Main Drain

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Main Drain

Postby nursery42 » Sat 27 Jul, 2013 08:58

hello everyone, would like a bit of direction if possible. Just had a villa built in Turkey with a 12mtr outside pool. First visit went well but have a question regarding the main drain in pool. The drain itself is located at the deep end of the pool but hasn't got any suction, although there is plenty of suction from the 2 skimmer drains. I've asked the pool maint man and he says the main drain is only used for draining the pool, which to me seems wrong. There are 2 pipes (with a valve in each) which are in a tee on the inlet of the pump (suction side i guess) i'm guessing but would think one pipe comes from the skimmer and one comes from the main drain. can anybody answer whether both these valves should be open (partly / fully). many thanks in advance for any help you can give me
nursery42
 

Main Drain

Postby mas985 » Sat 27 Jul, 2013 11:40

Main drains are not suppose to have a lot of suction for safety reasons. The covers are designed for distributed flow so that risk of entrapment is reduced. Plus if you have dual main drains, each handles half the flow so in most cases, the flow into the drain is barely perceivable. They are really only useful for circulation and even at that, only provide minimal benefit.

But to maximize flow rate and circulation, open all the suction valves 100%.
Mark
Hydraulics 101; Pump and Pool Spreadsheets; Pump Ed 101
18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, 450 sq-ft EPDM Solar Panel, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
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Main Drain

Postby 42 » Sat 27 Jul, 2013 12:00

hello mas985, we only have 1 main drain and there is no suction at all i.e. valve into tee for main drain to pump inlet is closed. would you expect the main drain isol valve to be at least part open ? or are you saying that it wouldnt make any difference to water / dirt collecting on bottom of the pool whether the valve was open or closed?
42
 

Main Drain

Postby mas985 » Sun 28 Jul, 2013 10:04

I would have at least a little flow through the main drain plumbing so it remains chlorinated but again, the main drain will not remove dirt from the bottom of the pool and it's only function is to help with the circulation of water in the deep end. If you don't have any deep returns or a return that you can point to the deep end, then I would leave the main drain line open or at least partially open.
Mark
Hydraulics 101; Pump and Pool Spreadsheets; Pump Ed 101
18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, 450 sq-ft EPDM Solar Panel, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
mas985
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Main Drain

Postby czechmate » Fri 16 Aug, 2013 14:33

mas985 wrote:Main drains are not suppose to have a lot of suction for safety reasons. They are really only useful for circulation and even at that, only provide minimal benefit.

Now, anyone that has a regular flushing toilet knows better than that. Main drain is piped with the same O.D. pipe as skimmers and even though it has to overcome the riser of around 9 feet it is helped by the pressure of water above it as well as the pump.
Notion, that it may be there for draining the pool is also an absolute nonsense, because the regular pool pump will burn well before it gets half the job done. ( its very design is not a sump pump).
If main drain was really so useless as some here proclaim, than why is the WC designed with drain on the bottom and not on the side like the skimmers in the pool ?? :roll:
Is it really so hard to understand? All pool sweep, worth the money, starting with the Polaris ( best on the market for the last 50 years) are designed and built on the very principle that main drain is an essential part of a maintenance of pool, which is a removal of a organic as well as non-organic debris.
Also, that eventually ALL debris that enters the pool surface will sooner or later end up on the bottom of the pool!
czechmate
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Main Drain

Postby free info » Fri 16 Aug, 2013 23:29

If you have 2 skimmers you would have 2 pipes going to the pump depending on how they plumb everything.
Do both your skimmers have 2 holes? If yes get skimmer floats that sit on the bottom of the skimmer that connects the holes so the main drain or drains work. Know it all depends on how the builder set up your system but it sounds like this is what you have.
free info
 

Main Drain

Postby czechmate » Sat 17 Aug, 2013 11:20

free info wrote:If you have 2 skimmers you would have 2 pipes going to the pump depending on how they plumb everything.
Do both your skimmers have 2 holes? If yes get skimmer floats that sit on the bottom of the skimmer that connects the holes so the main drain or drains work. Know it all depends on how the builder set up your system but it sounds like this is what you have.


You could not be more confusing here.
Skimmers are routinely piped separately, to allow choice between either one and the main drain suction at the 3 way valve ahead the pump intake.
That is, unless your uncle Teddy piped it while drinking margarita. :wink:
czechmate
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My Pool: 16 x 32 gunite21000 gal., Diamond Brite Blue, Swimquip XL pump, DE36

Main Drain

Postby mas985 » Sun 18 Aug, 2013 17:32

czechmate wrote:
Now, anyone that has a regular flushing toilet knows better than that. Main drain is piped with the same O.D. pipe as skimmers and even though it has to overcome the riser of around 9 feet it is helped by the pressure of water above it as well as the pump.

Hmm. Last time I checked most pools aren't shaped like a toilet. But let's compare the two.

A pool bottom is much flatter than a toilet bottom which is more funnel shaped so "debris" is more inclined to move toward the toilet hole than the main drain. The toilet has a fairly large hole for "debris" to pass while a pool main drain has many very tiny holes which only fine dirt can pass through. A toilet uses gravity to flush while a pool use a pump to create water flow. A toilet flushing looks nothing like a main drain running (i.e. no swirling action). The physics are substantially different between the two and the toilet is going to be much more effective at removing debris than a main drain.

Also, I don't disagree that a main drain can have decent flow rate but it is usually a little less than a skimmer because the pipe lengths are longer. But even it was the same, the flow rate doesn't do much in the way of debris removal. If you don't believe me, dump a small amount of dirt directly around the main drain and see how much dirt it actually collects (without the help of a cleaner or vacuum). You will be very surprised at how little is removed.

This is an interesting read: http://www.poolinspections.com/manuals/ ... drains.pdf

And this is an interesting video: http://www.youtube.com/watch?v=0IOUGhuKkSM

Many PBs around the country are now designing pools without MDs without any deleterious effects.
Mark
Hydraulics 101; Pump and Pool Spreadsheets; Pump Ed 101
18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, 450 sq-ft EPDM Solar Panel, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
mas985
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Main Drain

Postby Free Info » Sun 18 Aug, 2013 18:25

Both skimmers run off the same pump and have their own main drain. These skimmers probably have 2 holes
the front one goes to main drain the back one to the pump. The only way to connect the 2 holes is with a skimmer float that sit on top of the 2 holes in the skimmer and connects the suction to the main drain.
Free Info
 

Main Drain

Postby Pool User » Sun 18 Aug, 2013 18:33

If you have a valve that opens & closes main drain then open it so it functions.
Pool User
 

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