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Thread: Pump Positioning

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    Pump Positioning

    This maybe my dumbest question yet. Has any body ever tried putting a pump after the filter? So that the pump is pulling water through the filter instead of pushing it. Besides the the obvious problem of backwashing is there other reasons this should not be done?
    Intex Ultra Frame 14 x 42 3500 gals
    Intex SWG combo unit (2000 gph), screen door screening around cartridge:-).

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    spishex's Avatar
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    Re: Pump Positioning

    It's hydraulically more difficult for your pump to pull water than push it, so it makes more sense to put large strains like a filter on the pressure side. It's also less finicky as your filter accumulates debris for the same reason.

    That being said suction side filters are used regularly, just in hot tubs, not pools. The hydraulics are not so demanding in a tub because of the relatively small amount of water, the debris load is low because they're covered 99% of the time, and using suction side filters allows you to connect the cartridge to a simple PVC fitting rather than building/buying a pressurized housing for it.

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    Re: Pump Positioning

    This may or may not be an issue with modern filters, but I would imagine another issue would be structural. When they are designed, the concern is their ability to contain rather high pressures without exploding. I don't think they are designed to withstand high vacuum, and could potentially implode at such vacuum levels that could be experienced at the inlet to the pump. This would probably not be a problem if the filter was full of water, but if it were partially filled with air, the vacuum could be enough to cause problems. I'm just not certain that filters would be designed to deal with that sort of stress (unless the engineers anticipate some hooking up the system backwards!).
    38K in ground pool with attached spa. Current equipment: Easytouch 8 (521150) with IC-60 SWCG with web control by Autelis, 1x Pentair IntelliFlo 011018 pump (for filter), 1x Pentair 2HP WhisperFlo pump (for waterfall), 2X Pentair IntelliBrite 5G 12V lights, Pentair MiniMax400 NG Heater, Pentair SMBW2060 DE filter. Zodiac Barracuda MX8 cleaner on dedicated cleaner line. Lighting/home automation controlled by Insteon/ISY-99i.

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    spishex's Avatar
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    Re: Pump Positioning

    Rather than being in any kind of sealed chamber, a suction side cartridge is usually just in a "well" for lack of a better word. Sometimes directly under a skimmer basket, but often just behind a weir like you see here. Collapse can be a problem if the filters get clogged up, but no more so in my experience than a clogged pressure side filter since a pool/spa pump is capable of creating much more positive pressure than vacuum force.

    I'm not sure how this would change if you were to just hook a sealed pool filter housing up on the suction side of your pump, aside from making the system difficult to prime.

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    mas985's Avatar
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    Re: Pump Positioning

    Assuming that the head loss and flow rate through the filter is the same on the suction vs return side of the pump, which it should be, then the differential pressure between the input and output ports of the filter will be the same on either side of the pump as well. Absolute pressure will be different but that is irrelevant to the filter media.

    However, the biggest problem with moving the filter from the return side to the suction side is the head loss of the filter moves as well. So even though the operating point of the pump should remain the same, having more head loss on the suction side while reducing the head loss on the return side will make the pump much more prone to cavitation especially when the filter gets dirty. A spa can get away with this because there is high head loss in the jets so moving the filter to the suction side is not such an issue. Plus stand alone spas use cartridges which have much less head loss than other filter types.
    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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