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Thread: How do main drains work?

  1. #1
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    Default How do main drains work?

    I know you can tell I'm a newbie by this question - how do main drains work?
    I know (because I can see them) that we have two main drains in the bottom of our pool. But they don't seem to 'do' anything. Shouldn't the pump be pulling water from them? I see small bits of debris on the bottom of the pool, but they don't seem to be moving towards the drains, which seems to indicate to me that there is no movement through them back to the filter. Is there some setting - on the filter valve, maybe - that I need to set to get suction from them? I am getting plenty of suction through my new skimmers and I can feel powerful jets of water returning to the pool through the new 'eyeballs' I put on the returns. But I can't tell if anything is, or should be happening down there on the bottom.

    35,000 gal - 3 ft to 8.5 ft
    Sylvan pool (mid 70's maybe)
    shotcrete with new plaster/marble finish
    1.5 hp Hayward Super pump
    Nautilus NS 36 sq ft DE filter
    Polaris 380 with 1hp PB4 booster pump
    two skimmers/returns
    two main drains
    one Polaris port

  2. #2
    CarlD's Avatar
    CarlD is offline Super Moderator Vortex Adjuster CarlD 4 stars CarlD 4 stars CarlD 4 stars CarlD 4 stars CarlD 4 stars CarlD 4 stars
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    Default Re: How do main drains work?

    It should be very straight forward: The two drains each go into a pipe. They meet at a Tee or a Y into a single pipe. This joins with the pipes from the skimmer(s) and the low drain (if there is one) and where they meet, near the pump, they should each have a valve to allow you to turn them on, off, or valve them down.

    If the valves are ball valves, then the when the handle is in-line with the pipe it's open, when it's perpendicular, it's closed.

    The reason there are two drains is safety. If someone dives down and clothing or hair get in the drain, there won't be an unbreakable suction--it will divert to the other drain because they are connected. Simple and effective.

    If you don't have a valve for the bottom drains, then it is most likely the pipe is clogged or collapsed. The first requires a plumber's snake, the latter, repair.
    Carl

    16'x40' rectangle 19K gal AG vinyl pool; ; Hayward T210 sand filter and 2 Speed Superpump 1hp pump; Solar heated. Test kit: PS 233; PF:6.3

  3. #3
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    394

    Default Re: How do main drains work?

    There could be another reason for the observation that the main drain is not doing anything. It could be a function of friction losses in the piping. If there are two skimmers in addition to the main drain the pump is going to pull from the point of least resistance, so all the water may be coming from the skimmers. If you partially close the valves between the skimmers and the pump it will force the pump to draw more water from the main drain. As a test you might want to close off the skimmers totally, when you do that the pump should pull exclusively from the main drain.

  4. #4
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    Default Re: How do main drains work?

    Whoa, you people are great. I do have two valves, I just didn't know what I was looking at. When my plumber connected up our new Hayward Super Pump yesterday he replaced most of the piping and put in two new red handled ball valves. They are in line with the pipes which means that they are both open. How do I tell which is which (skimmer vs main drain)? Or doesn't it matter? I'm thinking I can just shut one off and go and look at the skimmers - if they are still pulling water - I'll know I've turned off the valve to the main drain and vice versa. I guess my concern is if I guess right and turn off the skimmers first and the pump is only pulling from the main drains and the drains are clogged or collapsed, that I will be putting great strain on the pump, or somehow make things worse.

    If it makes any difference to the answer, I'll tell you a little bit about the pool. We 'inherited' it with our house, and it was in poor shape. When we closed it up last year we knew that the coping was loose, the tiles were falling off and the skimmers were leaking badly. But as far as we could tell, the only leaking we had was at the skimmers. We would fill the pool halfway up the skimmer throat and the water level (rather quickly, like in a day or two) would drop to the bottom of the skimmers and stop. It didn't drop any further even if we left it for several days. So we felt it was worth renovating the pool since we were guessing that we had no major structural cracks or leaks around the drains at the bottom. We have since had it restored and had 35,000 gal of water trucked in two days ago. I have been watching the water level and there is minimal change (less than 1/4") and since it has been sunny and hot I am guessing that is due to evaporation.

    So I don't think there are any leaks down there, but if it is clogged and I put all the pressure from the pump on them, what will happen? Or am I just being a worry wort (entirely possible)?

  5. #5
    Join Date
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    Default Re: How do main drains work?

    Just close the valve about half-way and see what happens. Then slowly close it more until you can determine which is which.

  6. #6
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    Default Re: How do main drains work?

    Yup - that did it! I now know which is which. It probably seems silly, but I was afraid to touch anything. Thanks so much. Now on to posting my chemical questions....

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