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The steps to winterize the pool are simple:

  1. Drop the water level until it's below the skimmer, and also below the return lines.
  2. Blow air out of the skimmer and the return lines. To keep the figure simpler, we don't show those lines.
  3. Blow air out of the main drain line.

pool main drain winterization  2

You know the skimmer line and the return lines will be safe. Because you dropped the water level below the skimmer, you know that the water has no way to return to these pipes over winter.

But no one ever dives to plug the main drain line. How do you stop the water from bouncing back (blue area) in the main drain line until it reaches the water level in the pool, after blowing the main drain line?

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  • If water accumulates above the frost line, you can add a comparatively safe antifreeze, such as polyethylene glycol (seer amazon.com/Swimming-Pool-Anti-Freeze-Degrees-1-Gallon/dp/… for example). Oct 28, 2021 at 1:27
  • @DrMoishePippik I have a couple of gallons of the anti-freeze. I'm still trying to figure out how to use them. It should be simple enough with the skimmer line and the return lines (blow water out, seal skimmer, pour anti-freeze, reconnect at pump to make sure no rodent enters). I'm less sure with the main drain line. From the present answer, I need to blow the air and immadiately close the valve. There is no window for pouring the anti-freeze.
    – Sam7919
    Oct 28, 2021 at 12:38
  • To others reading, and for reference, there are apparently two kinds of anti-freeze. The one used in water-cooling systems in cars (methanol?) is unsafe to ingest, even in trace amounts. The other, plumping specific, anti-freeze (propylene glycol?) is safe to accidentally ingest in trace amounts (in a cottage after re-opening the pipes in spring, or in a swimming pool. The latter application is trickier. A farm house plumbing can be rinsed for a few minutes. No such rinsing is possible with a swimming pool.
    – Sam7919
    Oct 28, 2021 at 12:39
  • @DrMoishePippik Okay... no need to chat in comments. The point is important enough that it merits a sequel. I asked.
    – Sam7919
    Oct 28, 2021 at 12:56
  • I'll shortly delete these four comments.
    – Sam7919
    Oct 28, 2021 at 12:56

1 Answer 1

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If you blow air through the drain line and then seal the "pump end" the air will be trapped in the vertical part of the drain line and keep the water confined to the horizontal section of the drain pipe.

What I do is hook up my air compressor to the drain lines, blow the water out the skimmer line, and then seal that up from the skimmer end of the drain.

Next, I open up the floor drain line at the pump end, and direct the air through the "floor drains" until the water is mostly cleared from those. At that point, I cut off the valve from the pump end to the floor drains, to seal the air in the drain lines. They'll likely fill back up a little bit, but once the water traps and compresses the air in the vertical section of the drain line, no more water will get in.

Think of putting your finger over the end of a straw and then pushing it down into a glass of water... As long as your finger is sealing up the top end of the straw, the water can't get in.

The pipes under the deep end of your pool should be plenty deep enough to prevent the residual water in them from freezing.

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  • I see. Two questions though. The valve to the main drain line is water-tight, but I didn't know that it was expected to be air-tight too. For what you suggest to work, there cannot be microscopic air particles passing. (2) Adding anti-freeze is a good safety measure. It's not really necessary for the skimmer and return lines; they're completely empty, sealed, and there is no way for the water to return. It's the main drain that's again the issue. How might I pour anti-freeze if I need to blow and then immediately close the valve?
    – Sam7919
    Oct 28, 2021 at 3:04
  • The straw analogy is helpful, but (digressing to hydraulics) I sense that there must be some maximum diameter. If the "straw" becomes a 1 meter pipe, would the water never go up? Perhaps it just boils down to the air-tightness of the valve. The contained air would be slightly more pressurized, but then it would stabilize.
    – Sam7919
    Oct 28, 2021 at 3:06
  • 1
    Yes. The valve needs to be air-tight. That's the "magic" here. The size of the pipe is irrelevant. Air is always going to "want to go up" and water is always going to "want to go down"...so to speak. So, as long as you can prevent the air from escaping out the top of the vertical pipe (via the valve) there's no way the water can push the air out (or, if you will, no way the air will allow the water to fill the vertical pipe. The antifreeze is just extra precaution in case of a leak in the skimmer/return lines. Peace of mind is worth $20, etc.
    – gnicko
    Oct 28, 2021 at 12:59
  • 1
    Yes. That's it exactly.
    – gnicko
    Oct 28, 2021 at 13:12
  • 1
    I pour it in straight from the bottle. It dilutes with any residual water in the lines.
    – gnicko
    Oct 28, 2021 at 13:36

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