Pressure drops when AAV open

[plumb_know]

Hi Everyone,

to cut a long story short i have a customer that has had problems with pressure in their heating system. i have got the system to stop dropping pressure then they spotted a small leak from a manual air vent on cylinder flow to coil.

I replaced this with a auto air vent and left it open to remove air, but the system just keeps loosing pressure but not water from valve.

I have closed this up and now the pressure holds. In my theory there must be a small leak on negative side of pump which is drawing air in the system and this is being lost through AAV?

Could there be another explanation to why system pressure keeps dropping when AAV open, it is dropping from 1.5bar - 0 within 1 hour. As said earlier when AAV is closed pressure stops dropping.

Also how can i trace where exactly the air is being drawn into the system? Would bottle of leak sealer fix the problem?

Appreciate all your comments.

Thanks

 

[Chuck]

Also how can i trace where exactly the air is being drawn into the system?

IME, it's very rare for 'air to be drawn in' to a pressurised system. Several unlikely things have to be wrong simultaneously. Does the rate of ingress depend on the pump speed? Is the circulation okay? Can you tee a gauge into the pipework infront of the pump to demonstrate that the gauge pressure is negative at that point?

I'd start by checking how much "air" is really being vented through your AAV. Put a balloon (or condom) over it. A drop from 1.5 to 0 bar should be several litres of 'air' depending on the size of the expansion tank.

Most likely cause after work has been done is unpurged air trapped in the system by refilling. If the expansion tank is also not working correctly (incorrect size, precharge, not blocked, etc.) then you have an explanation for the symptoms.

Would bottle of leak sealer fix the problem?

IMO, leak sealer is a last resort to try and keep a system going for a couple of weeks in the winter until a new boiler can be installed. If you were to put it in at this point and the householder got someone who knew what they were doing you could end up footing the bill for an expensive repair.

EDIT: If the above doesn't give you a useful lead, a few more details about the system will help further discussion.

 

[John.g]

Hi Everyone,

to cut a long story short i have a customer that has had problems with pressure in their heating system. i have got the system to stop dropping pressure then they spotted a small leak from a manual air vent on cylinder flow to coil.

I replaced this with a auto air vent and left it open to remove air, but the system just keeps loosing pressure but not water from valve.

I have closed this up and now the pressure holds. In my theory there must be a small leak on negative side of pump which is drawing air in the system and this is being lost through AAV?

Could there be another explanation to why system pressure keeps dropping when AAV open, it is dropping from 1.5bar - 0 within 1 hour. As said earlier when AAV is closed pressure stops dropping.

Also how can i trace where exactly the air is being drawn into the system? Would bottle of leak sealer fix the problem?

Appreciate all your comments.

Thanks

 

[John.g]

IME, it's very rare for 'air to be drawn in' to a pressurised system. Several unlikely things have to be wrong simultaneously. Does the rate of ingress depend on the pump speed? Is the circulation okay? Can you tee a gauge into the pipework infront of the pump to demonstrate that the gauge pressure is negative at that point?

I'd start by checking how much "air" is really being vented through your AAV. Put a balloon (or condom) over it. A drop from 1.5 to 0 bar should be several litres of 'air' depending on the size of the expansion tank.

Most likely cause after work has been done is unpurged air trapped in the system by refilling. If the expansion tank is also not working correctly (incorrect size, precharge, not blocked, etc.) then you have an explanation for the symptoms.



IMO, leak sealer is a last resort to try and keep a system going for a couple of weeks in the winter until a new boiler can be installed. If you were to put it in at this point and the householder got someone who knew what they were doing you could end up footing the bill for an expensive repair.

EDIT: If the above doesn't give you a useful lead, a few more details about the system will help further discussion.

To check for leak:
AAV closed, boiler off, system cold & pressure initially 1.5 bar, is it still losing this 1.5 bar pressure within hour?

 

[Sharp Point]

What’s the make and model of the boiler?

 

[plumb_know]

Hi Everyone,

Thanks for replies,

Chuck:
Thanks for detailed reply. just to expand a bit further the system was fine for weeks before AAV installed, the AAV is at top of system so only released pressure from system to install and did not drain down system to carry out work so air introduced would have been minimal.
It has been loosing pressure like this since the AAV installed every day for week, i doubt this is air within the system from work carried out.

