# Maximum GPM on mixed sized piping

#### JamesW

TFP Expert
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.

AviatorBimmer

#### AviatorBimmer

##### Well-known member
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!

JamesW

TFP Expert

AviatorBimmer

#### mas985

TFP Expert
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.

AviatorBimmer

#### JamesW

TFP Expert
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%

AviatorBimmer

#### AviatorBimmer

##### Well-known member
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.

JamesW

#### JamesW

TFP Expert
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%

AviatorBimmer

Replies
7
Views
97
Replies
6
Views
124
Replies
10
Views
262
Replies
15
Views
234
Replies
1
Views
144