Hydraulic question for new pool

carlos31

Well-known member
Oct 18, 2007
104
Central Texas
#1
I have an under construction IG gunite 22 x 46 pool, 3.5 to 8 foot depth, ~42K gallons. I am planning (12) 1.5 inch wall returns, (4) 1.5 inch floor returns, 3 skimmers and 1 drain all at 2 inch. All pipe is to be run back to the pad into manifolds. All the runs are long (100 to 150 feet). The reason I have so many returns is to maintain circulation in odd places like the beach area, on benches, behind stools, etc. I figure I can always plug them if necessary but can never install them after gunite. I will probably use an Intelliflow pump. I may also loop two returns and use 2 inch pipe instead to reduce the number of runs. However, in either case I seem to have much greater pressure capacity than suction capacity. Am I headed toward a problem with cavitation or low return pressure?

I want to adhere to the big pipe/small pump theory. But I am not sure if 3 inch pipe for skimmers does any good if each end of the pipe is reduced to 2 inch. Any opinions are appeciated. Oh yes, I also have a WF, long run and 2 inch pipe but of course it will have its own pump.

Thanks for any advice.
 

waste

TFP Expert
LifeTime Supporter
Mar 29, 2007
4,160
Coastalish 'down easter'
#2
Carlos. welcome to TFP!!

I agree with your plan, but 12 returns may be 'overdoing' it as you plan for floor returns. The floor returns will add heat (if you have a heater) and chems from a feeder into the bottom part of the pool where they can be mixed with the water more efficiently. For the returns, I'd use enough to direct any debris towards the skimmers and leave ~ no dead spots.

Just my $.02 :)
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
12,172
Pleasanton, CA
#3
Generally, you want your suction equivalent piping to be larger than the return piping but other things can trump that.

So you have 16 1.5" runs pool to pad for the returns which is equivalent to approximately 1 run @ 4.3" diameter.

The suction side has 4 runs @ 2" which is equivalent to about 3.4" diameter.

Although the return side is effectively bigger pipe, given the relative sizes of the pipes, the majority of the head loss on the return will come from the pad equipment so that will make sure that the balance is correct. The head loss on the suction side should be very low so cavitation is unlikely. That usually happens for high suction loss.

Also, I always say it is easier to add head loss than it is to reduce it so worst case is you add a few ball valves for balancing. You may want that anyway with all of those returns so that you can control the flow to a group of returns.

Having a small section of 2" pipe vs long runs of 3" pipe is still ok. Head loss is dependent on the diameter and length of pipe so a short section of small pipe will have a minimal effect.

I think you are headed in the right direction and if I were to build a pool today I would use a lot of little pipes rather than few big pipes. This gives you a lot more control over flow and head loss.

Sounds like it's going to be a great pool. Good Luck.


If you want to run your own equivalent pipe diameters use the simple formula:

Eq Diameter = Actual Diameter * N ^ 0.38
where N = Number of equal length runs
 

carlos31

Well-known member
Oct 18, 2007
104
Central Texas
#4
thanks guys, I do have a couple of follow-up questions

Thank you for your responses. I have decided to use 2 inch pipe for returns with 2 returns on each two inch line and a valve for each line. I am still concerned about the 2 inch lines on the suction side, especially the one skimmer that is 160 feet and 5 90's from the pad. Maybe this should be a 2.5 or 3 inch line? maybe they should all be 3 inch but this doubles the pipe cost.

I am also considering moving the pad but the alternate location (while closer) is 5 feet higher than the water level. Any thoughts on this?

Also, I see 15 gpm suggested for spa returns but never see anything about pool returns. I have pool returns about 12 feet away from the skimmers based on what I have read elsewhere but I am not sure what the gpm a pool return should have.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
#5
There is no specific number of GPM for a pool return. They can go quite low while the spa is on and that will usually be fine. The issue with the pool is being able to get two or three turn overs in 24 hours. Divide the size of the pool by the number of minutes in a day, assuming the spa is off your system should be able to do two to three times that number of GPM.

I would go with the larger pipe, the pipe cost isn't that high compared to other things and it will help balance the system.
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
12,172
Pleasanton, CA
#6
Re: thanks guys, I do have a couple of follow-up questions

carlos31 said:
Thank you for your responses. I have decided to use 2 inch pipe for returns with 2 returns on each two inch line and a valve for each line. I am still concerned about the 2 inch lines on the suction side, especially the one skimmer that is 160 feet and 5 90's from the pad. Maybe this should be a 2.5 or 3 inch line? maybe they should all be 3 inch but this doubles the pipe cost.

