Constant Build Up of Air Pressure In Filter

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NullQwerty

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Apr 23, 2008
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Hey folks,

Had a very green pool opening. Finally got the pool water blue, but now just very cloudy as the filter tries to clean it out.

After changing the DE, my filter's pressure builds up and waterflow stops within 4 hours. To be expected of course, but something that seems a little strange is that when I turn the pump off, before I go to bump it (or switch out the DE), I open the air pressure relief valve, and there's always a good deal of air that gets released. I don't have any water loss. I don't have bubbles in the return line. I don't have an air bubble in the pump's water pot. I've got plenty of water in the pool (top of skimmer). But somehow air is getting in the filter. Where would it be coming from?

Often, I can just release the air, then close the relief valve again and start the pump, without even bumping the filter or changing the DE, and immediately I'll get water flow again and a drop in pressure by at least 5 psi (not always, but often).

So I started reading around about it. I can see that it should not be happening. But I don't know where the air would be coming from. I recently replaced the pump basket gasket and lubed it up and also lubed the other gasket on the pump near the motor housing. Could it be coming from elsewhere on the pump? Maybe the line between the pump and the filter?

One thing that does happen is I hear a weird air straining noise in the middle of the pump after I turn it off. Not sure exactly where it's coming from though. Is this most likely causing it, you think?

Thanks
 
If your filter is above water level and you open the releaf valve while the pump is OFF, air should be drawn into the filter and not released out of the filter. Are you saying that the filter is still under pressure when the pump is off? That would be very odd.
 
Is the filter below water level? If not, I don't see how a filter can hold pressure with the pump off. Are you sure it isn't drawing air in?
 
It's above water line. Right now I'm in the stage of running the pump 24 hours a day because I'm still in start up. So, when I turn off the pump, I'm opening the relief valve only 10 seconds later. It can take 20 seconds or more to get all the air out.

If I were to turn the pump off for a while without opening the relief valve, and keep it that way for like 30 minutes or more, there's a good chance that the pressure would relieve itself on it's own. Haven't tried it, but it wouldn't surprise me.
 
Do you have a check valve on the suction side of the plumbing or between the pump and filter? Also, what is the filter pressure when you shut the pump off? Can you see the pressure slowly decline?

It could be that the grids are so clogged up that it is not allowing anything including air through the filter but that would also require a check valve on the suction so that the pressure could not be relieved through that path. If everything was operating problem, the pressure in the filter should be released through the returns almost immediately when the pump is shut off.
 
No check valve. When I turn off the pump, the filter pressure gauge immediately drops to 0.

I think I can explain why it's not being released. It won't go back through the pump because the pump's hose leaves the pump at a lower elevation than where it enters the filter. So the air won't travel down. It's also not going to go into the pool through the return lines because I've got a salt system and so the plumbing comes out of the filter and then goes up into the cell (about 1' higher than from where it leaves the filter). So again, the air won't travel down the pipes, because it's gonna float. So instead the air will stay in the cell and float there. The water in the cell also won't return to the pool, because that would create a vacuum in the cell, but there's no additional air to pull in to replace the water, so it can't do it. So instead both the water and air just stays in the cell.

When I open the relief valve, all sorts of water will start moving out of the salt cell. Either back into the filter, or into the return lines...which also makes sense.

So, I could see how if air is being built up in the filter, it wouldn't go away when you turn the pump off. So, if you agree, the question ends up becoming, where is this air coming from. Only thing I can think of is that maybe I need to replace my pump's shaft seal assembly? Could that be drawing air in?
 
If the filter pressure goes zero, then the excess pressure is relieved out through the returns so the filter is no longer under pressure.

I think I can explain why it's not being released. It won't go back through the pump because the pump's hose leaves the pump at a lower elevation than where it enters the filter. So the air won't travel down. It's also not going to go into the pool through the return lines because I've got a salt system and so the plumbing comes out of the filter and then goes up into the cell (about 1' higher than from where it leaves the filter). So again, the air won't travel down the pipes, because it's gonna float. So instead the air will stay in the cell and float there. The water in the cell also won't return to the pool, because that would create a vacuum in the cell, but there's no additional air to pull in to replace the water, so it can't do it. So instead both the water and air just stays in the cell.
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That is not the case. Pressure will always be relieved from high pressure to low pressure. Pressure cannot be maintained in a plumbing system unless it is a closed system. A pool system is an open plumbing system and the pressure at the pool surface is 0 PSI so the filter will release the pressure to the pool and that pressure is equalized throughout the plumbing system. However, because the top of the filter is higher than the water level, there will be a slight vacuum and when you open the filter relief valve air will be sucked in.

Also, if the water flow rate is higher than the rate at which air rises in water, air will flow downwards. This why a pump primes and why the air can be purged from a solar system on a roof. In both cases, air is pushed downwards and out the returns.

When I open the relief valve, all sorts of water will start moving out of the salt cell. Either back into the filter, or into the return lines...which also makes sense.
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This confirms that air is going INTO the filter. Since there is slight vacuum at the top of the filter, when you open the valve, you are allowing air into the filter to displace the water which is then flowing out of the return. You only want to open the relief valve with the pump running so air is removed from the filter. This could be part of your problem.

With the pump running, open the relief valve until water squirts out. Then close the valve. Watch the pump basket. Is there any air in the pump basket? If so, the air is being drawn in from the suction side of the pump and passed through to the filter where it is accumulating.
 
Ok! Thanks for the help! I'll give it a shot tonight when I get home. Often what will happen is, after I turn the pump off and open the relief valve, water will start spraying out for 15 or seconds, and then the water stops and the air starts. But, you're probably right and it reverses flow direction...I just didn't notice. To confirm, I'll put a thin strip of paper in front of the valve to see which direction the air moves in (does it push the paper or pull it). It's possible that it's strong enough that what felt like air hitting my hands was actually suction.

Oh, and thanks for the tip on how to determine if the leak is on the suction side. I'm gonna pray it's not :D

Much appreciated!
 
Checked out the air flow. Couldn't tell, so I'm sure you're right and it's suction...I probably just always assumed the opposite, till it became fact in my head. One correction, I said when I turn the pump off the psi dropped to 0. I looked again. It actually just drops to 4 or 5 psi. I did the test you mentioned and didn't get the bubble in the pump basket. So, I went off to the pool store and grabbed a new seal shaft and diffuser gasket and replaced those. They didn't have a housing gasket, so I just lubed up the existing one. After installing those, the air noise I mentioned coming from the pump went away when the pump is turned off, so that's good.

One thing that is still there though is that when I turn the pump on and then turn it off (whether it was on for 30 seconds or 4 hours), if I then open the air relief valve, a bunch of air still transfers. Seems like it makes sense. But just checking...is that ok?

Thanks again for all the help! Hope I can return the favor one day.
 

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One thing that is still there though is that when I turn the pump on and then turn it off (whether it was on for 30 seconds or 4 hours), if I then open the air relief valve, a bunch of air still transfers. Seems like it makes sense. But just checking...is that ok?
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Yes, all filters above water level should do this.

I looked again. It actually just drops to 4 or 5 psi.
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This could be due to a bad gauge. Does it ever go to 0 PSI? Tap it and see if it drops down. You might need a new gauge.
 
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