Is main drain necessary?

[JimK]

There was some discussion in this thread about whether or not a main drain is necessary;

http://www.poolforum.com/pf2/showthread.php/25853-Anyone-heard-of-or-had-experience-with-flow-reversal

My pool has a main drain; I've never seen an IG pool without one so I just took it for granted we needed one when we had our pool built.

After reading that thread, I greatly reduced the flow the main drain to see what would happen (our setup has a valve that can be closed to shut off the MD). It does seem the skimmers are performing better, but it hasn't been very long and it could just be we just haven't had as much debris in the pool this past week.

So far I've not noticed any negative affects, though I do notice that when I reduce flow from the MD the filter pressure drops. Does this mean I'm not getting as much flow out of the returns?

For reference my pool has 2 skimmers, 1 MD, and 3 returns (4 if you count were the pressure cleaner hooks up).

I look forward to reading other's input on this topic. :)

[mas985]

You will probably see a lot of opinions on this but I think you know my position if you read the thread.

Without a main drain you will see better performance out of the skimmers. But the study I posted should be enough to convince you:

http://www.poolforum.com/pf2/showthread.php/25853-Anyone-heard-of-or-had-experience-with-flow-reversal?p=126169#post126169

[JimK]

You will probably see a lot of opinions on this but I think you know my position if you read the thread.

Without a main drain you will see better performance out of the skimmers. But the study I posted should be enough to convince you:

http://www.poolforum.com/pf2/showthread.php/25853-Anyone-heard-of-or-had-experience-with-flow-reversal?p=126169#post126169

Yes, I did read that (thank you for posting that :) ). Your posts are what have me pursuing how my pool behaves without the MD, or at least with flow from the MD greatly reduced.

Do you know if the pressure drop I'm seeing at the filter when restricting the MD flow means there's also less flow coming out of the returns?

I'm always learning something new here. :)

[mas985]

Turning off the main drain adds head loss to the suction (i.e two lines vs three) which in turn lowers flow rate but usually not by a lot. You should expect a slight drop in pressure because of this. However, the amount of total flow rate lost is less than the flow rate gain by the remaining skimmer so you come out well ahead from that perspective. I think overall, your pool will probably end up cleaner without the main drain, than with it.

But given your run time (12 hrs), you might want to read the article in my signature on pump run time. Most pools only need about 3-4 hours per day. You can save a lot of energy and cost by reducing pump run time.

[JimK]

Turning off the main drain adds head loss to the suction (i.e one line vs two) which in turn lowers flow rate but usually not by a lot. You should expect a slight drop in pressure because of this. However, the amount of total flow rate lost is less than the flow rate gain by the remaining skimmer so you come out well ahead from that perspective. I think overall, your pool will probably end up cleaner without the main drain, than with it.

But given your run time (12 hrs), you might want to read the article in my signature on pump run time. Most pools only need about 3-4 hours per day. You can save a lot of energy and cost by reducing pump run time.

Thanks. I'll take a look at that link in your sig.

With the SWCG I'm not sure if I could get away with only 3-4 hrs. Right now I have it set to 70% to maintain CL, though it is an older cell (in it's 7th season) which may explain the higher setting. When the cell was new I only needed to set it to about 40% if I recall correctly.

Which brings up a question. Is it better to run the SWCG at a lower setting (longer pump on time) giving the pool doses of CL throughout the day, or at 100% (shortest pump on time) dosing CL one time a day? (Lol, I've derailed my thread already!).

[mas985]

From the SWG perspective, 100% for 4 hours vs 50% for 8 hours is the same. From a FC perspective, it shouldn't matter either as long as the FC doesn't go below the minimum recommended FC level. So if you reduce pump run time and you know FC will dip some after the pump is off, then you just need to set the FC a little higher. However, with a SWG, you should have 60-80 ppm of CYA and so the FC should not drop that much when the pump is off.

I run my pump 4 hours per day and the SWG output is set to about 95% specifically because I am trying to reduce run time as much as I can.

[JimK]

I appreciate you sharing your experience with this. :)

I'm wondering how long my SWCG can stay off before CL dips below the minimum? I try not to let it dip below 5ppm with CYA maintained at 70-80ppm, and preferably maintain between 6-7ppm for a little wiggle room.

Am I correct that if right now I have to set the SWCG to 70% for 12 hours, then running at 100% I would have to run it 8.4 hrs (12*.7=8.4)? Am I figuring that right?

[mas985]

Yes, that is correct. As for the FC, just measure it one evening after the sun goes down and see what it is. But at 8 hours, that should still be enough to cover most of the daylight hours anyway.

[JimK]

Thanks. :)

I was looking up your SWCG and it looks like it uses the same cell, T-15 Turbo Cell, as my Aqua Rite (now Hayward, I believe).

[mas985]

Yes, mine uses the T-15 and currently on my 9th year. It produces about 1.45 lbs of chlorine per 24 hr period at 100%. So in 12 hrs at 70%, that would be about 1/2 lbs per day or about 3 ppm per day added to the pool which would be on the high side. So either your cell is indeed failing or you have something else in the pool that is consuming a lot of chlorine.

Have you done an overnight free chlorine loss test? You should not lose much chlorine overnight and if you do, then it could be something growing in the pool.

[JimK]

Wow, 9 years with the same cell?

Our pool was installed in March 2004 and included the Aqua Rite. In June 2008 the cell died with some warranty left on it. At that time the T-15 cell and controller came with a 5yr pro rated warranty. I've since learned that it likely died prematurely due to using it to shock the pool (long story.....I'll just say I don't use pool store recommendations anymore ;) ). The replacement cell is the one I have now (I learned my lesson and only use it to maintain CL level).

I did wonder if the reason for the higher setting I'm using now was something driving up demand, but my pool passes the overnight test and there's never any CC.

I didn't realize that a 3ppm loss a day was considered on the high side. I'll take a look through my records for times when the SWCG was off to see if I can tell how much I normally lose on a sunny day.

[JimK]

Looking back it seems like 3ppm loss on a sunny day is about normal for my pool. Perhaps this evening and tomorrow morning I'll repeat the overnight test to see if FC is still holding overnight.

On topic, I'm surprised no one else has weighed in on the main drain question.

[JimK]

Checked FC this morning....no FC loss overnight. ;)

Concerning the main drain, it does seem that throttling down flow through the MD has improved skimmer performance. That said, I'm not sure I'd want to be completely without one. There are times during late spring when the trees are dropping a lot of stuff and the skimmers clog up while I'm at work. With the MD there's still water flow preventing damage to the system.

Perhaps during times of lots of debris falling into the pool I'll keep the MD open and close it off the rest of the time.

[CarlD]

Just my 2 ¢... I have a 5 1/2' deep end, a 40' long pool, and only a low side-wall drain and skimmer at one end and it does fine without a bottom drain.