Since i have closed the nipple on the AAV about 4 days ago the customer has said pressure is fine now and no other factors have changed.

What i am struggling to look past is it is only when AAV is open this occurs surely if it were expansion vessel etc etc it should happen whether or not the AAV is open or closed. The only explanation i can think of is that some where air is being introduced to the system and the AAV was doing its job by venting it.

Otherwise could it be hydrogen being formed and vented?? but surely it wouldn't produce that quickly to produce that big a drop every day over week?

Also i checked the expansion vessel pressure & replaced PRV to fix initial drop they were having few weeks prior to installing the AAV.

John - all pipework is above ground and no sign of leak, which with this size drop should be showing some signs by now. Also since AAV closed no drop over 4 days.

Appreciate your thoughts

 

[plumb_know]

bog rodder - it is a worcester system boiler, greenstar cant remember the exact model off the top of my head.

 

[Rob Foster]

IME, it's very rare for 'air to be drawn in' to a pressurised system. Several unlikely things have to be wrong simultaneously. Does the rate of ingress depend on the pump speed? Is the circulation okay? Can you tee a gauge into the pipework infront of the pump to demonstrate that the gauge pressure is negative at that point?

I'd start by checking how much "air" is really being vented through your AAV. Put a balloon (or condom) over it. A drop from 1.5 to 0 bar should be several litres of 'air' depending on the size of the expansion tank.

Most likely cause after work has been done is unpurged air trapped in the system by refilling. If the expansion tank is also not working correctly (incorrect size, precharge, not blocked, etc.) then you have an explanation for the symptoms.



IMO, leak sealer is a last resort to try and keep a system going for a couple of weeks in the winter until a new boiler can be installed. If you were to put it in at this point and the householder got someone who knew what they were doing you could end up footing the bill for an expensive repair.

EDIT: If the above doesn't give you a useful lead, a few more details about the system will help further discussion.

Never ever leak sealer centralheatking

 

[Riley]

If you go back and open it again is there another air build up

 

[plumb_know]

Yes when i went back and topped it up before closing the AAV i got a bit of air come out again.

It has been over a week now and pressure is still stable as customer has not been in contact so must i can only see it being air or gases being released from the AAV as nothing else has changed apart from closing the nipple and no water coming from valve.

surely it wouldn't take over a week for remaining air in system to be vented out with AAV open as i have never had this issue before? Also for it to be dropping within the hour 1.5bar this can not just be general air in the system.

I am finding it hard to look past air or gases being introduced some way into the system at the moment as surely if it was under size expansion vessels, PRV discharge or leaks etc this would all occur with the AAV nipple closed also??

Please tell me if i am being blinkered to this view, but i just cant see any plumbing theories as to what else could explain the drop when the only difference making the system keeping pressure to dropping is AAV nipple being open??

Chuck:
"If you were to put it in at this point and the householder got someone who knew what they were doing you could end up footing the bill for an expensive repair."

Thanks for detailed response earlier, much appreciate the help but feel it is a bit harsh to imply i do not know what i am doing just because i am asking for some advice on one problem as i am sure you have done in your career.

I very rarely use leak sealer only as a last resort, most of the time this would be on property where they have a very small drop of pressure over weeks/months and all obvious things have been checked and they may have a large amount of pipework under concrete floor which is just no practical to try and locate such a small leak in large system. I personally have yet to have any problem's with it but have only used it about 3 times in 4 years.

I am sure you may not have meant to imply it in the way it came across but just to clarify i am under 30 and have had my business over 4 years and regularly solve problems that numerous other engineers have struggled solving for customer's but i do not claim to know everything and on occasion need some help, advice or another prospective from good people like yourself and others on this forum.

no hard feeling just didn't want people thinking i am total novice or DIY enthusiast appreciate all the help

 

[John.g]

Yes when i went back and topped it up before closing the AAV i got a bit of air come out again.

It has been over a week now and pressure is still stable as customer has not been in contact so must i can only see it being air or gases being released from the AAV as nothing else has changed apart from closing the nipple and no water coming from valve.

surely it wouldn't take over a week for remaining air in system to be vented out with AAV open as i have never had this issue before? Also for it to be dropping within the hour 1.5bar this can not just be general air in the system.

