My feedback on main drains in an above ground pool...

[mas985]

Many PBs are building pools now without main drains but not because they don't have a clue or because they are trying to save costs but because studies have shown that they really don't contribute much to the sanitation and cleanliness of a pool. Here is one such study done by the APSP: http://apsp.org/portals/0/PDFs/Case%...n%20Drains.pdf

The video at the end shows how ineffective a main drain really is even at very high flow rates. I have confirmed this myself with my own pool since I have a MD and have operated with it on and with it off and I saw absolutely no difference in water quality or bottom cleanliness. I once dumped a bucket of small debris around the MD and even with the pump running over 8 hours with ONLY the MD running, only the first couple of inches around the MD were free of debris. Because of the VGBA, MDs are now designed to have very low water velocity to reduce the risk of entrapment which makes these MDs even worse at drawing in debris. While you may have seen debris setting around the MDs, that could be due to the slope in the floor rather than the MD itself.

But there is another significant reason not use MDs. Skimmers require a certain flow rate in order to effectively draw in debris. Some manufactures recommend at least 15 GPM. With the addition of an MD, it steals water from the skimmer and there may not be enough suction on low speed of a two speed pump so running on high speed may be necessary which is about 4x the energy use. Same is true with a VS pump. More suction ports require higher RPM settings and higher energy costs.

Sorry but I really think you are fooling yourself into thinking that the MD is actually doing anything at all. The evidence simply doesn't support it.

[SunnyOptimism]

Wow! The YouTube video in the last citation at the end of that APSP study is pretty damning. With only 11" of water above the drain, to see the blue dye just float past it is pretty impressive. I think there's probably some effect on the residence time of the dye over the drain by how far you position the main drain from the skimmer but the drain definitely seems to provide very little suction.

The only good I ever see my MD possibly doing would be to act as a secondary source of water to the pump should the skimmer become clogged. However, I have yet to find a dead raccoon plugging up my skimmer basket

[mas985]

A wall port is more effective for equalization purposes because it is less likely to get clogged with leaves as would a MD.

[Anonymous [GDPR] European in the UK]

mas985, I was going to lead Dennis slowly to the same direction but you just slam dunked the discussion! ;-)

If you take a kitchen sink and do lots of washing up the dirt is everywhere. pull out the plug and let the water run away, it will even form a vortex at high speed but only when the last 1/4" of water drains away to 0 does dirt on the base of the sink flow down the drain otherwise it stays put and you have to wash it away afterwards. same in a pool. you have to push the debris down the holes so the rest of the time it's circulating pretty much the clean stuff.

If the circulation via the M/D improved Dennis then your skimmer can't handle the flow, a second or third skimmer would take the strain off the pump and dirt enters from the top so taking it out before it can sink to the bottom is the best answer.

Leaves etc do stick to the walls of the pool, this is due to the boundary effect, water at the edge maybe stationary or even going in the opposite direct to the flow created by the return jets. Increase this flow with your big ol pumps and the boundary layer gets bigger so more debris becomes trapped at the perimeter.

This can again be seen in a dirty bowl of washing up water, go mad winding it around in a circle and you may see the perimeter dirt stationary or going the opposite way. Moving the water gently produces a smaller boundary layer.

It's true skimmers don't work very well on low flow, but that's again because the water needs to push the debris into the skimmer rather than trying to pull the water in. It would help a lot if there was a really good skimmer on the market but pool equipment has never needed to be efficient because if you bolt on a big enough pump, no matter how bad the plumbing water will arrive at the other end so bad design is the normal with pool equipment. I am working on a better design of skimmer as a project, it takes time and I could used some CFD software to shorten down my experimental stage.

The other point I would like to make is what is the point of the turnover rate? or what is the point of better circulation?

Chlorine kills most harmful things in seconds, they are dead and that happens if the chlorine is available and well mixed which doesn't mean high turnover or circulation. What chlorine doesn't kill needs to be filtered out but regular high rate sand filters and element filters and even DE if the rate is too high doesn't filter these out because many are too small so they just get recirculated around and around, just a waste of electricity because you are not achieving anything. Chlorine has done it's job and the rest of the dirt and skin cells etc gets broken up in the rough treatment moving through the filters that are too course to remove the other stuff.

Slower filtration is better filtration, high rate is just straining the big stuff and using lots more expensive electricity to do it.

If a Chef wants a clear consomme they let it flow through the filter slowly to produce an almost clear stock, if they applied 1 bar pressure to push it through it would come out dirty.

In regards to debris, only some debris will float. Only what floats will get sucked into the skimmer. Anything else that falls to the bottom will stay there,

Most debris floats at least for a while, the surface tension on the water supports a fair amount of dust, leaves, pollen etc until it gets water logged and then sinks, that is the optimum time to remove it but you can only do this if the system is running. That means if you switch off your pump at night and there is a storm/strong winds or even mild wind, by morning you'll be out there with the cleaner or robot.

