Position of AAV

Wearb

I was at a boiler yesterday and they are having trouble with air getting into the system. Not a lot but enough to necessitate the bleeding of one radiator of a little air every 2 or 3 weeks.


It has an outside SE grant boiler with the circ pump within the heatpac. The pump is on the hot pipe coming from the top of the boiler. Also on the top of the boiler is the AAV. Could there be air being sucked in through this because of the pump position? Do those AAV bottles have any feature to prevent this?


I have closed the cap on the AAV and asked customer to open it (with system off) weekly and if no air, then monthly etc. It will be interesting to see if this clears the problem.


Anyway, any thoughts on the above please?

sullzz

Wearb wrote: »

I was at a boiler yesterday and they are having trouble with air getting into the system. Not a lot but enough to necessitate the bleeding of one radiator of a little air every 2 or 3 weeks.


It has an outside SE grant boiler with the circ pump within the heatpac. The pump is on the hot pipe coming from the top of the boiler. Also on the top of the boiler is the AAV. Could there be air being sucked in through this because of the pump position? Do those AAV bottles have any feature to prevent this?


I have closed the cap on the AAV and asked customer to open it (with system off) weekly and if no air, then monthly etc. It will be interesting to see if this clears the problem.


Anyway, any thoughts on the above please?

Air in water out.
I think you need to ask yourself where the water is going.

Wearb

sullzz wrote: »

Air in water out.
I think you need to ask yourself where the water is going.

I am not sure if I understand your reply. There isn’t any water getting out, any water displacement is absorbed by the expansion tank...if that’s what you meant.

John.G

Wearb wrote: »

I was at a boiler yesterday and they are having trouble with air getting into the system. Not a lot but enough to necessitate the bleeding of one radiator of a little air every 2 or 3 weeks.


It has an outside SE grant boiler with the circ pump within the heatpac. The pump is on the hot pipe coming from the top of the boiler. Also on the top of the boiler is the AAV. Could there be air being sucked in through this because of the pump position? Do those AAV bottles have any feature to prevent this?


I have closed the cap on the AAV and asked customer to open it (with system off) weekly and if no air, then monthly etc. It will be interesting to see if this clears the problem.




Anyway, any thoughts on the above please?

Based on my (limited) experience(s) of a relations sealed system with the AAV mounted on a T piece at the highest point on the system which is the coil entry; then yes in this case at any rate as the cap is kept closed normally and opened very occasionally to vent any air in the system. On the two occasions that I've drained down the system I have opened the AAV cap and can hear air being drawn in through the AAV, I would assume that it is then acting as a vacuum breaker once the expansion vessel contents are drained down and the its diaphragm is hard up against the expansion vessel end/bottom and not exerting any pressure on the remaining system contents.

Regarding air being drawn in your customers case above due to the circ pump position I would say yes it is possible but far less likely in a sealed system, is this system sealed or vented?, if a vented system like my own then a high mounted rad can run at a slightly negative head and draw in some air over a period, my attic rad does this and to vent it (about twice a year) I just do it when the circ pump is off as the F&E tank then supplies enough head to push out the very small volume of air. (My circ pump is mounted on the Boiler return)

Sorry, I had put the edit in your post by mistake.
Edit: I see you said it has a expansion tank, so then a vented system?.

I read somewhere or other that someone who had a long standing problem with air ingress finally tracked it down to the circ pump isolation valve gaskets which he changed to rubber as the fibre ones apparently were drawing in small quantities of air over time with no sign of leakage.

Wearb

I have moved the other posts in this thread to a new thread as they were about 2 different things.

See Circulating Pump Settings.
https://www.boards.ie/vbulletin/showthread.php?t=2057936384

It doesn’t take much for a AAV to play up and if it’s under negative pressure it can draw in air.

Wearb

gary71 wrote: »

It doesn’t take much for a AAV to play up and if it’s under negative pressure it can draw in air.

Close to the suction side of a pipe I could be sure of it being under negative pressure, but I wondered if the large capacity of the boiler reduced that effect.
So many boilers are setup like this nowadays, ie pump on hot pipe.

