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Boiler pressure gyrating

Does what it says on the tin
MrFoolish
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Boiler pressure gyrating

#389221

Postby MrFoolish » February 23rd, 2021, 3:25 pm

My combi boiler pressure stayed pretty nominal during the cold weather, but now things have warmed up a bit, it seems to be gyrating somewhat.

A couple of days back, I noticed it wasn't much above 0 bar, so I topped it up to 1 with the fill loop. Later that evening, when the heating came on, it was around 2 and some water had come out the bottom (overpressure relief?)

Today it is back down to about 0.25 when cold.

But it seems to be working ok.

I haven't noticed any leaks. I haven't bled the radiators recently.

Thoughts? Thanks.

Itsallaguess
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Re: Boiler pressure gyrating

#389240

Postby Itsallaguess » February 23rd, 2021, 4:14 pm

MrFoolish wrote:
Thoughts?


Those are all classic signs of your expansion vessel not currently doing it's job...

Difficult to tell if they're goosed or just need inflating, but the usual test is to release the system pressure and inflate the expansion vessel with a bicycle pump, and then re-pressurise the system.

If it works, it just needed inflating, but if you end up back at square one then it usually either needs replacing if that's a simply job, or if that's difficult to do then consider just adding an additional expansion vessel of a suitable size elsewhere on the system if you've got the space for one and access to install it.

Cheers,

Itsallaguess

MrFoolish
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Re: Boiler pressure gyrating

#389244

Postby MrFoolish » February 23rd, 2021, 4:20 pm

Thanks. I guess that expansion vessel must be inside my boiler (Remeha Avanta Plus) cos I've not seen it elsewhere? Will have to consult the manual to see if it looks like a job for an amateur....

Alternatively, I do have a contract for repairs so will need to read the small print to see if they cover doing this.

Itsallaguess
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Re: Boiler pressure gyrating

#389261

Postby Itsallaguess » February 23rd, 2021, 5:02 pm

MrFoolish wrote:
I guess that expansion vessel must be inside my boiler (Remeha Avanta Plus) cos I've not seen it elsewhere?

Will have to consult the manual to see if it looks like a job for an amateur....

Alternatively, I do have a contract for repairs so will need to read the small print to see if they cover doing this.


The expansion vessel looks to be on the right hand side of your boiler as you're looking at it - you can see it in the picture below, it's the big red vertical thing on the right hand side - and it looks like the inflation-valve is in the middle on the left hand side that's facing you in the photo below (it looks like it's got a red cap on it in this photo) -

Image

Source - https://www.expert-boiler-repair.co.uk/other/repairs_archive/#remehaavantaplus24c

You should consult the boiler manual to ensure the correct re-inflation pressure if this is something you're considering giving a go, and the system should be de-pressurised before doing so..

Cheers,

Itsallaguess

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Re: Boiler pressure gyrating

#389270

Postby staffordian » February 23rd, 2021, 5:19 pm

Just a suggestion should it turn out that the expansion vessel has had it.

Consider having an external one such as this...

https://www.toolstation.com/expansion-v ... AREALw_wcB

somewhere in the pipework rather than having the existing one replaced, unless it's all covered by your maintenance agreement, of course.

First, it will probably be a lot cheaper, second, it will avoid the labour cost of dismantling the boiler to access and swap the old one, and finally it will probably be easier to get to if it needs re-pressurising in the future.

We did this when ours went and it sits in the loft, easy to get to if needed. We've since had a new boiler but we were advised to leave the extra vessel in place. I don't think it does any harm to have two of them.

MrFoolish
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Re: Boiler pressure gyrating

#389310

Postby MrFoolish » February 23rd, 2021, 7:29 pm

Thanks all.

I see from my manual that the expansion vessel needs charging to 1 bar. I'll at least have a peek at it in the next day or two and see if I fancy being brave with it. Was thinking I could use my electric car tyre inflator but that 12V lead ain't going to reach... hmm.

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Re: Boiler pressure gyrating

#389353

Postby UncleEbenezer » February 23rd, 2021, 10:38 pm

MrFoolish wrote:Thanks all.

I see from my manual that the expansion vessel needs charging to 1 bar. I'll at least have a peek at it in the next day or two and see if I fancy being brave with it. Was thinking I could use my electric car tyre inflator but that 12V lead ain't going to reach... hmm.


