at 1.7 bar, no change at all. Is this normal? If so, why? Or do I have a defective flow or return valve, so am not isolating the rad circuit when I think I am?

If it is the boiler, what could it be that the Vaillant engineer missed? A slow leak from the Schrader valve on the expansion vessel?

[BullDog]

Expansion vessel problem. Perhaps. Sounds like it to me.

[Itsallaguess]

Following diagnostic advice found trawling another forum (Screwfix), I increased boiler pressure to 2 bar, then shut off the rad circuit flow and return valves. Four days later boiler pressure had dropped to 1.8 bar, two days after that to 1.7 bar.

Must be the boiler, I thought. I then opened the flow and return valve.

The pressure in the rad circuit should have been 2 bar, so on opening the valves the boiler pressure reading should have increased to somewhere between 2 and 1.7 bar.

It didn’t, remaining at 1.7 bar, no change at all. Is this normal? If so, why? Or do I have a defective flow or return valve, so am not isolating the rad circuit when I think I am?

If it is the boiler, what could it be that the Vaillant engineer missed? A slow leak from the Schrader valve on the expansion vessel?

I'm glad you mentioned the expansion vessel at the end there, because it was what I began thinking of whilst reading your earlier diagnostic processes...

If there's a slow leak on either the expansion vessel itself, or the air-valve to it, then there's the possibility that there's been little or no actual loss of water, and all you've been doing up to now has been 'making up' the lost pressure from the expansion vessel as part of your earlier diagnostic steps.

What this should have shown recently though, if it's the case, and you've topped up enough water, is a higher-than-normal swing in system pressure when running from cold to hot, whilst you've been firing the boiler for central heating use over a decent length of time for it to get up to full 'tick-over' operating temperature. I think you might have touched on that when you mention the in-use pressure reading going up to 2.3 bar at one point, after running for 30 minutes, although it's not quite clear what the 'cold' pressure was before you started that particular running-up process.

If this is the case, and it is, or has been, a leak on the expansion vessel, then eventually (even if you're not there already...) you will have a completely empty expansion vessel, you'll then top up the system pressure when 'cold', and then on the next run you'll see the system pressure *probably* swinging quite rapidly upwards, and perhaps then going over the pressure-relief limit, where over-pressure system-water might then be ejected via the over-pressure relief valve going to the pipe outside, so the fact that you've not *yet* seen any water from that external pipe is what suggests to me that you might not *yet* have completely-flattened the leaking expansion vessel, and you might up to now have just been topping-up into it's still-dropping-pressure 'gap', and you should perhaps still keep your eye on the bag placed over the external pipe outlet if you continue to top the system up and then do spot fairly wild pressure-swings from a lower-pressure cold-start...

Like Bulldog - if I were a betting man then I'd be looking closely at that expansion vessel and signs of related issues via rapidly-rising system pressure when the radiators get hot, given that you've not spotted any signs of leaks elsewhere...

Cheers,

Itsallaguess

[Elephant14]

Thanks for your replies, Bulldog and Itsallaguess
I’ve checked the pressure on the expansion vessel twice, once with the boiler drained, to check what pressure the Vaillant engineer had left it at (1 bar) and once after my 2 bar pressure test (about 1.5 bar) when the boiler reading was 1.7 bar. This was using my foot operated tyre pump which has an analogue gauge attached. Not as precise, but adequate for checking. Inadvertently I released some pressure (subsequently repressured) and what came out was air, not water. So I think the membrane is probably all right – as it should be after only 18 months. The valve is another matter. Maybe I’ll use some needle nose pliers and give it a light twist. Whatever, I’ll certainly keep an eye on the boiler pressure reading.

All this assumes it is the expansion vessel and I hope it is. As matters stand though, given the lack of response of the pressure reading at 1.7 bar to the opening of the flow and release valves with their (supposed) 2 bar pressure in the rad circuit, I still have no conclusive evidence that it is the boiler at fault and not the rad circuit.

