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Thread: Help selecting replacement in-ground pool pump

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    Help selecting replacement in-ground pool pump

    My pool's main pump is dying, probably won't limp along for much longer. See here for details. I've determined my best option is to replace it.

    I'm trying to learn as fast as I can, there is a lot that goes into selection! I've read Hydraulics 101 and I think I understand most of it.

    Here is all of the information about my pool that I think is applicable.

    27,000 sq feet (free form design, quickly slopes to 9 ft deep, estimate from previous pool owner, have not attempted to verify)
    Filter Pressure is 5.0 to 6.0 PSI with a freshly cleaned filter. Currently 9.0 PSI, last cleaned in April, no backwash since then, almost time.
    48 sq. ft. Nautilus D.E. Filter, maximum flow unknown, wore off data plate.
    Pool has two skimmers and one main drain.
    Polaris with Booster Pump.
    Existing Pump: 1.5 HP, SF 1.30, 3450 RPM, 115 V 60 Hz, 18.4 Amps, See here for more.
    No spa. No water features.

    Here is my plumbing setup: (please omit comments on socks with sandals)

    EDIT: Pipe sizes were wrong before, fixed now

    Aside from it being energy inefficient, I've been very happy with my pool's overall performance. I typically only backwash twice year, with one cleaning in the winter/spring. I backwash when the pressure climbs to 15 PSI. Right now I am running the main pump for 9 hours a day, and that may be a little overkill. I run it less when it's cooler. The water stays very clean and clear.

    Using the formulas in Hydraulics 101, I determined that my:

    Return Dynamic Head = 2.31 * 5.0 + 3 = 14.55 feet

    Suction Dynamic Head = 14.55 / 30 = 0.5 feet

    Minimum Flow Rate = 27,000 gal / 8 hrs = 56 GPM

    So Pump Head = 14.55 + 0.5 = 15 which seems extremely low... I know that my pool is older (built in eighties) and seems to be lower pressure than newer pools, but 15 feet of head is really low.

    Please help with recommendations for a new pump. If I left out any important information, please let me know. I'd like to order a new pump before my current pump quits for good.

    Thanks,
    Robert
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

  2. Back To Top    #2
    JasonLion's Avatar
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    Re: Help selecting replacement in-ground pool pump

    If your pump is currently wired for 115 volts, I would go with a Pentair WhisperFlo WFDS-3 3/4 HP dual speed. If it is wired for 230 volts then the WFDS-4 1 HP dual speed.
    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

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    Re: Help selecting replacement in-ground pool pump

    Thanks Jason. I was just looking at that pump as recommended in this thread

    My pump is wired for 115 Volts. I measured the voltage at the timer output and it is 120 V.

    My current pump must be way oversized at 1.5 HP. Then again I suppose I would have to look at its pump curve to be sure.

    Do you think my pump head estimate is way low? Is it very uncommon to have a pool with a clean filter pressure of only 5.0?

    Robert
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

  4. Back To Top    #4
    JasonLion's Avatar
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    Re: Help selecting replacement in-ground pool pump

    Historically having the filter pressure that low was rare, but with the increased use of 2" pipe and multiple runs in each direction from the equipment pad to the pool it has become much more common.

    Also, I believe that you got the suction dynamic head estimate wrong, but not by enough to really matter.

    There are various theories of pump sizing. From my point of view it is probably just a little bit too large. It appears to be up rated, and if so would only be 1 HP full rated. The WhisperFlo is full rated. You didn't mention the brand, so I hesitate to guess at the pump curve, but the WhisperFlo is a particularly good pump. A 3/4 HP WhisperFlo will match most 1 HP pumps. I suspect the WhisperFlo won't move quite as much water, but it should actually be quite close.

