# Thread: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

1. ## pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

I know 1.5" PVC can carry about 44 GPM at about 6-7 ft/s while 2" PVC can carry about 78 GPM at a similar speed. If two 1.5" join to become one 2" at the suction side. What is the final flow?

44 + 44 = 88
or
44 + 44 -> 78 (limited by the final 2" pipe)

How about two 2" join into one 2"?

Does the flow speed increase when you join them?

2. ## Re: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

There is no limit to the flow based on pipe size ... the velocity is a sizing guideline, but not a physical limit.

Flow speed is based on cross-sectional area.
1.5" pipe is ~1.76 sqin
2" pipe is ~ 3.14 sqin

So two 1.5" pipes has more area than one 2".

The actual flow rate depends on the total head loss which is a function of the pump and the entire plumbing system.

None of this really matters for any practical purpose. Why are you asking out of curiosity?

3. ## Re: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

You said flow speed is based on cross-sectional area. I disagree. It should be based on the pressure difference between two ends. Think about it. If there is no pressure difference between two ends (gravity force is a type of pressure by the way), the water does not flow in the pipe. As result, there is no water speed. When there is head pressure, there is water speed. The water flow is then calculated based on a given pressure and internal pipe size.

I am just thinking if it is worth the effort to upgrade those 1.5" pipes coming out from the skimmer boxes to 2". Maybe I am too obessed with high efficiency... LOL

4. ## Re: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

OK. Maybe I did not say that correct and I did not state the obvious that a pressure difference is required. The bigger the pipe, the lower the head loss, the higher flow rate you will get and at a lower velocity.

If you always have two 1.5" suction lines running, then up-sizing them will likely make little difference. If you only have one 1.5" line, then increasing the pipe to 2" might be more noticeable.

My pool used to only have one 1.5" suction line from the skimmer (drain connected to skimmer). I happened to find an abandoned suction line from a hole in the wall (I think used to be for a waterfall pump) and have now plumbed that wall suction line (also 1.5") together through a 3-way valve which then up-sizes to a 2" line into the pump. On high speed, if I have both suction lines open, I can see a small pressure increase in the filter, so I am getting a higher flow rate which helps for my solar. I may add a flow meter to get some hard numbers out of curiosity.

When I am not running solar (like now) and mostly running on low speed, I actually close the wall suction line so that I get higher flow through the skimmer on low pump speed. I might actually try to automate that valve to turn in conjunction with my solar valve to get the best skimming I can on low speed.

5. ## Re: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

Water velocity (ft/sec) in a pipe at a specific GPM is proportional to it's area according to this formula:

Water Velocity (ft/sec) = 0.4085 * GPM / DIA^2

Also with regards to dynamic head, although it is true there cannot be flow without a pressure difference, there is also no pressure difference without flow. The pump creates the flow rate and the flow rate creates the pressure difference in the pipe. But the opposite is also equally true. The pump creates pressure and the pressure creates the flow rate in the pipe. In my mind, it is difficult to say which causes which.

But up sizing the pipe is the best way to reduce head loss but it may not always reduce energy costs. It depends on what you do. When the pipe size increases, head loss decreases and so the pump will produce more flow rate. However, that additional flow rate causes more energy draw from the pump so unless you reduce run time, there is little benefit to up sizing the pipe. Unfortunately, there is evidence that required pump run time has little to do with flow rate so you may or may not be able to reduce run time at the higher flow rate.

6. ## Re: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

Ah, I actually meant to mention the fact that lowering the head loss and thus increasing flow rate may actually raise the electrical cost.

Thanks, Mark

7. ## Re: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

What a coincidence! I also found a capped bottom drain line in the wall. My main drain is currently plumbed to the skimmer but I wonder if I should use that capped line for better pool circulation.

I don't understand why you close the drain line at low speed. I assume you only have one skimmer and you want better surface skimming at the expense of bottom circulation, right?

8. ## Re: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

Originally Posted by scott_home
I don't understand why you close the drain line at low speed. I assume you only have one skimmer and you want better surface skimming at the expense of bottom circulation, right?
Exactly, low speed generally causes the skimmers to work less efficiently. Also, there is little benefit to main drain circulation. It can even out the temperature some but that is about it.

9. ## Re: pipe sizing question: 1.5" + 1.5" = 2" or > 2" ?

Actually, my floor drain is attached to the single skimmer and then a float diverter is used to isolate the floor (I leave this mostly open so little if any is drawn from the floor) and then I have a single pipe running to the pump. The capped line I found is actually through the wall under the waterfall (I thought it was a return jet but then realized it never got any flow).

But like Mark said, on low speed, I shut off my wall suction port so I get more flow through the skimmer but, this gives me LOWER TOTAL flow because of only the single suction pipe open.

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