[Itsallaguess]

I’ve checked the pressure on the expansion vessel twice, once with the boiler drained, to check what pressure the Vaillant engineer had left it at (1 bar) and once after my 2 bar pressure test (about 1.5 bar) when the boiler reading was 1.7 bar.

This was using my foot operated tyre pump which has an analogue gauge attached. Not as precise, but adequate for checking. Inadvertently I released some pressure (subsequently repressured) and what came out was air, not water. So I think the membrane is probably all right – as it should be after only 18 months.

The valve is another matter. Maybe I’ll use some needle nose pliers and give it a light twist. Whatever, I’ll certainly keep an eye on the boiler pressure reading.

All this assumes it is the expansion vessel and I hope it is. As matters stand though, given the lack of response of the pressure reading at 1.7 bar to the opening of the flow and release valves with their (supposed) 2 bar pressure in the rad circuit, I still have no conclusive evidence that it is the boiler at fault and not the rad circuit.

Agreed, and that useful additional information regarding the earlier pressure-vessel tests does of course put some doubt around that being the issue, but as you point out, usually with pressure-vessel issues, eventually and with additional top-ups, you're almost certain to see wild swings in cold-to-hot system pressure, so that should be a good indicator at some stage just on it's own...

Regarding the test you carried out by closing the system-pipework isolation valves, this is usually an early test to confirm boiler-side or radiator-side leaks. Given that the boiler is still under warranty, did the visiting engineer also do a similar isolation test whilst he was there?

You don't mention a mag-filter at all - did they fit one as part of the boiler installation, and has that been considered as a potential source of leaks?

Vaillant boilers usually have automatic-air-vents, similar to this one -

https://www.dhsspares.co.uk/product/vaillant-061707-automatic-de-aerator-1372633

These can sometimes leak enough to drop system pressure, so lightly wrapping yours with tissue-paper might at least help to discount that, although I'd also probably put a few sheets of newspaper completely under the whole boiler if possible, just as a belt-and-braces check for 'when you're not there' leaks that might be going on...

Lastly, and I don't mind being the guy to have to ask, but are you 100% sure that the external pipe you've previously bagged is from the boiler pressure relief valve? I only mention this because it's quite common for there to be similar-looking stubby external pipework from nearby toilet overflows, so I just thought I'd mention it, you know, just in case....:O)

Cheers,

Itsallaguess

[Itsallaguess]

One other thing that's a fairly simple test, is to consider a main heat-exchanger rupture, where system-side water might be leaving the heat-exchanger pipework and subsequently dropping into the condensate-removal process, which is one of the few ways you can lose radiator-system water without ever seeing it inside the house...

As you say - with an 18 month old boiler you'd like to think it's unlikely, but if you can pressurise the system and leave the hot water and central heating completely OFF for a while, then that should mean there should be no 'proper condensate' generated during that period, and as such, an additional plastic bag over the external condensate drain eventually seeing water might help to indicate a heat-exchanger issue...

Cheers,

Itsallaguess

[BullDog]

One other thing that's a fairly simple test, is to consider a main heat-exchanger rupture, where system-side water might be leaving the heat-exchanger pipework and subsequently dropping into the condensate-removal process, which is one of the few ways you can lose radiator-system water without ever seeing it inside the house...

As you say - with an 18 month old boiler you'd like to think it's unlikely, but if you can pressurise the system and leave the hot water and central heating completely OFF for a while, then that should mean there should be no 'proper condensate' generated during that period, and as such, an additional plastic bag over the external condensate drain eventually seeing water might help to indicate a heat-exchanger issue...

Cheers,

Itsallaguess

In an almost new Vaillant boiler that would probably be a manufacturing fault. Snag is, it's actually probable that such a heat exchanger leak would only manifest itself when the heat exchanger is hot. I've seen such scenarios myself and it's not easy to fault find.

I know it's not helpful, but it's at times like this I'm very grateful to have resisted the "encouragement" to replace my open vent Y plan system with a condensing or pressurised system boiler.

Good luck to the OP with his/her problem.

[Elephant14]

Thanks for your reply and all the trouble you’re going to. It seems you agree that the boiler pressure should have shown some reaction when I opened the flow and return valves.

