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Thread: pump sizing question

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    pump sizing question

    Hi,

    I want to get a new pump and I have been researching and it is confusing.....

    I have 1 1/2 inch piping,,,,,1 suction line coming from the pool strainer basket about 50 feet from the pump....
    there are 2 return lines and the inlet jets are about 20-25 feet from the pump...
    no waterfalls or spa....
    100 sq. ft. cartridge filter which has on the label 100 gpm max water flow..... pool is 9,500 gallons

    my current pump is 1 hp. and i do not know the gpm that it pushes..

    In my research I find written that with 1 1/2 inch piping and feet of head at 40-50, the max flow rate is 50 GPM...

    My problem is The Penair Whisper flo full rated 3/4 hp. pump is rated 70 GPM and the 1/2 hp.pump is rated 55 GPM.........

    The Dura Glas 3/4 hp. is rated 70 gpm....

    People say I would be crazy to get only a 1/2 hp. pump, and I would not be happy....
    I wanted to get the 3/4 hp. Whisper flo, but at 70 GPM and 1 1/2 inch pipe, I don't want to hurt anything...
    I called a few service places in my area and they say they install 1 1/2 hp to 2 hp pumps everyday in pools with only 1 1/2 inch pipe....
    Should I really worry about GPM

    Please advise
    Thanks
    Ken
    14 x 32 x 6 (ave depth) 20K gallons - IG gunite pool. Challenger 1.5 hp pump and cartridge filter.

  2. Back To Top    #2
    JasonLion's Avatar
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    The maximum flow rate for PVC pipe doesn't mean a whole lot. You can easily run at much higher flow rates and won't normally have any problems, even long term. The flow rate limit on the filter is far more important and should not be exceeded.

    Have you thought about getting a two speed or variable speed pump? Either one would save a noticeable amount of electricty by running at a lower flow rate than the single speed pumps can while still having high speed available when it is needed.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  3. Back To Top    #3
    mas985's Avatar
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    The 1 HP pump you currently have could be an uprated pump which means that it is the same as a 3/4 HP full rated pump. One way to tell is look on the label of the pump for a service factor. Also, do you have a make and model number for the pump.

    Also, what is your current filter PSI (assuming your guage works well)?

    One concern I have with your setup is that you only have one 1 1/2" suction line. This creates a lot of suction head and you will want a pump that can handle that well.

    Depending on the pump (full rated or up rated) you could have as much as 70 feet of head. A lower HP pump or 2/variable speed pump would be much more efficient and save you money in the long run.

    With a 3/4 Whisperflo, you could have as much as 62 feet of head @ 58 GPM with that pumbing setup.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater, ThePoolCleaner

  4. Back To Top    #4
    Thanks for the help....
    The current psi on the cartridge filter is now 14...........New and cleaned psi is 12....I clean it every month and the psi never goes above 16....The element is 2 years old....

    The motor says energy efficient,,and is a Century Centurion..It was installed in 1995 as part of a Jacuzzi magnum plus pump...All the numbers are gone due to years in the elements here in Florida....I am assuming it is a 1 hp.I called the builder and he feels back in 1995 for a 9,500 gallon pool like mine that they used a 1 hp. (no waterfalls and no spa )

    I only have the 1 suction line and the 2 return lines...And the filter and I'm sure some 90 degree elbows in the pvc pipe....

    I called Pentair to ask about the Whisperflo...They said at 40-50 feet of head the 3/4 hp whisperflow pump is 70 GPM and the 1/2 hp whisperflow is 55 GPM... They are both full rated..
    Am I safe with both?? With the 1 1/2 inch pipe...
    Which would you choose????
    Would the whisperflo 1/2 horsepower be too small ??? I see some people on this forum have 20,000gallon pools and only a 3/4 hp. pump....
    Thanks
    Ken
    14 x 32 x 6 (ave depth) 20K gallons - IG gunite pool. Challenger 1.5 hp pump and cartridge filter.

  5. Back To Top    #5
    mas985's Avatar
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    Pentair was likely quoting averages but your plumbing is probably on the higher end of the head scale. So your flow rates are likely to be much less than what Pentair was quoting.

    The problem is with the single suction line. This creates a lot of head loss and in fact, I estimate that the suction head is higher than the return head which is usually not a good situation to run under. It makes for a very loud running pump and can sometimes cause air to get into the pump basket.

    Based upon your plumbing setup, for the 3/4 HP Whisperflo, I estimate about 62 feet of head and 58 GPM. For the 1/2 HP, I would estimate 52 feet of head @ 53 GPM.

