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Thread: Why is my suction dynamic head so high?

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    Why is my suction dynamic head so high?

    I have a basic 15,000 gallon pool with 1.5" lines. Three return jets tee'd somewhere underground off of one return line from the pump. On the suction side, there is one skimmer and one bottom drain that feeds through the skimmer. No water features or other fancy stuff. The filter is a new StaRite System2 PLM-300.

    This pool had a Pentair Pinnacle 1.25 SFHP pump on it. That pump produced a suction of 21 inHg at the bottom of the volute and a filter pressure of 7 psi, for a RDH of 19.17 and a SDH of 23.73. According to the pump curve, this was pumping about 65 gpm -- too fast for the 1.5" lines.

    I downsized the pump to a Pentair Superflo 0.95 SFHP. It produces a suction of 14 inHg at the bottom of the volute and a filter pressure of 6 psi, for a RDH of 16.86 and a SDH of 15.82. According to the pump curve, this is pumping about 50 gpm -- still slightly too fast, but this is the smallest pump in the Superflo line.

    Why is the suction dynamic head so high? These numbers are *way* higher than the estimates that I would expect based on the formulas in the sticky thread in this section.

    I think I understand how most of this works based on the excellent information found here, but I'm obviously missing something.

    Thanks.

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    mas985's Avatar
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    Re: Why is my suction dynamic head so high?

    High suction head could be caused by a blockage in the suction line somewhere but the numbers you are quoting are not that uncommon for 1.5" plumbing. With a cartridge filter, low head loss, and very few pad fittings, the head loss on the suction and return sides can be similar with the suction being quite high.

    Is the pump loud and/or do you get a lot of air in the pump basket? One solution is to reduce the diameter of the return eyeballs to add some return head/pressure. The upside is this will increase the exit velocity of the water helping with circulation. The downside of course is a little less efficiency.
    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: Why is my suction dynamic head so high?

    Quote Originally Posted by mas985
    High suction head could be caused by a blockage in the suction line somewhere but the numbers you are quoting are not that uncommon for 1.5" plumbing. With a cartridge filter, low head loss, and very few pad fittings, the head loss on the suction and return sides can be similar with the suction being quite high.

    Is the pump loud and/or do you get a lot of air in the pump basket? One solution is to reduce the diameter of the return eyeballs to add some return head/pressure. The upside is this will increase the exit velocity of the water helping with circulation. The downside of course is a little less efficiency.
    The new Superflo is about as loud as the old Pinnacle was (which is pretty quiet), even though it's smaller and I expected it to be quieter. It does pull about an amp less, though. It's dataplate says 4.5A, and that's what it's doing. The Pinnacle dataplate said 7.1A, but it was only pulling about 5.5A. Neither one of them has any real air in the pump basket, although with the new Superflo I can see some tiny bubbles in a vortex. The Pinnacle had no detectable bubbles or vortex. I assume the difference here is just due to the way the volute is constructed.

    Interestingly, I moved the Pinnacle to my spa circuit, which has completely separate plumbing with a 2" inlet with 1.5" return. The Pinnacle runs much quieter there -- I assume that's primarily the result of the 2" inlet line (even though it gets reduced to 1.5" to enter the pump). The actual numbers on the spa match your estimate formula exactly, with the SDH being 10% of the RDH.

    Thanks much..

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    Re: Why is my suction dynamic head so high?

    Quote Originally Posted by mas985
    High suction head could be caused by a blockage in the suction line somewhere but the numbers you are quoting are not that uncommon for 1.5" plumbing. With a cartridge filter, low head loss, and very few pad fittings, the head loss on the suction and return sides can be similar with the suction being quite high.

    Is the pump loud and/or do you get a lot of air in the pump basket? One solution is to reduce the diameter of the return eyeballs to add some return head/pressure. The upside is this will increase the exit velocity of the water helping with circulation. The downside of course is a little less efficiency.
    Sounds like the old problem of suction lines too small. Ideally suction lines should be bigger than returns for exactly the reasons you have posted. reducing the returns may balance the equation but in reality it's a waste of energy and the turnover time will be greater still. Shame you are unable to increase the suction line size.

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    susa's Avatar
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    Re: Why is my suction dynamic head so high?

    after having run many models of single speed pumps, this was one of many reasons I installed the VF and despite some crazy downsize GPM automation limits, in manual mode the VF offers limitless capabilities suitable for any pipe diameter, pressures and flow

    in my current mode of running pump 24 hours/7 days it uses just 95 watts @ 850 rpm and is totally silent and vibration free. when I use programming features (20 GPM & 15 GPM), the pump automatically executes a prime as needed which is the only time I can hear it, for about 20 to 30 seconds and then it revs down and resumes stealth mode silence.

    ....will never ever install a single speed pump again....

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    Re: Why is my suction dynamic head so high?

    Well I wouldn't say never as the customer gets what they want but I'll put forward a good case. I actually want my 9 year old pump to fail so I can fit one

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