As long as you have individual pipes going from the skimmers or main drain all the way back to the pad, you should be ok with 2" pipes. If you have a one very long run while the others are short then there could be slight inbalance in flow rates between the lines but it should not be that significant especially when you run at lower speeds.

I am also considering moving the pad but the alternate location (while closer) is 5 feet higher than the water level. Any thoughts on this?

Most pumps will prime up to 10' of elevation on the suction side so you should not have any problem. Once the pump is primed, it really doesn't matter after that. A check valve before the pump will help so that you do not lose all the prime when opening the pump basket.

Also, I see 15 gpm suggested for spa returns but never see anything about pool returns. I have pool returns about 12 feet away from the skimmers based on what I have read elsewhere but I am not sure what the gpm a pool return should have.
The only concern for return flow rates is to get enough movement of the water for the skimmers to work properly. There has been some talk recently about the Intelliflo at the lowest speeds not having enough flow to move the water towards the skimmers. I think the lowest flow rate is around 10 GPM and if you split that between the returns it is probably less than 3 GPM per return which is not a lot and probably why there is not enough water movement. Something to consider.
 

carlos31

Well-known member
Oct 18, 2007
104
Central Texas
#7
floor return lines

Thanks, I am going to oversize everything and adjust with valves or by sizing the return sockets appropriately. The floor of my excavation is solid rock or native gravel (limestone).

Does anyone know if floor return lines need to be completely encased in the gunite or can they just sit on the rock?

The lines either need to be on the ground or up against the underside of the floor rebar. I am guessing it doesn't make any difference, but would rather go with some experience.

Thanks,
 

waste

TFP Expert
LifeTime Supporter
Mar 29, 2007
4,160
Coastalish 'down easter'
#8
Carlos, the feed lines can be below the crete. What we always did was to run a larger line to the first 2 and then reduce down to the second set to keep the pressure to each bank ~ even. I realize that this is somewhat cryptic -- if you need more detail, just ask :)
 

carlos31

Well-known member
Oct 18, 2007
104
Central Texas
#9
Thanks Waste,

On wall returns I have 2 returns sharing a 2 inch pipe (balanced versus in series) and I will put a valve on each line at the pad to control flow to each set of 2 returns. I could also change the eyeball size for each return. I was was just going to use pvc ball valves. Do you recommend that I use the SxS solvent style or the threaded style with union fittings? If I only need to replace the valve every 15 years I would rather use SxS.

I will likely use the Hayward SP1425 style floor return. The floor returns aren't as easy to adjust at the return and I really need to plumb these in series. So based on what you are saying in your reply I could do the following. Run four floor returns in series by connecting the first two with 2 inch and the second two with 1.5 inch pipe. Am I on the right track?
 

waste

TFP Expert
LifeTime Supporter
Mar 29, 2007
4,160
Coastalish 'down easter'
#10
Carlos, the way we did our floor returns 4 would be a 'box', 1 return at each corner -- I assume that yours will be run in a line on the center line of the pool. If that's the case, run the feed line off center and TEE in in the center of the 2 centermost returns - you add an extra 90 but it will help equalize the flow to the 4. After you TEE off into the first 2 floor returns, you can reduce to the other 2 to help equalize the pressure (perhaps one of the 'pipe sizing experts' can tell you the optimal place to reduce down -- I can't, but on some pools we'd run 2" into the first set and then reduce down to 1.5" after a ~ 3' run, but on others we'd reduce down right after the we teed into the first set. :? ) It'd be like your wall returns, run balanced, not in series.

As for the valves, I prefer the SxS over the threaded, just because the threads always seem to leak before the valve actually fails and it's a PITA to have to retape them, if the valve itself fails- you're cutting and gluing anyway :wink:

I hope I explained the plumbing a little better this time, if not - we'll take another go at it tomorrow :lol:
 

carlos31

Well-known member
Oct 18, 2007
104
Central Texas
#11
thanks waste,

i think i get the picture. i like the H idea with four returns on one pipe. I assume it looks like this.

R R
! !
! !
main feed_______!____________!
! !
! !
R R