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To keep water out of a swimming pool's main drain line over winter, you blow the water until you get a nice big fountain (which requires that you use an air pump powerful enough to overcome the water column that you have) and immediately seal the valve. Similar to how no water will enter a straw that you insert in a glass after pinching, this will keep the water out of the main drain line.

To be extra-safe, it's best to not just blow the water out of the pipes, but to also pour anti-freeze.

Talking over a figure is best. (I've already used the storage at imgur.com, and so this is hardly wasteful.)

pool main drain winterization

How do I use anti-freeze on a main drain line? It is simple enough with the skimmer line and the return lines (blow water out, seal the skimmer and the exit lines, pour anti-freeze, reconnect at pump to make sure no rodent enters).

It's far less clear with the main drain line. I understand that I need to blow the air out of the main drain line and then immediately close the valve. There is no window for pouring the anti-freeze.

Note 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. The anti-freeze just gets diluted. On the plus side, it does degrade and vanishes with time. I'm adding this important point here for the reference of others, but do comment if you find I got any detail wrong.

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  • BTW- images are always good, and imgur is SE's hosting company, so no need to worry about storage space
    – FreeMan
    Commented Oct 28, 2021 at 14:03
  • Methanol has been used for cars, but in many places ethylene glycol is common. This is also toxic, so you're right to think of propylene glycol (which is even used in foods). I've pointed it out because the similarity in the chemical names belies the vast difference in toxicity
    – Chris H
    Commented Oct 29, 2021 at 12:37
  • @ChrisH 1/2 Ironically, it would be immensely easier if manufacturers labeled their wares with the chemical name. Then the only issue would be to distinguish ethylene from propylene. Thanks for the hint; I wasn't aware—there's of course also ethanol, and there it's again tricky because there is trade-grade ethanol —as well as cosmetics-grade ethanol (the former can contain dangerously high levels of methanol, apparently). What they actually do is write, in gigantic letters, "Anti-Freeze".
    – Sam7919
    Commented Oct 29, 2021 at 16:37
  • @ChrisH 2/2 Selling in the plumbing section doesn't help. One has to read the labels, but they are not required to list the "ingredients" since it's not meant for consumption in the first place! And presence in the plumbing section leaves it ambiguous whether it's for sewage pipes or fresh water pipes.
    – Sam7919
    Commented Oct 29, 2021 at 16:37
  • @Sam absolutely. It's often worth googling the exact product name and "msds" or "safety data sheet" though even some of that can be a trade secret and the same name can be used for different compositions in different countries.
    – Chris H
    Commented Oct 29, 2021 at 20:14

1 Answer 1

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You can't.

There's no way, that I know of, to pour in anti-freeze and close the valve before the water gets back in the drain line. You don't really need anti-freeze in this case because of the depth of the pipes. They will be (roughly as deep as the deep end of the pool) at least 3 feet or more underground.

As far as I know, you'll want to use propylene glycol for your pool. Not automotive antifreeze. Propylene glycol is what they sell as "swimming pool antifreeze" and the advantage is, as I understand it, that it will actually break down and dissipate on it's own when you open the pool in the spring without interfering with the other pool chemistry.

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