No pressure when running on main drain

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marmaladejackson

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Jun 7, 2014
5
North Richland Hills
Hi All,

This is my second summer with this pool. Most of the time I have the valves set so its running 50/50 skimmer/drain. While I was working with it last weekend, I noticed that if I turned the valves to favor the main drain, pressure bottomed out and the filter skimmer would fail to fill with water. although it looked like there was good water flow coming in. If I turned the valves back to 50/50 or 100% skimmer, things would return to normal. I did the shaving cream test on the exposed pipework and didn't detect any air being sucked in at the joints. I took a video of whats happening for better clarification.


I frequently ran 100% on main drain last summer and while the pressure would drop a psi or two over the skimmer, it did nothing close to what it is doing now. Does anybody have an idea of what might be wrong or how I can further troubleshoot?

Thanks and kind regards,
Brian
 
Welcome to TFP!

That does look like an air leak somewhere in the plumbing between the main drain and the valve you are turning in the video. There are some suggestions on fixing suction side air leaks in Pool School. Finding a leak can be tedious, especially if it turns out to be underground.
 
Stop and think a second... If you're drawing air, you should have air in the tank. Run that same test and as soon as you switch back to skimmer, open the air bleed on the filter and see what comes out. Do it a couple times.

You could have a collapsed or clogged line to the drain and what you're seeing isn't air in the pump basket, but nothingness. A vacuum. I get the same thing when the vacuum head hits the hump where my pool floor slopes and sucks itself flat against the concrete, starving the pump.

Yes, a vacuum is possible. It's just much lower pressure than atmosphere. You know the pump is capable of that, because the lower pressure is what allows atmospheric pressure to push water into the pump. The pump doesn't suck water; it just lowers the pressure enough that atmospheric pressure can shove the water uphill against gravity to fill it. You suck on a straw, pressure drops, atmospheric pressure pushes the liquid uphill. Cap the straw with your finger and suck. There will be a point where the straw is under a vacuum, and then it collapses. You've never sucked all the air out of a coke bottle and had it hang from your lip? What's in the bottle? Maybe some liquid, and a lot of nothing.
 
marmaladejackson said:
I actually have been looking at that. Not much air comes out of the bleed. OK assuming I have a clog, is there a way to clear that?
Click to expand...
Shut everything down. Move the valve to full main drain. Open the pump strainer, remove the basket, shove a garden hose down the inlet pipe from inside and seal it up with a rag (or use a drain king bladder) and have someone turn on the hose full blast. If it's a bunch of debris down in the drain area, you should see it start coming up into the pool.

There's no chance your drain cover is easily removed, or has big openings? I recall someone a year or two ago that discovered a matchbox car caught at the inlet to the 3-way valve. But theirs came through the skimmer.
 
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