Main Drain

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nursery42

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


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mas985
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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, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
42

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?
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mas985
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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, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
czechmate
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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!
free info

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.
czechmate
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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:
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mas985
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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, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
Free Info

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.
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Main Drain

Postby Guest » Sun 18 Aug, 2013 18:33

If you have a valve that opens & closes main drain then open it so it functions.
czechmate
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Main Drain

Postby czechmate » Tue 20 Aug, 2013 19:03

mas985 wrote: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.

But you are so predictable!
I wrote this piece specifically for you. And you jumped right on it. :wink:
The suction in any pool return branch is related to distance from the pump and of course the depth in the case of the main drain. My main drain is half the distance of the skimmer I use.
(The second skimmer I do not use, because filtering clean water that is spitted by inlet to the surface right next to it, I consider a waste of electricity as well as unnecessary prolonging the cycle).
Also, you completely forgot about the selector valve.
I built my first pool in 1974, so please do not patronize me with your suggestions to "watch action of the main drain". I have installed the new safety cover 2 years ago, with the new plaster and it sucks in parts of oleander leaves and the flower stems as well!
Now to the point:
With the selector valve aimed on main drain only you can actually clean fine sand dust (settled in your pool after the sand storm), just gently using the wide brush toward the main cover. You see I have done it and not just once!
It may take an hour or two, but it would take a regular pump 8-10 hours, with the pool sweep activation 10-16 hours and in the same pool without a main drain at least 3 days!!
I sincerely admire your willingness to make a spectacle of yourself here in such a clear cut argument.
Maybe you have your original opinion about the "global warming" supported by a video as well.
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mas985
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Main Drain

Postby mas985 » Wed 21 Aug, 2013 10:23

czechmate wrote:But you are so predictable!
I wrote this piece specifically for you. And you jumped right on it.
As are you. Remember, you directly responded to my post first after which I responded back. But isn't that the way a forum works?

Anyway, I am glad that your main drain has worked out for you but that hasn't been the case for many people. Personally, I would rather redirect flow to the skimmer to pull debris off the surface before it hits the pool bottom. But when the dirt hits the bottom of my pool, the main drain removes only a few inches around the drain which I don't believe is very useful. It doesn't improve much even when I redirect all of the flow through the main drain. This is not uncommon among pool owners.

With the selector valve aimed on main drain only you can actually clean fine sand dust (settled in your pool after the sand storm), just gently using the wide brush toward the main cover. You see I have done it and not just once!
Of course, if you use a brush it will be more effective, that was not my point. My statements where based on the main drain working in isolation without help from the pool owner. But regarding this method, there are many people on various forums who have attempted this with a clean-up at opening only to have the main drain or suction line clog with debris. So you really have to be careful with this method to only push debris that is small enough to pass easily through the covers and lines.

It may take an hour or two, but it would take a regular pump 8-10 hours, with the pool sweep activation 10-16 hours and in the same pool without a main drain at least 3 days!!
My cleaner keeps the floor spotless with only 4 hours of run time per day so I don't think this is the case for all cleaners. My cleaner is by far more effective than the main drain alone and if I don't use the cleaner for a week, the pool floor is littered with debris so the main drain really doesn't do much by itself.


But neither my opinion nor yours changes the APSP position that main drains do not help very much and in case you did not bother to read their report here are a few quotes:

The single most commonly held, yet mistaken, belief regarding drains is that they “vacuum or
suck in dirt” and somehow “clean the floor.” They do not. Remember the candle experiment?
Pool drains are no different. If a leaf is placed two inches from the opening of a flowing two-inch
pipe, it will never enter the pipe unless it is pushed in – either by a brush or by a stream of water
specifically directing it to the pipe opening. In fact, a pool left un-vacuumed for several weeks is
complete proof that the drain alone cannot clean the settled debris. Some debris does enter the
drain, by chance, but the pool will still require routine vacuuming in order to remove settled
debris. Since vacuuming, manually or via automatic cleaner, is a routine part of pool
maintenance, why rely on a drain that might remove “some” of the settled debris, but also leaves
bathers exposed to a proven lethal hazard?


You may disagree with them and that is fine but everything that they mention has been my experience and the experience of many others. They have also backed up their position with some detailed analysis and experiments so I would not dismiss it out of hand so easily.
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, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
czechmate
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Joined: Sat 16 May, 2009 09:20
My Pool: 16 x 32 gunite21000 gal., Diamond Brite Blue, Swimquip XL pump, DE36
Location: Texas

Main Drain

Postby czechmate » Thu 22 Aug, 2013 09:50

"You can lead a camel to the water but you can't make it drink" :wink: .
Free info

Main Drain

Postby Free info » Fri 23 Aug, 2013 13:21

It may take an hour or two, but it would take a regular pump 8-10 hours, with the pool sweep activation 10-16 hours and in the same pool without a main drain at least 3 days!!
I sincerely admire your willingness to make a spectacle of yourself here in such a clear cut argument.
Maybe you have your original opinion about the "global warming" supported by a video as well.czechmate
Swimming Pool Superstar


Why not just vacuum the pool it takes 10 mins compared to 1-2 hours of brushing to the drain?
So this is the big deal that you can get your main drain to work as long as you brush debri/dust towards it?
When you have it set normally it doesnt do it so you do your thing that works for you, ok.
The new pools being built today do not even have this option anymore as they use skimmers with 2 holes.
Law of physics applied.

Main Drain

Postby Law of physics applied. » Sat 24 Aug, 2013 09:24

1. Location of clean water returns and skimmer intakes are almost at the same level at the top of the pool. Just cleaned water then has reenters the skimmer again heading for the filter. There is no physical reason for it to go elsewhere.

2.Anytime the pool level rises above the most optimum suction force of the skimmer, which varies from 1-2 inches, the suction is getting diminished to the point that at the rise of 2 inches even small grass clippings will flow by the skimmer at the mere distance of 2inches and will not enter.

3. When 2 skimmers are deployed the situation worsens for two reasons. More, already cleaned water enters the filtering system and the suction at the main drain becomes too negligible to function as designed. At this point due to shared intake with other skimmer half the debris will flow by the skimmers without being sucked in.

4.During the winter months, in southern climates, where hard freeze comes often as a surprise and where a larger percentage of the pools exists, main drain protects the equipment from accidental freeze.
Warmer water stored at the bottom of the pool is used to prevent pool from freezing, while running on freeze-stat. While surface water is being super chilled by winds, bottom water warmed by the deep concrete and warmer surrounding soil, is effectively mixed to supply filter and water heater preventing freeze of piping and equipment.

5. Conundrum that remains to be explained here is this: If the main drain is so ineffective as to pull anything as small as a leaf laying next to it, why is it deemed by the "pool experts" to present such a clear danger to a swimmer in motion?

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