Wearb

Wearb wrote: »

Close to the suction side of a pipe I could be sure of it being under negative pressure, but I wondered if the large capacity of the boiler reduced that effect.
So many boilers are setup like this nowadays, ie pump on hot pipe.

Be interesting to see if closing it has a impact

I went to a new install yesterday that was under negative pressure and had terrible air problems , the installer had accidentally forgot the flow and connected to the same pipe as the return.

sullzz

Wearb wrote: »

I am not sure if I understand your reply. There isn’t any water getting out, any water displacement is absorbed by the expansion tank...if that’s what you meant.

Ah fair enough, don't know why but i presumed it was a sealed system.
As Gary said, it'll be interesting to see what effect closing the AAV will have.

Wearb

sullzz wrote: »

Ah fair enough, don't know why but i presumed it was a sealed system.
As Gary said, it'll be interesting to see what effect closing the AAV will have.

But even in a sealed system, would the vessel not work the same as the expansion tank?

I know that it would be less likely to have a negative pressure zone in that case.

Wearb

gary71 wrote: »

Be interesting to see if closing it has a impact

I am curious myself. though the new pump might contribute to solving the problem also by having the ability to vary its load if they happen to shut off a few rads. It would have been a more accurate experiment if the same pump had remained.

I went to a new install yesterday that was under negative pressure and had terrible air problems , the installer had accidentally forgot the flow and connected the same pipe as the return.

I don't get that

sullzz

Wearb wrote: »

But even in a sealed system, would the vessel not work the same as the expansion tank?

I know that it would be less likely to have a negative pressure zone in that case.

I wouldnt think it would be as extreme to the extent that a rad would need to be vented as often as they are doing at the moment.

Wearb

Wearb wrote: »

I am curious myself. though the new pump might contribute to solving the problem also by having the ability to vary its load if they happen to shut off a few rads. It would have been a more accurate experiment if the same pump had remained.





I don't get that

The boiler had no flow going to the rads and cylinder, the flow pipe was connected directly to the boiler return pipe work, only circulating between the boilers flow and return

Wearb

Wearb wrote: »

But even in a sealed system, would the vessel not work the same as the expansion tank?

I know that it would be less likely to have a negative pressure zone in that case.

I believe the cold feed is the neutral point normally and with a sealed system it’s the mid-way point of the pipe work( but I wouldn’t bet my house on it)

The vessel IS the neutral point on regular boilers with external components and the Mid-way point with combis and system boilers with incorporated expansion vessels.

Wearb

gary71 wrote: »

The boiler had no flow going to the rads and cylinder, the flow pipe was connected directly to the boiler return pipe work, only circulating between the boilers flow and return

Got it. Didn't take long to heat that circuit

John.G

gary71 wrote: »

I believe the cold feed is the neutral point normally and with a sealed system it’s the mid-way point of the pipe work( but I wouldn’t bet my house on it)

The vessel IS the neutral point on regular boilers with external components and the Mid-way point with combis and system boilers with incorporated expansion vessels.

I think that if the pump is pumping away from the expansion vessel that the pump inlet is at the E.vessel head (pressure) and the pump outlet is at the E.vessel head + the pump diff head and if the pump is pumping towards the E.vessel then the pump inlet is at the E.vessel head - the pump diff head and the pump outlet is at the E.vessel head.? If true then the siting of the expansion vessel is of some importance.

Wearb

John.G wrote: »

I think that if the pump is pumping away from the expansion vessel that the pump inlet is at the E.vessel head (pressure) and the pump outlet is at the E.vessel head + the pump diff head and if the pump is pumping towards the E.vessel then the pump inlet is at the E.vessel head - the pump diff head and the pump outlet is at the E.vessel head.? If true then the siting of the expansion vessel is of some importance.

With open vented system the misplacment of the vent and f/e pipe in relation to the pump used to cause lots of problems.

Re above: My ( perhaps wrong) thinking is that (with EV behind pump intake) with pump off the whole system has to be of equal pressure. Pump starts and immediately there is a pressure increase at flow side and drop at intake.
The EV if set at a little below system pressure would take up the pressure drop at the intake.

I might not have that correct and anyway most EV s aren’t setup like that. They are set at or a little above system pressure That’s what I’ve found when I check them.