Maximum GPM on mixed sized piping

JamesW

TFP Expert
Mar 2, 2011
24,022
If I set the rpm so it gives me 25 GPM, will it be actually faster in the 1 1/2" section of piping? If so, then I should maybe run it a bit lower so it actually is 25 GPM in the 1 1/2" section?
The gpm will be the same, but the velocity will be more in the 1.5" plumbing.

25 gpm is ok for 1.5" plumbing.
 
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AviatorBimmer

Well-known member
Mar 29, 2021
76
Florida
Pool Size
14300
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-30 Plus
The gpm will be the same, but the velocity will be more in the 1.5" plumbing.

25 gpm is ok for 1.5" plumbing.

Thanks, I just love this website and everyone's willingness to help! 💪🏼
 
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mas985

TFP Expert
LifeTime Supporter
May 3, 2007
14,033
Pleasanton, CA
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
GPM is a constant throughout the plumbing path regardless of the change in pipe diameter. What changes is the water velocity (i.e. ft/sec). But given the low flow rates, you really don't need to be concerned with water velocity.
 
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JamesW

TFP Expert
Mar 2, 2011
24,022
Note that if you have multiple suction or return lines, the flow will be distributed between the lines such that the head loss in each line is equal.

If the lines are the same diameter and length, the flow will be equally distributed.

If the lines are different diameters and/or lengths, the flow will be unequally distributed.

For example, if you have (7) 100 foot long return lines at 3/4", 1", 1.25", 1.5", 2", 2.5" and 3", the flow will be distributed between the lines at about the following:

3/4"........1.5%
1"............2.7%
1.25"......5.4%
1.5"........7.8%
2".........15.4%
2.5"......24.4%
3".........42.8%
 
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AviatorBimmer

Well-known member
Mar 29, 2021
76
Florida
Pool Size
14300
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-30 Plus
Note that if you have multiple suction or return lines, the flow will be distributed between the lines such that the head loss in each line is equal.

If the lines are the same diameter and length, the flow will be equally distributed.

If the lines are different diameters and/or lengths, the flow will be unequally distributed.

For example, if you have (7) 100 foot long return lines at 3/4", 1", 1.25", 1.5", 2", 2.5" and 3", the flow will be distributed between the lines at about the following:

3/4"........1.5%
1"............2.7%
1.25"......5.4%
1.5"........7.8%
2".........15.4%
2.5"......24.4%
3".........42.8%

Great info once again! This pool came with the house, so I can only attest to what is above ground.

The pool has a skimmer and main drain for suction.

For returns, it has 4 return jets and 2 spa jets above a set of steps.

On the pad, I can see 2 pipes coming from the suction side of things and 2 return pipes. Return pipes I'm guessing, one pipe supplies the spa jets while the other pipe splits into the 4 return jets. All 1 1/2" piping.

BTW, my pump is running at 1150 RPM to maintain 25 GPM at 79 watts.
 
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JamesW

TFP Expert
Mar 2, 2011
24,022
If you have any combination of different size pipes, you can determine the distribution of flow using the previous numbers.

For example, if you have (3) equal length return or suction pipes at 1.5", 2" and 2.5", the formula is

7.8 ÷ (7.8 + 15.4 + 24.4) = 16.4% for 1.5".
15.4 ÷ (7.8 + 15.4 + 24.4) = 32.4% for 2".
24.4 ÷ (7.8 + 15.4 + 24.4) = 51.3% for 2.5".

1.5"........7.8%
2".........15.4%
2.5"......24.4%
 
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