I suspect you may be where I was on the boiler learning curve a couple of months back[1].

You don't charge to one bar with a tyre inflator. You just open a particular tap and the water rushes in raising the pressure. Then you hope the pressure stays up, 'cos if not you have a problem.

If you google, you can find various explanations including youtube videos, though by no means all are comprehensible.

Having said that, gyrating as you describe is AIUI a different problem to what I had. It can probably be repaired, though I couldn't guess at the price.

[1] I had some fantastic help, not least from our Foolish expert Mike (if he contradicts what I say, you definitely believe him and not me). I ended up getting a new boiler, which was installed yesterday.

MrFoolish
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Re: Boiler pressure gyrating

#389394

Postby MrFoolish » February 24th, 2021, 6:58 am

Funnily enough, I didn't see anything in the manual about inflating the vessel with a pump, though maybe I missed something.

Anyway, I did nothing yesterday except bleed all the radiators and reset the pressure to 1 bar using the fill loop. Looking just now, it was sitting at 1.5 which seems an improvement on yesterday morning. Perhaps it will just settle down, he said hopefully.

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Re: Boiler pressure gyrating

#390838

Postby SteelCamel » February 28th, 2021, 10:48 am

UncleEbenezer wrote:You don't charge to one bar with a tyre inflator. You just open a particular tap and the water rushes in raising the pressure. Then you hope the pressure stays up, 'cos if not you have a problem.

That's the point MrFoolish got to - and the pressure doesn't stay up, it shoots up really high then drops to zero. This means the expansion vessel isn't doing its job.

It's basic physics that when you heat something it expands. Water doesn't expand very much, but it does expand. This raises the pressure, the volume, or both. In the old-fashioned heating systems with a tank, the water level in the header tank (which is open to the air) rises slightly to accommodate this - that is, the volume increases and the pressure stays the same. In a sealed, tankless system there's nowhere for the water to go. So an expansion vessel is needed. Usually this is inside the boiler, though external ones can be added - either because the one in the boiler is failed and too much trouble to replace, or it's a very large system and the one in the boiler isn't big enough.

It's a small cylinder with water in one side and air in the other, and a rubber membrane to keep them apart. When the water expands, it pushes on the membrane - and as air is a gas, the air compresses quite easily, allowing the rubber to stretch and accommodate the extra volume of water. When the water cools, the air pressure pushes back and the water goes back into the pipes. So normal behaviour is that when you turn the heat on, the pressure goes up a bit as the air compresses, then back to 1 bar when it cools.

There's a few ways it can fail, but they all basically amount to the vessel having no air in. In that case, there's no air to compress (and water being a liquid doesn't compress easily), so the pressure goes really high and the relief valve dumps water out to prevent the pipes bursting. Then when it cools, there isn't enough water (as some got dumped) so the pressure drops to near-zero.

The easiest to fix is simply that the air leaked out over time. In which case you can drain the water to relieve pressure, then pump some more air in. And yes, the vessel has the same valve as a tyre, and you pump it up with a bike pump (it's pretty small, a bike pump is enough, and easier to get into place than a car tyre pump). The other common fault is that the membrane is split - if water comes out of the valve when you try to check the air pressure this is a clear sign of a leak. In this case you need to replace the vessel - or add another, as sometimes replacing requires taking the boiler to pieces. The other possibility is that the pipe into the vessel is blocked - in this case the vessel has enough air in but still doesn't function as the water can't get to it.

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Re: Boiler pressure gyrating

#390858

Postby Mike4 » February 28th, 2021, 11:31 am

SteelCamel wrote:
UncleEbenezer wrote:You don't charge to one bar with a tyre inflator. You just open a particular tap and the water rushes in raising the pressure. Then you hope the pressure stays up, 'cos if not you have a problem.

That's the point MrFoolish got to - and the pressure doesn't stay up, it shoots up really high then drops to zero. This means the expansion vessel isn't doing its job.

It's basic physics that when you heat something it expands. Water doesn't expand very much, but it does expand. This raises the pressure, the volume, or both. In the old-fashioned heating systems with a tank, the water level in the header tank (which is open to the air) rises slightly to accommodate this - that is, the volume increases and the pressure stays the same. In a sealed, tankless system there's nowhere for the water to go. So an expansion vessel is needed. Usually this is inside the boiler, though external ones can be added - either because the one in the boiler is failed and too much trouble to replace, or it's a very large system and the one in the boiler isn't big enough.

