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