Combi boiler losing pressure but with a twist

Combi boiler losing pressure but with a twist

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leef44

Original Poster:

4,958 posts

168 months

Friday 22nd November 2024
quotequote all
I have a Worcester Bosch condensing boiler fitted two and half years ago when I moved in. It is now losing all its pressure every time I turn the heating on or use the hot water after a while.

The boiler man says he checked the integrity of the boiler so it must be a leak in the pipes or joins in the house. I think it's the boiler and not the pipes in the house.

My query is that when we were away it did not lose any pressure. When I stop using the heating then it does not lose any pressure. But even if I just use the hot water it loses pressure after a couple of hours. And it's only got this bad now the weather has turned cold. When the weather was warmer it was not this bad.

Could this be due to something wrong with the boiler? Is there something I should be asking the boiler man to check (I'm a numpty with these things).

For reference, the house previously had a vented system. It was built in 1990 and has 10mm radiator pipes which are buried in the concrete floor and solid walls.

Countdown

44,619 posts

211 months

Friday 22nd November 2024
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I thought the same when my CH system was losing pressure (i.e. it MUST be the boiler as it only loses pressure when the boiler is running).

It turned out to be a leak under the floorboards in the hallway..

otolith

61,753 posts

219 months

Friday 22nd November 2024
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We were losing pressure from the joint to a filter. It wasn't losing a huge amount of water, just dripping, but it was enough. Since it was fixed, it has stayed up.

leef44

Original Poster:

4,958 posts

168 months

Friday 22nd November 2024
quotequote all
Countdown said:
I thought the same when my CH system was losing pressure (i.e. it MUST be the boiler as it only loses pressure when the boiler is running).

It turned out to be a leak under the floorboards in the hallway..
OK this doesn't sound good for me. I think I'm in this situation. We're talking over a litre of water top up each day at the moment in this cold weather. This has been going on for months but I cannot find any damp patches at all.

Countdown

44,619 posts

211 months

Friday 22nd November 2024
quotequote all
leef44 said:
OK this doesn't sound good for me. I think I'm in this situation. We're talking over a litre of water top up each day at the moment in this cold weather. This has been going on for months but I cannot find any damp patches at all.
frown

Mine was much worse. 2bar to empty in about 2 hours. In the end I had to get a specialist leak trace company out (which cost £700) and they only managed to get a rough idea where the leak was. Fortunately it turned out to be close enough and a plumber managed to pinpoint it and fix it. I gave him £250 because he managed to avoid damaging the laminate floor.

tux850

1,934 posts

104 months

Friday 22nd November 2024
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leef44 said:
The boiler man says he checked the integrity of the boiler so it must be a leak in the pipes or joins in the house.
I'm not sure why he'd think that with a combi as the primary water loop (to which the pressure gauge is connected and measuring) is entirely within the confines of the boiler when in hot water mode.

Where does the condensate drain to? Can you access it? Whilst it will drip under normal operation I am wondering if there's a leak in the primary heat exchanger (i.e. inside the combustion chamber) and the leak water is passing out via the condensate drain hence not resulting in any other telltale signs.

HalfManHalfJaffaCake

80 posts

65 months

Friday 22nd November 2024
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A ‘boiler man’ should have checked this but just in case, try refilling the boiler and then watch the pressure gauge - it should rise as the water heats up then stay static, but if it keeps rising until it exceeds the maximum it will then open the pressure relief valve and release the water. If you can find the outlet for this valve - e.g. a copper pipe going outside - look to see if it’s dripping.

This combination of symptoms points to the expansion vessel - either faulty or with a leaking top-up valve (literally a car-type schrader valve). Not a big deal either way, easy to replace if there’s room to remove it with the boiler in situ, or it can be bypassed to an external expansion vessel.

Of course it could be a hidden leak slowly rotting your house from the inside out…..fingers crossed it isn’t!

leef44

Original Poster:

4,958 posts

168 months

Friday 22nd November 2024
quotequote all
tux850 said:
leef44 said:
The boiler man says he checked the integrity of the boiler so it must be a leak in the pipes or joins in the house.
I'm not sure why he'd think that with a combi as the primary water loop (to which the pressure gauge is connected and measuring) is entirely within the confines of the boiler when in hot water mode.

Where does the condensate drain to? Can you access it? Whilst it will drip under normal operation I am wondering if there's a leak in the primary heat exchanger (i.e. inside the combustion chamber) and the leak water is passing out via the condensate drain hence not resulting in any other telltale signs.
Thank you, this was the original thinking. They yanked up the pressure to 3.0 bar in the closed heating loop and found no leaks. But I don't understand why it should leak when only the hot water system is being used.

The condensate pipe goes to the ground outside and then below the concrete floor outside so difficult to check unless we disconnect the discharge pipe and collect the water.

My last house had a similar issue where I had to keep topping up but the condensate pipe was disconnected due to the frozen pipe outside. I was amazed how much water was coming out - several litres a day. Hence me thinking it is the boiler.


leef44

Original Poster:

4,958 posts

168 months

Friday 22nd November 2024
quotequote all
Countdown said:
leef44 said:
OK this doesn't sound good for me. I think I'm in this situation. We're talking over a litre of water top up each day at the moment in this cold weather. This has been going on for months but I cannot find any damp patches at all.
frown

Mine was much worse. 2bar to empty in about 2 hours. In the end I had to get a specialist leak trace company out (which cost £700) and they only managed to get a rough idea where the leak was. Fortunately it turned out to be close enough and a plumber managed to pinpoint it and fix it. I gave him £250 because he managed to avoid damaging the laminate floor.
Do you have the name of the specialist? I'm just not ready to start ripping up LVT flooring, digging up concrete flooring and breaking up the walls for vertical pipes. Although I'm still not completely sure it's not the boiler.

