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Thread: Is blowing out main drain in an inground pool really necessary? Closing pool.

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    Join Date
    Jul 2015
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    Sault ste Marie, Ontario, Canada
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    Is blowing out main drain in an inground pool really necessary? Closing pool.

    I am trying to figure out if blowing out the water in our main drain pipe in our inground pool is really necessary. Our pool is 10 feet deep in the deep end and water would never freeze at that depth even with our harsh winters here in Northern Ontario (Canada).

    Our neighbour has had a pool for more than 30 years. It is an inground vinyl pool with a 8' deep end. He has NEVER done anything related to the main drain when winterizing/closing his pool and has never had any issues. He does clear water from the return and skimmer lines (but does not use antifreeze). He helped us close our pool last fall and so we did nothing to the main drain (didn't occur to me that there was even a way to do that) and it was fine. When we opened the pool in the spring we had a very difficult time getting water back into the pipes (including the main drain) and priming the pump. This suggests to me that there was a lot of air in the main drain pipe even without taking any action to clear it.

    Also, my thought was that if the water does not freeze at that depth, and the drain at the bottom of the pool is open (which it always is), if water did start to freeze in the main drain pipes, wouldn't it push water back into the pool. In other words if some ice formed, it would not form down to a ten foot depth and therefore any pressure caused by expanding ice would just push the water that was not frozen back into the pool. If you leave the valve for the main drain (near the pump) open, that would give additional room for freezing water to go to.

    There may be an obvious flaw in my logic that I am missing. Would appreciate hearing others perspectives on this.
    22' x 44' rectangular IG vinyl liner swimming pool with SWG, 10' depth in deep end, 44,000 gallons. Pump: Pentair Superflow 1.5 HP High Performance Pump. Filter: Pentair Sand Dollar Filter. Location: Ontario, Canada

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    Patrick_B's Avatar
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    Re: Is blowing out main drain in an inground pool really necessary? Closing pool.

    The water level in your maindrain line should be no higher than the water level in the pool after it's drained below the skimmers, to whatever that point is. I can't think of any way you could blow out and isolate the water in that piping, so there shouldn't be any issue with a properly purged/winterized pool system.

    Priming a pump usually requires a little effort on a "new" startup so I dont think you experienced anything unusual. Once the pool is full, you have to help the pump by sometimes repeated priming the lines and pump with the garden hose.
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    Join Date
    Jul 2015
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    Sault ste Marie, Ontario, Canada
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    Re: Is blowing out main drain in an inground pool really necessary? Closing pool.

    Great. That makes me feel better about it. We will proceed as we did last year - blow out the skimmer and return lines only (after lowering water level to below the returns and skimmer) and do nothing with the main drain.
    22' x 44' rectangular IG vinyl liner swimming pool with SWG, 10' depth in deep end, 44,000 gallons. Pump: Pentair Superflow 1.5 HP High Performance Pump. Filter: Pentair Sand Dollar Filter. Location: Ontario, Canada

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    Catanzaro's Avatar
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    Re: Is blowing out main drain in an inground pool really necessary? Closing pool.

    When we opened the pool in the spring we had a very difficult time getting water back into the pipes (including the main drain) and priming the pump. This suggests to me that there was a lot of air in the main drain pipe even without taking any action to clear it.
    I have main drains, 2 skimmers (can close one at a time, including the main drain) and 2 return lines (4 return jets on each side). When I open my pool, I close off one return jet, the main drain and one skimmer. I remove the Gizmo in the skimmer and fill water up with a hose though the skimmer plumbing only for a few minutes. Then turn everything on and prime the system. Within a minute, the skimmer is moving along perfectly and within another minute all the air is out. At this point, then I remove the Gizmo from the other skimmer and slowly open the Jandy valve to prime that side (If I start losing pressure, I quickly close and slowly open again). Once the skimmers are primed, then slowly I move on to the main drain.

    This works quite well for me as I isolate each section. Please make sure that there is water at least 1/3rd of the way in the skimmer. If not, you can work on the main drain first and then after the pool fills up, open the skimmers one at a time. When everything was open the first year, it took a long time.

    - - - Updated - - -

    Regarding your main drain, I personally feel better blowing out the main drain and crating the "air lock" and in addition I add a rubber plug in the pump basket.
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