Regarding the test you carried out by closing the system-pipework isolation valves, this is usually an early test to confirm boiler-side or radiator-side leaks. Given that the boiler is still under warranty, did the visiting engineer also do a similar isolation test whilst he was there?

I don’t know. In my original post I said he was there for two hours, but my OH, who has a much better memory than me, says 45 minutes to an hour. I would think that the minimum time for such a test is 6 hours, so by that criterion, it looks unlikely.

You don't mention a mag-filter at all - did they fit one as part of the boiler installation, and has that been considered as a potential source of leaks?

A mag filter (Spirotech MB3 with ‘Vaillant’ stamped into the metal) was put in, and certainly I hadn’t considered it as a source of leaks. I’m going up to the house tomorrow and will run my fingers round the associated pipework.

Vaillant boilers usually have automatic-air-vents
These can sometimes leak enough to drop system pressure, so lightly wrapping yours with tissue-paper might at least help to discount that,

I’ve checked that the airvent is open, and checked the visible horizontal metal surfaces for droplets and any signs of water staining, but that’s all. The tissue paper would go around the grey knob?

a few sheets of newspaper completely under the whole boiler

I had a cloth towel under it for a few weeks, and it showed no signs of damp. I’ll put in a double layer of paper towels. Problem is, even though I’ve been going up to the house every 2 or 3 days, water can evaporate in the intervening period.

Lastly, and I don't mind being the guy to have to ask, but are you 100% sure that the external pipe you've previously bagged is from the boiler pressure relief valve?

100% sure, yes: the boiler is fixed to an external wall and I’ve traced the bagged pipe (15mm) to the heating expansion relief valve.
I have no problem with being asked any question at all. This has already delayed my son’s occupancy by a month: we can’t put down carpets till I’m totally sure the leak is not in the pipework within the first floor.

[Itsallaguess]

It seems you agree that the boiler pressure should have shown some reaction when I opened the flow and return valves.

It's difficult to be sure, as you didn't seem 100% confident that you'd actually isolated the flow and return legs, either by way of a possibly defective valve, or possibly something else, which is why I asked if the engineer had also carried out what's usually a pretty standard process for them to look at early on with these types of issues.

I agree that given the short time he seems to have been there, then he might not have carried it out, so perhaps something to consider if you do get back in touch with them for a re-visit at any point...

Regarding the automatic air-vent, I'd just put some tissue or paper bunched and cupped under it - just something that might then visually indicate a taking-up of water over time, if possible. As you say - it's difficult when not watched like a hawk, and especially with what is often warm or hot water that then more readily wants to evaporate quite quickly, unless it's a relatively large leak...

I think it's definitely worth considering the condensate pipe as a means of system-water escape, so hopefully there's a few things in all this that you can consider as avenues for investigation alongside another potential call to Vaillant. If you don't find anything yourself, then I'd be asking how a definitive 'it's not the boiler' decision has been made without a longer-term pressure-holding test with the boiler fully isolated...

Cheers,

Itsallaguess

[Elephant14]

Ah, cross posting there.
Heat-exchanger rupture.
Not wanting to complicate the matter, I didn’t mention that I had additional isolation valves put in on the incoming and outgoing pipework when I had the system installed. When I ‘isolated’ the rad circuit from the boiler 7 days ago, I just used the extra valves I had put in. Yesterday, after opening these and getting no reaction on the boiler pressure reading, I repressured to 2 bar, then shut both the standard valves and the extra valves on flow and return, so 4 in total.
So I have another isolation check going. I want to leave that for at least 6 days, to get a definitive result.

Obviously I can’t heat the system up, as Bulldog suggests, but would it be worth putting a bag on the end of the condensate drain if the boiler circuit is isolated from the rad circuit?

[Itsallaguess]

Not wanting to complicate the matter, I didn’t mention that I had additional isolation valves put in on the incoming and outgoing pipework when I had the system installed.