I am finding it hard to look past air or gases being introduced some way into the system at the moment as surely if it was under size expansion vessels, PRV discharge or leaks etc this would all occur with the AAV nipple closed also??

Please tell me if i am being blinkered to this view, but i just cant see any plumbing theories as to what else could explain the drop when the only difference making the system keeping pressure to dropping is AAV nipple being open??

Chuck:
"If you were to put it in at this point and the householder got someone who knew what they were doing you could end up footing the bill for an expensive repair."

Thanks for detailed response earlier, much appreciate the help but feel it is a bit harsh to imply i do not know what i am doing just because i am asking for some advice on one problem as i am sure you have done in your career.

I very rarely use leak sealer only as a last resort, most of the time this would be on property where they have a very small drop of pressure over weeks/months and all obvious things have been checked and they may have a large amount of pipework under concrete floor which is just no practical to try and locate such a small leak in large system. I personally have yet to have any problem's with it but have only used it about 3 times in 4 years.

I am sure you may not have meant to imply it in the way it came across but just to clarify i am under 30 and have had my business over 4 years and regularly solve problems that numerous other engineers have struggled solving for customer's but i do not claim to know everything and on occasion need some help, advice or another prospective from good people like yourself and others on this forum.

no hard feeling just didn't want people thinking i am total novice or DIY enthusiast appreciate all the help

Its a strange one alright, If the system is fitted with a external EV (modern gas boilers have integral ones and you may not be qualified to access it) then I would drain down the system, ensure that the EV pre pressure is 1.0 bar, I would then check that there is no blockage in the line (including any flexible piping) from the EV to the tie in point to the system, I would then reconnect and fill the EV to 1.5 bar (cold) and retest with AAV open. If same problem then persists then it more or less proves that you are expelling air/gas, ~ 0.5 to 1 litre/hr as the EV would have had a reserve water volume of ~ 1.5 to 2.5 litres and this would have to enter the system before the EV bladder would be hard up against the end of the EV at 1 bar, of course the water side pressure will then fall very rapidly to zero bar as the EV is having no more effect.
I note all your reasoning which IMO is sound enough but you simply have to rule out any EV or associated blockage problems 100% IMO before going any further.
One indirect test you can do if not already done so is to note the EV pressure with system cold and again note it when system is fully heated, it should rise ~ 0.5/1.0 bar.

This method is not foolproof of course as I found on a relations system. He was topping it up 2 or 3 times a day, the pressure was then rising from 1.5 bar to 2.8 bar when hot but not lifting the PRV, I found the EV had no air pressure which resulted effectively in having no expansion volume whatsoever (EV full of water), the only conclusion that I came to re the PRV not lifting was that he had lots of entrained air in his system which was taking the expansion, he has a mix of rads and UFH.

 

[Riley]

Well a radiator is a “get you by expansion vessel in emergency”

 

[Riley]

Its a strange one alright, If the system is fitted with a external EV (modern gas boilers have integral ones and you may not be qualified to access it) then I would drain down the system, ensure that the EV pre pressure is 1.0 bar, I would then check that there is no blockage in the line (including any flexible piping) from the EV to the tie in point to the system, I would then reconnect and fill the EV to 1.5 bar (cold) and retest with AAV open. If same problem then persists then it more or less proves that you are expelling air/gas, ~ 0.5 to 1 litre/hr as the EV would have had a reserve water volume of ~ 1.5 to 2.5 litres and this would have to enter the system before the EV bladder would be hard up against the end of the EV at 1 bar, of course the water side pressure will then fall very rapidly to zero bar as the EV is having no more effect.
I note all your reasoning which IMO is sound enough but you simply have to rule out any EV or associated blockage problems 100% IMO before going any further.
One indirect test you can do if not already done so is to note the EV pressure with system cold and again note it when system is fully heated, it should rise ~ 0.5/1.0 bar.

This method is not foolproof of course as I found on a relations system. He was topping it up 2 or 3 times a day, the pressure was then rising from 1.5 bar to 2.8 bar when hot but not lifting the PRV, I found the EV had no air pressure which resulted effectively in having no expansion volume whatsoever (EV full of water), the only conclusion that I came to re the PRV not lifting was that he had lots of entrained air in his system which was taking the expansion, he has a mix of rads and UFH.