It's a small cylinder with water in one side and air in the other, and a rubber membrane to keep them apart. When the water expands, it pushes on the membrane - and as air is a gas, the air compresses quite easily, allowing the rubber to stretch and accommodate the extra volume of water. When the water cools, the air pressure pushes back and the water goes back into the pipes. So normal behaviour is that when you turn the heat on, the pressure goes up a bit as the air compresses, then back to 1 bar when it cools.

There's a few ways it can fail, but they all basically amount to the vessel having no air in. In that case, there's no air to compress (and water being a liquid doesn't compress easily), so the pressure goes really high and the relief valve dumps water out to prevent the pipes bursting. Then when it cools, there isn't enough water (as some got dumped) so the pressure drops to near-zero.

The easiest to fix is simply that the air leaked out over time. In which case you can drain the water to relieve pressure, then pump some more air in. And yes, the vessel has the same valve as a tyre, and you pump it up with a bike pump (it's pretty small, a bike pump is enough, and easier to get into place than a car tyre pump). The other common fault is that the membrane is split - if water comes out of the valve when you try to check the air pressure this is a clear sign of a leak. In this case you need to replace the vessel - or add another, as sometimes replacing requires taking the boiler to pieces. The other possibility is that the pipe into the vessel is blocked - in this case the vessel has enough air in but still doesn't function as the water can't get to it.


A very thorough explanation, thank you.

But I'm not sure it is entirely clear where I have highlighted, so here is some more detail. For the expansion vessel to have lost all it's air on one side of the membrane, it will necessarily have completely filled with water on the other side of the membrane. This means that just adding air via the schrader valve won't get you very far because as you have already pointed out, there is nowhere else for the 5 or 10 litres of water filling the expansion vessel to go when you add the air. So you can only squeeze a tiny amount of air in before the pressure rises to 1.0 bar, and full functionality will not have been restored.

To get back to full functionality one needs to keep a drain tap on the heating circuit open whilst adding the air. This is allow the surplus 5 or 10 litres on water in the expansion vessel to be expelled from the system and the full volume of the vessel to be pumped up with air to 1.0 bar.

When this has been achieved, the water pressure (system pressure) gauge will still read zero but the air pressure behind the schrader valve will have risen to and read a nice stable 1.0 bar. At this point close the drain tap, then open the filling loop and add what should be a litre or two of water to bring the system pressure up to 1.0 bar too, and the job is done. If a stable 1.0 bar of air pressure in the vessel cannot be achieved and the air pressure keeps falling whilst the drain tap is open, the membrane has ruptured and a new vessel is required.

All this needs to be done for best results with the boiler and radiators cool, by the way.


(Edit to clarify a couple of sentences.)

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Re: Boiler pressure gyrating

#390939

Postby MrFoolish » February 28th, 2021, 5:51 pm

Thanks for the comments.

I've now had a couple of attempts at this. The first time when I connected the pump, some water came out. Not exactly Niagra Falls, perhaps an egg cup or two. I wimped out of draining the boiler from the outlet underneath as it didn't look very user friendly. Instead I used the bleed point on a downstairs radiator (not sure if this is legit!) and waited until the dial on the boiler dropped to zero. I then closed the radiator bleed. It only took about one press of the foot pump until its dial and the boiler's dial read 1 bar. Instinctively this didn't seem right. Running the boiler that evening, I think the gyrations were more contrained but still not right.

So now, taking heed of Mike's comments, I've just had another go, this time keeping the radiator bleed open as I pumped. It probably took 3 times the amount of air this time and I had to add some water back in. The heating has now been on about half and hour and the pressure is at 2.0 bar. I suspect it will go up further and that the pressure vessel has a leak but will keep an eye on it.

I see my service contract does not cover periodic recharging of the vessel. But presumably it should cover a vessel failure. So suspect I'll be calling them out before too long.

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Re: Boiler pressure gyrating

#390990

Postby Fingers » February 28th, 2021, 10:27 pm

If your system happens to have a radiator you do not use, it could be used as an expansion vessel. Close its valves, drain it of water then reopen one valve but do not let any air out. Problem solved, only expense is time and trouble.


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