Main drain not sucking full volume of water

jonnyrr77

Active member
May 1, 2013
33
#1
Hello. When I have my skimmer valve and main drain valve open, the pump basket is full of water. When I try to run main drain only, water flows, but it struggles to maintain a full basket and air is introduced into the system. Obviously, my assumption is that air is getting in somewhere. But, why is air NOT getting it at all when the skimmer line is open too? Shouldn't air be getting in regardless if the main drain valve is involved? How does the issue go completely away when the skimmer valve is open? Secondly, I have checked all the unions above ground to see where air may be getting in and don't see any. Are there other causes of this issue? I.e. debris in and around the main drain (pool is still cloudy so I can't see if anything is down there) or air stuck in the line some how? I used a compressor to blow out the main drain for the winter. I use a little contraption to blow out the main drain until I see a good volume of air bubbles coming up. Could this have caused anything? Thank you very much for the help.
 

Richard320

TFP Expert
LifeTime Supporter
Jan 6, 2010
20,330
San Dimas, CA (LA County)
#2
Before you go :crazy: and start pulling your hair out, check that it is indeed sucking air and not just starving for water.

How? Easy. Run things normally so that the pump basket is full and air-free. Open the filter bleeder and let any trapped air escape. Do that twice a few minutes apart. Then switch to drain only and wait a few minutes until you see the air and froth through the lid. Open the air bleed again. If air comes out, you're sucking air. If water squirts out immediately, it's just starving. That air you see in the pump basket isn't air; it's just emptimess. In that case it just means that the pump is capable of pulling more water than the underground plumbing to the main drain alone can supply. I get that sometimes when I'm vacuuming. The reduced diameter and the length of the ose and the drag from all those ribs combine to starve the pump.

The cure is to crack the skimmer valve just enough to keep the pump basket full.

If air comes out when you test, then you need to seek out a suction side leak. I'd start by coating the valve and connections with shaving cream and see if it gets suckedin. You'll see foam in the basket and maybe even out the returns.
 
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jonnyrr77

Active member
May 1, 2013
33
#3
Thank you very much. Appreciate the thoroughness of your answer. I will give this all a go and see if I can get it corrected. Thank you.

Before you go :crazy: and start pulling your hair out, check that it is indeed sucking air and not just starving for water.

How? Easy. Run things normally so that the pump basket is full and air-free. Open the filter bleeder and let any trapped air escape. Do that twice a few minutes apart. Then switch to drain only and wait a few minutes until you see the air and froth through the lid. Open the air bleed again. If air comes out, you're sucking air. If water squirts out immediately, it's just starving. That air you see in the pump basket isn't air; it's just emptimess. In that case it just means that the pump is capable of pulling more water than the underground plumbing to the main drain alone can supply. I get that sometimes when I'm vacuuming. The reduced diameter and the length of the ose and the drag from all those ribs combine to starve the pump.

The cure is to crack the skimmer valve just enough to keep the pump basket full.

If air comes out when you test, then you need to seek out a suction side leak. I'd start by coating the valve and connections with shaving cream and see if it gets suckedin. You'll see foam in the basket and maybe even out the returns.
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
12,262
Pleasanton, CA
#4
If you see air in a pump basket, it is sucking air. A pure vacuum in a pump basket is impossible.

What is the filter pressure when you switch to MD only? The MD could be blocked or perhaps just have a lot of head loss which is causing an air leak.