OP is gas safe

 

[Harvest Fields]

In an open forum?

 

[John.g]

Its a strange one alright, If the system is fitted with a external EV (modern gas boilers have integral ones and you may not be qualified to access it) then I would drain down the system, ensure that the EV pre pressure is 1.0 bar, I would then check that there is no blockage in the line (including any flexible piping) from the EV to the tie in point to the system, I would then reconnect and fill the EV to 1.5 bar (cold) and retest with AAV open. If same problem then persists then it more or less proves that you are expelling air/gas, ~ 0.5 to 1 litre/hr as the EV would have had a reserve water volume of ~ 1.5 to 2.5 litres and this would have to enter the system before the EV bladder would be hard up against the end of the EV at 1 bar, of course the water side pressure will then fall very rapidly to zero bar as the EV is having no more effect.
I note all your reasoning which IMO is sound enough but you simply have to rule out any EV or associated blockage problems 100% IMO before going any further.
One indirect test you can do if not already done so is to note the EV pressure with system cold and again note it when system is fully heated, it should rise ~ 0.5/1.0 bar.

This method is not foolproof of course as I found on a relations system. He was topping it up 2 or 3 times a day, the pressure was then rising from 1.5 bar to 2.8 bar when hot but not lifting the PRV, I found the EV had no air pressure which resulted effectively in having no expansion volume whatsoever (EV full of water), the only conclusion that I came to re the PRV not lifting was that he had lots of entrained air in his system which was taking the expansion, he has a mix of rads and UFH.

Should have added above that his AAV was opened at all times but he hadn't tried closing it, he just rang me.

 

[plumb_know]

sorry for delayed response been hectic.

i had already attended & checked the expansion vessel, recharged it & replaced pressure relief as was initially loosing pressure from the PRV.

since then the pressure has been fine for about a week or so and customer had not had to top it up once. Then there was a leak from the manual bleed valve on primary to unvented cylinder (rubber washer leaking).

i replaced this with auto air vent as was at top of system and this is when the pressure began dropping. No other thing has been changed apart from releasing pressure in system (about 1 bucket of water) cutting old air vent off & putting AAV in replace for it.

Still not had a call from customer since closing AAV so assume it has not dropped pressure since.

but there was no water leak from the new AAV so could only be air or gasses it was expelling.

 

[John.g]

Extraordinary, as I said previously if the EV was pre pressurised to say 0.7 to 1 bar and the filling pressure was 1.3 to 1.5 bar or so then the AAV MUST be releasing well over a litre of air/hour in order for the EV system pressure to fall to zero.

 

[Stigster]

The symptoms being caused by the AAV are certainly weird and not what you would expect to cause these pressure drops. Maybe it's a red herring? Is the filling loop left connected, or an integral loop which is letting by, causing a pressure increase (overnight when mains pressures tend to rise maybe) which then discharges from the PRV?

This large and fairly rapid depressurisation surely can't all be happening at the AAV? Have you tried putting the finger off a rubber glove over the end of the discharge pipe to see if water is lost there when there's nobody watching it?

Certainly a very strange one!

 

[plumb_know]

Hi Thanks for the replies everyone,

Yes the filling loop was left connected as they had to keep filling it up, but it wasn't letting by as i disconnected it to release the pressure out of the system.

Also in theory if this was causing it, it should be over pressurising and releasing the pressure from PRV now surely?

It has stumped me, but if i get called back for further work i will let you all know what i find.

Really appreciate all the help everyone, got the brain thinking

 

[SameoProblemeo]

Did you manage to completely figure it out in the end?

Hi Thanks for the replies everyone,

Yes the filling loop was left connected as they had to keep filling it up, but it wasn't letting by as i disconnected it to release the pressure out of the system.

Also in theory if this was causing it, it should be over pressurising and releasing the pressure from PRV now surely?

It has stumped me, but if i get called back for further work i will let you all know what i find.

Really appreciate all the help everyone, got the brain thinking

 

[plumb_know]

Hi, never heard from the customer again so assume it was the AAV, close them now once i have vented the system. All i can think of was that the system was deteriorating and giving off gases which were being released by the AAV, i did repair pin holes in pipes under concrete previously so were signs system had seen better days.