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Thread: Why would a main drain have a plug in it?

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    Why would a main drain have a plug in it?

    Some background: I bought a house a couple months ago with a pool, which made me a first time pool owner. I've been lurking the site for several months now, and started off with the BBB method (in fact, it's all I've ever known ). Pool chemistry is exactly where I want it.

    Anyway, now I'm starting to get more familiar with the plumbing; finding out where pipes go/what valves do as nothing is labeled. I've determined that since I've owned the pool, the main drain has been shut-off at the equipment pad. So inflow is split 50/50 between the skimmer and the vacuum line, which has had a Hayward Navigator (which I had to fix) attached to it full-time. I verified the main drain is off by diving down to it and covering it up with my hands... nothing.

    So, at the equipment pad, I turned the valve to an even split between skimmer and main drain, and turned the vacuum line off. When I did this, the pump lost prime for a few seconds, and I saw a little air in the leaf canister, but it seemed recover and get prime shortly after.

    Wanting to check my work, I dove down to the main drain again, and covered it with my hands. I felt very light suction, but not really what I was expecting. Looking through the drain cover, I saw what looks like a drain plug threaded into the main pipe opening. I didn't have time to get the scuba gear to remove the cover and get a good look at it yet, but obviously there was some water getting through it.

    My question is, should this plug be there? If so, what is the purpose? If not, should I remove it?

    Also one final note, I noticed what looked like the same plug rolling around in the skimmer (so not inserted) with two holes drilled in it. Any ideas on what was going on here? Advice on what I should do?

    Thanks in advance for the help!
    Greg
    18000 Gallon In Ground Pool
    Gunite, Cartridge Filter, 1.5HP Single Speed Pump.

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    Mod Squad JohnT's Avatar
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    Re: Why would a main drain have a plug in it?

    First and most importantly, be careful messing with the drain while the equipment is on. If you don't have a modern cover, you can be trapped by the suction.

    Welcome to TFP!

    There isn't much noticable suction on a properly protected main drain. Modern covers are designed to distribute the flow over a large area to reduce entrapment risks. It wouldn't be surprising that you saw some loss of prime when first opening the drain valve.

    The plug in the drain may be the hydrostatic relief valve, which allows water to flow into the pool from the ground when the pool is empty for maintenance. Without such a valve, the pool could float on the groundwater and be seriously damaged.

    The plug in the skimmer may be someone's efforts at balancing water flow to make a cleaner work. Hard to say.
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    JasonLion's Avatar
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    Re: Why would a main drain have a plug in it?

    Welcome to TFP!

    There are a number of possibilities. Perhaps the drain plumbing leaks, so they plugged the drain to stop the leak. It could also be left over from winterizing. Or it might be plugging a spare hole in the fitting, and water is actually drawn into a pipe connected someone else inside the drain. It could also be for no reason at all (more common than you might think).
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    Re: Why would a main drain have a plug in it?

    Thanks guys for the quick response and ideas!

    It sounds like I have a bit more investigation to do (with the pump off). John, after looking up what a hydrostatic relief valves look like, you may be on to something there. Afterall, I am in Florida next to a small lake.

    I know for sure water is getting through, however, I turned off the main drain before I left it today because I wasn't sure what the "obstruction" was and didn't want to damage anything. Knowing that a proper main drain shouldn't have a lot of suction, do you think it's OK to turn on the main drain (with the skimmer) at this point?

    Really appreciate the help!
    18000 Gallon In Ground Pool
    Gunite, Cartridge Filter, 1.5HP Single Speed Pump.

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    Re: Why would a main drain have a plug in it?

    Every bottom drain that I have seen had 2 holes, one connected and one plugged.
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    Re: Why would a main drain have a plug in it?

    The second hole may be on the side and you can't see it. Mine are. I have the bottom holes plugged in the drain, but I installed a perforated tube under the drain when it was built to help ground water get in if i need to drain the pool. My side ports are plumbed to a T and then to the pump. Hard to see with teh cover still on probably.
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    Re: Why would a main drain have a plug in it?

    In the future you could test water flow in the main drain with a colored liquid, like say water with food coloring in a water bottle....
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    Re: Why would a main drain have a plug in it?

    Quote Originally Posted by jtech1
    The second hole may be on the side and you can't see it. Mine are. I have the bottom holes plugged in the drain, but I installed a perforated tube under the drain when it was built to help ground water get in if i need to drain the pool. My side ports are plumbed to a T and then to the pump. Hard to see with teh cover still on probably.
    I second this. I plugged the hole in the bottom of mine, which is what you see when looking into the drain. The open hole is on the side towards the other drain, connected via a T
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    Re: Why would a main drain have a plug in it?

    Thanks, everyone, for the tips! I'm sure that's what I'll find when I'm able to get down there and take a good look.
    18000 Gallon In Ground Pool
    Gunite, Cartridge Filter, 1.5HP Single Speed Pump.

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