We don't run our pools 24/7 because of cost. I have gone to the opposite end of the rainbow with testing this. My pool runs 24/7 the water is the cleanest it has ever been. The little dust cloud that gets pushed up when manual vacuuming, the one where the pool is clearer before you start cleaning? That dust cloud is no more because the water is moving constantly so more goes into the skimmer (downside is you have to empty the skimmer more often)
Now many people say my water is crystal clear but eyesight is bad, plenty more to come out of the water to make it really clear when tested with a turbidity meter. Now the electricity I use on my pool is 30 watts when no one is swimming, that is still enough to have the water moving and skimmer working and when swimmers are in the pool I push it to 69 watts. I can afford to run my pool 24/7 on that low level of electricity and moving water is better than stationary. The flow rate is slower but that means the filter catches more although I do use better filter media than sand (4 micron filtration but with flocculation as well to remove sub micron particles) so more of what the filter catches is not recirculated and is back washed away leaving cleaner water and using less chlorine although that is cheap anyway in the big scheme of things.

I sold my pool robot 3 seasons ago, it's not needed any more.

The pools I work on with main drains get re plumbed and I put a 3 port on them to use them as inlets unless draining or cleaning is taking place. Putting the heated water back through the main drains does wonderful things for warming up and de stratifying the water.

[DennisP]

Honestly, most of this is about actual experience and what you have to deal with. There is a huge push away from main drains, primarily from the safety aspect of it. It is just like people disliking guns. Most say that because they fear it. Main drains are due all the bad press with people getting hurt over the ages, but with dual main drains that concern is all but eliminated in a residential pool.

I understand the science of boundaries, skin effect, wall adhesion, etc, etc... I keep my pool covered at all times with a solar cover when not in use. A skimmer in that aspect doesn't do much and is prone to clogging by the cover and anything else. If anything with the pump on low I notice mostly small stuff accumulating in there. The main drains are cut in full along with the skimmer.

If anyone would care to re-read my first post on this, I gave all the pertinent information as to what I am experiencing and why I am doing it and what the advantages are to me, personally. To argue any of that is just stupid. I never said it only picks up this and if it keeps the floor clean or not. I know it doesn't. It pulls in anything that is suspended in the water it pulls through it. As I stated I can usually cut the main drains in full, stir the water up around them and have them pull in anything small enough to work through the grills. Everything else gets vacuumed (which isn't much once the spring cleanup and fall leaves are done).

In regards to using the main drains as a feed for heated pool water, it also works in reverse, pulling cool water from the bottom center of the pool and allowing circulating warm water form the returns to work towards the center. Simple water flow dynamics dictate that. And with a strong circular current in the pool (using dual returns and pointing them at a 45 degree angle down and to the right creates quite a bit of current in the pool), which is always present since you heat with the pump on high, you have the best case of allowing for the cool water to be sucked in the main drains.

Again, do it or not, that is all personal choice. I understand pool professionals not wanting to put them in. I understand why they tell their clientele they aren't worth it. But for people like myself that operate a pool the way I do, it isn't even a matter of "should I or shouldn't I?", it is a matter of it is the best solution to deal with draining, heating, cleaning and overall circulation and flow. It is just amazing how people try to say it doesn't do anything when it is obviously pulling water from the lowest point in the pool in the center. Plus it is hilarious when most people that argue you shouldn't put one in are the ones that have them in their own pools already. I could care less if you don't find a value or a need and if you have yours cut off all the time, it makes zero difference to the benefits I am personally experiencing in the operation of my own pool.

If anyone cares to have the same advantages, they are welcome to install them as well. To argue the majority of the above posts is beyond futile and ignorant, since none of what was said applies to anything I stated. I appreciate the argument on the academic level, but in an applied sense, in my pool, for my purposes, none of it matters.

Oh, and in fall when the leaves are flying, it makes a huge difference to still be able to filter the water via the mains drains if the skimmer clogs with leaves... I had it happen last fall when the kids were swimming when it was 50 degrees outside... The pump was on high, the pool heater was running heating the pool and the skimmer was totally filled with leaves... If the main drains would not have been able to supply the water properly the heater would have shut off due to either low flow or too high of heater temps and the kids wouldn't have been able to enjoy it as long as they did. Without main drains that situation would have been ruined since the pool would have started to get cold quickly (and a 3-4 degree drop with the kids playing is a big deal to them being able to enjoy themselves).

[J_Palmer_Cass]

Oh, and in fall when the leaves are flying, it makes a huge difference to still be able to filter the water via the mains drains if the skimmer clogs with leaves... I had it happen last fall when the kids were swimming when it was 50 degrees outside... The pump was on high, the pool heater was running heating the pool and the skimmer was totally filled with leaves... If the main drains would not have been able to supply the water properly the heater would have shut off due to either low flow or too high of heater temps and the kids wouldn't have been able to enjoy it as long as they did. Without main drains that situation would have been ruined since the pool would have started to get cold quickly (and a 3-4 degree drop with the kids playing is a big deal to them being able to enjoy themselves).

If you have a skimmer and a "main drain" located on the pool sidewall (AKA two inputs), a clogged skimmer will have no effect on circulation and flow rate. Over thirty years experience with that setup.

As far as mixing heated return water with pool water, all you need is an eyeball fitting connected to the heated water return. Aim it down a touch and the heated water mixes with the lower level cooler pool water.

I prefer that the "main drain" be located on the pool sidewall rather than on the pool bottom.