J6542

2,711 posts

59 months

Friday 22nd November 2024
quotequote all
HalfManHalfJaffaCake said:
A ‘boiler man’ should have checked this but just in case, try refilling the boiler and then watch the pressure gauge - it should rise as the water heats up then stay static, but if it keeps rising until it exceeds the maximum it will then open the pressure relief valve and release the water. If you can find the outlet for this valve - e.g. a copper pipe going outside - look to see if it’s dripping.

This combination of symptoms points to the expansion vessel - either faulty or with a leaking top-up valve (literally a car-type schrader valve). Not a big deal either way, easy to replace if there’s room to remove it with the boiler in situ, or it can be bypassed to an external expansion vessel.

Of course it could be a hidden leak slowly rotting your house from the inside out…..fingers crossed it isn’t!
This is what is up with mine, my plumber mate has recommended fitting the external expansion vessel to fix the problem. Mine has been faulty for a while but it’s only since the heating has been going on that I have needed to top up the system. I don’t think the pressure builds enough from just heating water, so the system hasn’t needed topped up

tux850

1,934 posts

104 months

Friday 22nd November 2024
quotequote all
HalfManHalfJaffaCake said:
A ‘boiler man’ should have checked this but just in case, try refilling the boiler and then watch the pressure gauge - it should rise as the water heats up then stay static, but if it keeps rising until it exceeds the maximum it will then open the pressure relief valve and release the water. If you can find the outlet for this valve - e.g. a copper pipe going outside - look to see if it’s dripping.
Yes, I should've also said that I was assuming the boiler man has already checked the PRV outlet but of course shouldn't really make assumptions.

leef44

Original Poster:

4,958 posts

168 months

Friday 22nd November 2024
quotequote all
J6542 said:
HalfManHalfJaffaCake said:
A ‘boiler man’ should have checked this but just in case, try refilling the boiler and then watch the pressure gauge - it should rise as the water heats up then stay static, but if it keeps rising until it exceeds the maximum it will then open the pressure relief valve and release the water. If you can find the outlet for this valve - e.g. a copper pipe going outside - look to see if it’s dripping.

This combination of symptoms points to the expansion vessel - either faulty or with a leaking top-up valve (literally a car-type schrader valve). Not a big deal either way, easy to replace if there’s room to remove it with the boiler in situ, or it can be bypassed to an external expansion vessel.

Of course it could be a hidden leak slowly rotting your house from the inside out…..fingers crossed it isn’t!
This is what is up with mine, my plumber mate has recommended fitting the external expansion vessel to fix the problem. Mine has been faulty for a while but it’s only since the heating has been going on that I have needed to top up the system. I don’t think the pressure builds enough from just heating water, so the system hasn’t needed topped up
Thank you for that thought. Unfortunately I believe the boiler man did test this. It didn't go to maximum so no pressure release.

When I told him about my old house boiler losing pressure he said that it was most likely the expansion vessel so this was one of the first things he looked at.

98elise

29,892 posts

176 months

Friday 22nd November 2024
quotequote all
If its a slow leak you could try a chemical leak sealer

https://www.screwfix.com/p/fernox-f4-central-heati...


leef44

Original Poster:

4,958 posts

168 months

Friday 22nd November 2024
quotequote all
98elise said:
If its a slow leak you could try a chemical leak sealer

https://www.screwfix.com/p/fernox-f4-central-heati...
thank you, yes they did that during the summer and it did work for a while. It was losing 0.2 bar a day back then and this reduced it to 0.1 bar a day. So there was definitely a leak relating to the piping in the house but I'm wondering whether this is a red herring.

Now with the much colder weather it seems to leak big time even when just using the hot water and not using the heating system. If both hot water and the heating system are not used then the boiler pressure remains stable for days.

Jeremy-75qq8

1,401 posts

107 months

Friday 22nd November 2024
quotequote all
Faulty pressure relief valve

leef44

Original Poster:

4,958 posts

168 months

Friday 22nd November 2024
quotequote all
Jeremy-75qq8 said:
Faulty pressure relief valve
This is what I was thinking but the boiler man says he's checked this.

BlackZeD

812 posts

223 months

Friday 22nd November 2024
quotequote all
Sounds like the expansion vessel has a split diaphragm.
Not using hot water won't affect it as the water has nowhere to go.
When the heating comes on it will pressurise and possibly open the PRV, therefore losing water again, if it gets that far.
As soon as you use hot water it will also bleed past the diaphragm and lose pressure.

leef44

Original Poster:

4,958 posts

168 months

Friday 22nd November 2024
quotequote all
BlackZeD said:
Sounds like the expansion vessel has a split diaphragm.
Not using hot water won't affect it as the water has nowhere to go.
When the heating comes on it will pressurise and possibly open the PRV, therefore losing water again, if it gets that far.
As soon as you use hot water it will also bleed past the diaphragm and lose pressure.
Although they did test the expansion vessel back in the summer, this is making me think it might be worth getting them to do the test again now for both the expansion vessel and the PRV. And this time to get them to walk me through it.

OutInTheShed

11,524 posts

41 months

Friday 22nd November 2024
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I made an adaptor to pressurise the system with a tyre pump.
Sometimes you can hear an air leak where water is too subtle.

I was only testing my ugly soldering on old pipe in awkwards places, but I've lent the adaptor to someone with greater needs, and not got it back yet!

Phooey

13,091 posts

184 months

Friday 22nd November 2024
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Sounds like a leak in the pipe work within the concrete floor. A combi will operate under pressure hence it loses more water when heating is on, and less when off. We have this problem with ours although the pipes in the floor are mid-1970s when there was a common issue with copper pipes or unprotected pipes in screed.