    A 1/2 HP would be more than enough as it would provide a 6 hr turnover. However, I usually don't recommend the 1/2 HP Whisperflo since it uses the same motor as the 3/4 HP which means the efficiency of the pump will be reduced and GPM/Watt will go down.

    I would suggest a 2 speed 3/4 HP so that you could run on 1/2 speed most of the time and still have a 12 hour turnover. Therefore you could run 24 hours/day in the heat of the summer and still have 2 turnovers. That way too you have high speed available when needed to help clean up the pool.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater, ThePoolCleaner

  6. Back To Top    #6
    mas 985
    Thanks for the info
    I have been going crazy reading every site I can find and calling many places,. Many contradict each other....

    The whisperflow Dual speed in the full rated 3/4 hp. requires (115v). The 1 hp whisperflow dual speed takes either 115 or 230 v...I now have 230 v running my motor...But at full speed the 1 hp. at 50 ft. of head puts out 90 GPM according to Pentair.

    Is it easy to switch over from 230v to 115 v to accomodate the 3/4 hp whisperflow dual speed, or should I just go with the whisperflow 1/2 hp single speed....
    Why do you think my ft of head is greater than 50 ?
    Thanks
    Ken
    14 x 32 x 6 (ave depth) 20K gallons - IG gunite pool. Challenger 1.5 hp pump and cartridge filter.

  7. Back To Top    #7
    mas985's Avatar
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    The reason your head loss is greater than 50 is because of the size of your pipe. 1 1/2" pipe has more head loss for a given GPM than larger pipe. Head loss is highly dependent on the diameter of pipe. If Pentair did not take into account the size and configuration of your plumbing, their estimate is only a very rough guess and in this case, way off.

    Take a look a this post and chart. Your plumbing is probably similar to scenario A. So you can see how head loss changes with the size of pump that you use. Larger pumps will have higher head loss for a given plumbing system. The other scenarios are for increasing pipe diameter so as you go from A-G, head loss drops and flow rates increase.

    Let me give you another example. I have 2 1/2" suction pipe and 2" return pipe and my head loss with a 1 HP pump is about 56' of head @ 88 GPM (see scenario D on the chart). So with the same size pump and 1 1/2" plumbing, the head loss goes up to 75' of head @ 57 GPM (scenario A on the chart). So the size of the pipe makes all of the difference.

    Also, a 2 speed pump is not much more expensive than a 1 speed but you will save so much more energy costs. For the same number of turnovers, you will save about 35% in energy cost by running at 1/2 speed rather than full speed even though the 1/2 speed will need to run twice as long. You will probably save the extra cost in the pump in less than a year.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater, ThePoolCleaner

  8. Back To Top    #8

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    Quote Originally Posted by mr ken
    mas 985
    Thanks for the info
    I have been going crazy reading every site I can find and calling many places,. Many contradict each other....

    The whisperflow Dual speed in the full rated 3/4 hp. requires (115v). The 1 hp whisperflow dual speed takes either 115 or 230 v...I now have 230 v running my motor...But at full speed the 1 hp. at 50 ft. of head puts out 90 GPM according to Pentair.

    Is it easy to switch over from 230v to 115 v to accomodate the 3/4 hp whisperflow dual speed, or should I just go with the whisperflow 1/2 hp single speed....
    Why do you think my ft of head is greater than 50 ?
    Thanks
    Ken
    For pool pump wiring, normally a 230V pump is connected with three wires, 2 hots and an earth ground.
    For 115V, you need 1 hot, 1 neutral and 1 earth ground.
    So, you need to have neutral available, to connect a 115V pump. I have seen people "cheat", and use one of the hots, and earth as neutral, to power a 115v circuit, which might be ok for a low current device, but I would definitely not recommend for a pump.

    The other issue is that the breaker box is wired for 230v, not 115V. I am not sure what would happen if you took one of the hots, and neutral (if available), and hooked up a 115V pump without making changes at the breaker. But, I would think fixing the breaker box would be pretty simple.

    But, I am looking at the same thing, I think a 1hp dual speed whisperflo is a little bigger than I really need, a 3/4 is probably enough, but I will probably go with the 1hp so I don't have to mess with the 230/115V wiring.

    Randy
    20,000 gal fiberglass pool, Raypak 266k BTU Lo Nox gas heater, Purex 2048 48 sq ft DE filter, 1.5hp pump. iWave wireless remote, Pulsafeeder chlorine pump, Safety cover on most of the time

    seperate spa, with own pump, cartridge filter, Teledyne heater

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