When I ‘isolated’ the rad circuit from the boiler 7 days ago, I just used the extra valves I had put in. Yesterday, after opening these and getting no reaction on the boiler pressure reading, I repressured to 2 bar, then shut both the standard valves and the extra valves on flow and return, so 4 in total.

So I have another isolation check going. I want to leave that for at least 6 days, to get a definitive result.

Obviously I can’t heat the system up, as Bulldog suggests, but would it be worth putting a bag on the end of the condensate drain if the boiler circuit is isolated from the rad circuit?

If the boiler isn't being used at all whilst it's currently pressurised and isolated, then yes, it's definitely worth putting a bag over the condensate pipe-end.

If the isolated boiler doesn't lose pressure then it shouldn't get wet beyond a few spare drops.

That doesn't discount the possibility of losing water via the condensate pipe when the system is back up and running hot, but for the sake of a plastic bag, it's definitely worth doing whilst cold as well, because if the isolated boiler does lose pressure whilst cold, and the bag doesn't get wet, then you're not losing any system water via the condensate-pipe route, so it would at least reduce the remaining possibilities...

Cheers,

Itsallaguess

[Elephant14]

Thank you, I'll do that today.

[Elephant14]

I checked the Magnaclean for leaks: none visible.

However ... boiler pressure is steady at 2.0 bar after 3 days. So preliminary conclusion is that the boiler is not losing pressure; the loss is from the radiator circuit, and that one of the extra flow and return valves I had put in is defective.
I'll leave it a few more days before reaching a definite conclusion. As before, I went around to all the rads and checked for leaks: nothing.

[Itsallaguess]

I checked the Magnaclean for leaks: none visible.

However ... boiler pressure is steady at 2.0 bar after 3 days.

So preliminary conclusion is that the boiler is not losing pressure; the loss is from the radiator circuit, and that one of the extra flow and return valves I had put in is defective.

You've had a cold boiler though, so there"s a chance it could still be the source of a pressure loss, but only when running hot...

If the boiler maintains cold pressure, it would be worth opening up the isolation valves and leaving the whole system at 2 bar with the boiler still off, just to see if there's any whole-circuit pressure loss in a cold state...

Cheers,

Itsallaguess

[Elephant14]

The previous loss was with a cold boiler too.

Yes, I'll certainly be watching the boiler pressure reading when I open the values: but that'll be only in another 3 - 4 days, so that I'm matching the previous loss test: 2.0 to 1.7 in 6 days.

[Dicky99]

A voice from the past here, but on an old topic. I see names from the old MF, offering competent advice, so am hopeful

We had a new Vaillant EcoTEC 825 boiler put in 18 months ago, together with a new central heating system, 15 and 22mm copper pipes, soldered joints, all as standard. The house is a small two up, two down, suspended timber floors, all traditional build. We have 3 rads first floor, 4 rads ground floor. The house is being renovated, so the system has had little use

Two months ago the boiler pressure dropped. 1.4 bar to 0.6 bar in three days. I checked the radiator circuit thoroughly, all compression connections at the rads, lifted the ground floor floorboards so I could see the oversite concrete to check for damp patches, checked ground floor ceilings under the lines of the CH pipe runs. No sign of damp staining or damp patches at all.
So I thought it must be the boiler. We called Vaillant as the boiler is still under guarantee. The engineer came round (within three days, good service) spent about 2 hours running tests and checking everything. He said the boiler is fine, it must be in the rad circuit. I told him I’d checked everything, but he said it’s not the boiler.

However, the pressure drop after his visit was much less, so he clearly did something. I suspect he found a leak just at the point where the heating flow pipe connects to the boiler. The nut shows green staining, which I think is recent. He probably didn’t mention it because he didn’t (as a Vaillant representative) want to take responsibility for anything other than the boiler. Anyway, I went around the rad circuit again (5th time at least), checking everything, and as before found no sign of leaks. I also put a plastic bag under the pressure relief outlet pipe and ran the CH for 30 minutes, with pressure readings going up to 2.3 bars – no water collected.

