Confused about reconciling GPM ratings and standards.

mvoltin

Active member
Aug 1, 2016
36
atlanta, GA
Starting to build a new pool (actually, halfway done) and having trouble reconciling piping recommendations based on GPM.

Based on what I have read, 2" piping only supports around 80GPM (or 63GPM on suction and 84GPM on return side). There are other small variations across websites but its universally somewhere around 80GPM but more conservative would be to aim around 63GPM (based on suction side).
Numbers for 1.5" are around 35GPM suction and 50GPM return.

QUESTION:
Pentair 1.5HP VS pump can go up to 100GPM (see the picture from brochure below), yet, it has unionized fittings of 1.5" and 2" based on the installation manual. I called Pentair and was told the actual opening is 1.5" - so, it should support up to 50GPM from everything I have read. They couldn't provide any reasonable explanation ("that's how it is").

Can anybody help explain what I am missing? Why pump rated above 80GMP has fittings less than 2.5"?

Thank you

P.S. I have cartridge filter that is rated at 150GPM and it has 2.5" fittings.

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Last edited:

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,732
Pleasanton, CA
Starting to build a new pool (actually, halfway done) and having trouble reconciling piping recommendations based on GPM.

Based on what I have read, 2" piping only supports around 80GPM (or 63GPM on suction and 84GPM on return side). There are other small variations across websites but its universally somewhere around 80GPM but more conservative would be to aim around 63GPM (based on suction side).
Numbers for 1.5" are around 35GPM suction and 50GPM return.
2" will support pretty much any flow rate. However, it is recommended to stay below 84 GPM on the return side and 63 GPM on the suction side of the pump. Again, it is easy to exceed those values and nothing catastrophic occurs but as flow rate increases, head loss increases and pump efficiency decreases. On the suction side, high velocity increases chance of entrapement.
 

JamesW

TFP Expert
Mar 2, 2011
20,712
Why pump rated above 80GMP has fittings less than 2.5"?
In some areas, code requires that the suction be kept below 6 feet per second and the returns be kept below 8 feet per second. Any good builder will follow these limits. Once you exceed these limits, the resistance is increasing exponentially and the law of diminishing returns comes into play.

Suction has a hard limit where the water will begin to cavitate. This needs to be strictly avoided. Returns don’t have a strict hard limit, but the 8 feet per second rule should be observed anyway.

Technically, the fittings probably should be at least 2.5” for a pump that can exceed 63 gpm.

Realistically, most systems won’t require over 63 gpm.

If you require over 63 gpm, you should go with an Intelliflo VS over the Superflo VS.

The current VS pumps were designed decades ago as single speed (3,450 rpm) pumps when there was much less understanding of good hydraulic design. Most builders just installed a 1 to 2.2 hp pump with almost no actual hydraulic design. They just had crude rules of thumb for guidance. This resulted in energy hog pumps consuming anywhere from 1,000 watts to 2,500 watts.

Builders mostly didn’t worry about this because they were mostly unaware of the cost since they did not have to pay the electric bill.

Most current VS pumps can be run using anywhere from 150 watts to about 700 watts depending on the flow requirements.

Most current VS pumps are just the original single speed pump with a variable speed motor attached.

In my opinion, the pumps need to be redesigned with a top speed of about 2,750 rpm vs. the current 3,450 rpm to reduce noise. 3,450 rpm is only really useful for very high head situations, which are and should be rare to nonexistent.

New laws are going to prohibit single speed pumps for many filtration systems.
 

mvoltin

Active member
Aug 1, 2016
36
atlanta, GA
Thank you, these responses make sense in general but let me restate the question:

the highly rated late model VS pump (link provided above) has 1.5” fittings, yet it states to produce 90+ GPM at 3000rpm.

Is it possible to be accurate? Pump with 1.5” fitting producing 90GPM?
Based on what i have read, 50GPM is the upper recommended flow for 1.5” pipe (and i assume for the fitting too).
 

JamesW

TFP Expert
Mar 2, 2011
20,712
The pump can be plumbed with 1.5" pvc pipe going into the pump union or a 2" pvc fitting going to the outside of the pump union fitting.

For 90 GPM, you would use a 2" x 2.5" bushing and immediately transition to 2.5" pvc pipe or bigger depending on the pipe length.

There is only a very short section where it is 2".

So, it won't degrade performance too much.

Technically, it should probably be 2.5". I would consider it a design flaw.

If the flow requirements will exceed 63 gpm, you should go with an Intelliflo.
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,732
Pleasanton, CA
Is it possible to be accurate? Pump with 1.5” fitting producing 90GPM?
Based on what i have read, 50GPM is the upper recommended flow for 1.5” pipe (and i assume for the fitting too).
Again, there are no flow rate limits for pipes. Only recommendations.

So a pump with 1.5" fittings can produce as much flow rate as the pump's head curve allows. The pump's fitting size has almost no bearing on the flow rate. However, the rest of the pool's plumbing has a big effect on flow rate.
 

mvoltin

Active member
Aug 1, 2016
36
atlanta, GA
Again, there are no flow rate limits for pipes. Only recommendations.

So a pump with 1.5" fittings can produce as much flow rate as the pump's head curve allows. The pump's fitting size has almost no bearing on the flow rate. However, the rest of the pool's plumbing has a big effect on flow rate.
Thanks. I comprehend what you are saying but don’t understand. I thought you had to look for a weakest point in the system and the 1.5” is the “weakest point”. It’s almost 3 times smaller than 2.5” cross-section and would require lots of pressure to develop velocity needed to keep up with the rest of the system’s flow of 90GPM.

(for example, pressure washer flow, even with all of its pressure Pushing water, is much lower when compared to a garden hose. There is only one small restriction towards the end of the pressure washer and that single restriction limits the overall flow rate.

i am just thinking out loud; i realize that it’s probably more complicated than that. Things don’t always make sense
Thanks for your explanation.