    The big savings comes from being able to run on low speed, which moves half as much water per unit time using 1/4 the power, so a 50% savings moving the same amount of water.
    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

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    mas985's Avatar
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    Re: Help selecting replacement in-ground pool pump

    RobertTx,

    5 PSI seems very low for that pump and two suction lines. Is the equipment pad elevated above the water level?
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Help selecting replacement in-ground pool pump

    The brand of the pump is Century Centurion, or AO Smith. See here.

    So does the Pentair WFDS-3 3/4 HP dual speed actually have a on it for switching between high-speed and low-speed? If so, then I should be able to install it with the existing wiring, right? I could leave it on low speed all the time and use my existing timer.

    I found this pump for sale at 'Bubbas Pools'. Seems to be much cheaper than anywhere else. Has anyone had dealings with them?

    Thanks,
    Robert
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

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    Re: Help selecting replacement in-ground pool pump

    mas985,

    The ground is pretty level between my pool deck and the location of the equipment pad. Water level is 6" below the pool deck, and equipment pad has the pump about 4" above ground level. So the pump is ~ 1 foot above water level. The pad is not in great shape. Here is a picture.
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

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    Re: Help selecting replacement in-ground pool pump

    Has the pool always run at 6 PSI?

    It could be due to the high head loss of the backwash valve which is not reflected in the filter PSI.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Help selecting replacement in-ground pool pump

    As far as I know it has always been 6 PSI after a filter clean. I have only owned the pool/home for about 2 years. The previous owner indicated that it has always been 6 PSI.

    The backwash valve seems to work fine... Would there be any other symptoms due to higher-than-normal head loss at the backwash valve?

    Robert
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

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    Re: Help selecting replacement in-ground pool pump

    What I meant was that some backwash valve designs have poor head loss characteristics. It is not because there is something worng with the valve but it is simply that it is a design which causes a large amount of head loss. The only way to know for sure is to measure the pump pressure at the pressure side drain plug if there is one. Another way is to put a pressure gauge on the hose spigot at the output of the pump. If that is a lot higher pressure than the filter pressure, the backwash valve is causing a lot of head loss.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Help selecting replacement in-ground pool pump

    I went outside to look at the backwash valve and it says '10 PSI maximum head loss' right on it. It is a Hayward Vari-Flo.


    So if we assume it has an 8 PSI drop that means my actual pressure at the filter pressure gauge is more like 13 PSI. **EDIT: actually at the pump outlet, the pressure at the gauge is what it reads. ** I guess that means my total head is closer to 35 than 15.

    Shouldn't the Return Dynamic Head formula take the backwash valve drop into account? I assume most people have one in-line between the pump outlet and their filter. I wonder what the typical drop (head loss) is. It says 10 PSI max, but the nominal drop is probably different for each position of the valve.

    Robert
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

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    Re: Help selecting replacement in-ground pool pump

    Shouldn't the Return Dynamic Head formula take the backwash valve drop into account?
    No. The filter pressure is related to the head loss of only the plumbing that comes after the gauge not before the gauge. So the pipe that connects the pump to the filter and the backwash valve is not included. Normally, the head loss in these items is not that bad and doesn't affect the measurement much. However, the Vari-flow line of valves tend to be on the high side.


    I assume most people have one in-line between the pump outlet and their filter.
    Pools with cartridge filters, like mine, do not have backwash valves. But even those that do, some backwash valves are quite good and add hardly any head loss. The Jandy never lube has less than 1 PSI at 80 GPM.


    I wonder what the typical drop (head loss) is. It says 10 PSI max, but the nominal drop is probably different for each position of the valve.
    The backwash valve head loss is dependent on flow rate like any other piece of plumbing. From the picture, it looks like the maximum flow rate is 60 GPM which probably is also where the maximum head loss is determined of 10 PSI. You can scale the head loss by the square of the GPM for lower flow rates but with that pump, you are probably exceeding 60 GPM but it is hard to tell.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Help selecting replacement in-ground pool pump

    Thanks mas985, your explanations have been very helpful. I guess if I really want to be thorough with selecting the correct pump, I should find a way to measure the pressure at the pump outlet, before the backwash valve. That would allow me to better estimate the total head of my pool.