Following diagnostic advice found trawling another forum (Screwfix), I increased boiler pressure to 2 bar, then shut off the rad circuit flow and return valves. Four days later boiler pressure had dropped to 1.8 bar, two days after that to 1.7 bar. Must be the boiler, I thought. I then opened the flow and return valve. The pressure in the rad circuit should have been 2 bar, so on opening the valves the boiler pressure reading should have increased to somewhere between 2 and 1.7 bar. It didn’t, remaining at 1.7 bar, no change at all. Is this normal? If so, why? Or do I have a defective flow or return valve, so am not isolating the rad circuit when I think I am?

If it is the boiler, what could it be that the Vaillant engineer missed? A slow leak from the Schrader valve on the expansion vessel?

Any chance your observations are a bit too granular? I recently cleaned my magnaclean and lost a fair bit of water in the process so fed some Fernox in and recharged the system to about 2.5 bar cold. Over a couple of days use it dropped to about 2 bar. I checked the Magna and no leaking joints and recharged back to 2.5. It's dropped again to 2 but over a few weeks and is now stable at that level. I choose put that down to refilling with fresh oxygenated water which needs to vent out.
Any chance you're faffing about a bit too much and you just need to run it a few weeks and see if the problem is indeed progressive or if it stabilises in use?

[quelquod]

From Elephant14:
“ Following diagnostic advice found trawling another forum (Screwfix), I increased boiler pressure to 2 bar, then shut off the rad circuit flow and return valves. Four days later boiler pressure had dropped to 1.8 bar, two days after that to 1.7 bar. Must be the boiler, I thought. I then opened the flow and return valve. The pressure in the rad circuit should have been 2 bar, so on opening the valves the boiler pressure reading should have increased to somewhere between 2 and 1.7 bar. It didn’t, remaining at 1.7 bar, no change at all. Is this normal? If so, why? Or do I have a defective flow or return valve, so am not isolating the rad circuit when I think I am? ”

You can find anything in forums (here too! ). Water is pretty incompressible and (normally) the radiator circuit will contain negligible air (which is easily compressed). The result of this is that it takes a minimal loss/gain of water from the radiator circuit to lose/gain any pressure whilst it takes a moderate loss/gain of water from the boiler to lose/gain much pressure because of the expansion vessel. So your test would normally give the result you got - equalising the pressures would only transfer a tiny amount of water to the boiler circuit and negligible pressure change there.

[Elephant14]

Thanks Dicky99 and Quelquod
Dicky99: Recharging to 2.5 bar: that’s bold. If your pressure in use goes over 3 bar, my understanding is that your pressure release valve is activated and in my experience with a Worcester Bosch, that means a new one: the old one never works properly again.
I do seem to have a pressure loss which is progressively reducing, and if the boiler was in my own home, I would just keep using it and see if it stabilized. Unfortunately it’s in my son’s house and it’s holding up the renovations: we can’t put down first floor carpets until we’re sure the pipes in the depth of the floor are not leaking. Investigations so far (no stains on ground floor ceilings) indicate they are not, but equally, the investigations don’t show where the leak is. I’d like to find that first. I also really don’t want to put Fernox in a system that is practically new.

Quelquod: I take your point about the relative compressibilities of water and air. So you’re saying that what I had, no change in pressure reading in going from (a presumed) 2.0 bar in the rad circuit to 0.7 bar in the boiler, is to be expected.

The trouble with this game is that there are just too many variables!

[quelquod]

Quelquod: I take your point about the relative compressibilities of water and air. So you’re saying that what I had, no change in pressure reading in going from (a presumed) 2.0 bar in the rad circuit to 0.7 bar in the boiler, is to be expected.

Yes, basically only the air compresses and the amount in the boiler circuit vastly outweighs that in the radiator circuit, so when combining them you’re left with more or less the pressure in the former.

[Elephant14]

Thanks, that's helpful.

Is there a pressure gauge with a flexible hose I can buy to screw into a rad, so I can check the pressure once I've isolated the system? Easiest would seem to be to unscrew a bleed valve, then screw in the gauge. Screwfix have some, but usually used on mains taps. Would I be able to get an adapter for the connection?