    Below, curves A and F are for the WFDS-3.


    Here are a few scenarios for my 'best guess' total head.

    Total Flow
    Head Rate
    40 80 ** close to optimum efficiency
    50 70
    60 60

    I need a minimum flow rate of 56 GPM to filter all 27,000 gal in 8 hours. I can get that with the WFDS-3 with a head loss of less than 60. I have read that 96 GPM is the absolute maximum I should put through the DE Filter, and I get a lot more dynamic loss at higher flow rates as well. So I probably would really like a flow rate between 60 and 70 GPM.

    So with all of this, it does seem that the WFDS-3 would be a good fit. If I get it, I will run it at low speed for 16 hours a day at first, then maybe try and see if I can run it less.

    Still trying to find out if this pump has a switch on it or if I will have to wire one to it.

    Thanks,
    Robert
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

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    Re: Help selecting replacement in-ground pool pump

    So I've been reading about horsepower ratings with different service factors... It appears that the Pentair WFDS-3 is the same exact pump as the WFDS-24. Is this correct?

    ..................HP......SF......SFHP
    WFDS-3......3/4......1.67.....1.25
    WFDS-24...... 1......1.25.....1.25

    Also, will my Polaris booster pump have any issue operating with the Pentair on low-speed?

    I'm getting close to pulling the trigger on the Pentair WFDS-24.

    Thanks again for everyone's help! This is a great site.

    Robert
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

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    mas985's Avatar
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    Re: Help selecting replacement in-ground pool pump

    Quote Originally Posted by RoberTX
    Thanks mas985, your explanations have been very helpful. I guess if I really want to be thorough with selecting the correct pump, I should find a way to measure the pressure at the pump outlet, before the backwash valve. That would allow me to better estimate the total head of my pool.
    This will get you the plumbing head loss but if you don't have the pump head curve, you won't be able to determine the plumbing flow rate to go with it. And without the flow rate, you won't be able to generate a plumbing curve that you can overlay on top of the WFDS-3 head curve to get a NEW operating point.

    All of this is somewhat academic and not really necessary for pump sizing. Nearly any size IG pump will give an 8 hour turnover for a 27k pool so your objective should be to get the smallest two speed available. The WFDS-3 is smallest two speed available in the Pentair Whisperflo line. However, the Hayward SuperPump SP2607X102S is a slightly smaller pump plus has a switch in the back.


    So I've been reading about horsepower ratings with different service factors... It appears that the Pentair WFDS-3 is the same exact pump as the WFDS-24. Is this correct?
    Yes

    Also, will my Polaris booster pump have any issue operating with the Pentair on low-speed?
    No
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Help selecting replacement in-ground pool pump

    So the Pentair WhisperFlo dual speed pumps do not have a switch to change between low and high speed? I have read that it may be necessary to use the high speed setting in order to prime the pump. I would also use high speed to backwash. If that is the case, then is the Pentair WhisperFlo typically installed with an external switch?
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

  17. Back To Top    #17
    mas985's Avatar
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    Re: Help selecting replacement in-ground pool pump

    I am not certain if you can get an end cap with a switch in it for the Whisperflo but there are other options. A dual speed timer which is a good idea anyway. Or you can wire your own wall switch box for two speed.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

  18. Back To Top    #18

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    Re: Help selecting replacement in-ground pool pump

    Having a switch on the motor itself is not a deal breaker. I can just get a single pole double throw switch like this one and wire it inside my timer box. I might look into an environmentally sealed one, but if it's inside my timer box, it should be fine...

    Robert
    27,000 Plaster IG Pool
    DE Filter, 48 sq ft
    Pentair WhisperFlo two speed 1 HP Pump

  19. Back To Top    #19
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    Re: Help selecting replacement in-ground pool pump

    The Pentair dual speeds tend to come with a speed switch, but you don't need to use the switch they include if you don't want to.
    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

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