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Thread: is there a minimum backpressure for filtering?

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    is there a minimum backpressure for filtering?

    I just got my new Intellifo VS hooked up and programmed. I had to increase the default priming time from 20 seconds to 2 minutes, since it wasn't pulling enough water into the pipes before dropping down to low speed.

    Now that I have that solved, I am running 2x a day for 4 hours @ 1000 rpm b/c I was told that is the most efficient setting. The pump reports it is consuming 125 Watts of power at the speed. The pressure gage on my sand filter shows 0 psi. There is water moving, I can feel suction in the skimmer and weak flow out of some of the return jets. When I run high speed for vacuuming I set 20 psi across the filter. So is there a minimum pressure for filtering, or is lower always better, as long as you are moving water?
    25K gal IG plaster pool w/ spillover jacuzzi, 12 Gulf Solar coils (leaking/currently bypassed), ancient Raypak propane heater (rusted/currently bypassed), Pentair 001018 VS pump, 4.9 sq ft Triton II sand filter, Taylor K-2006 FAS-DPD kit

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    Mod Squad jblizzle's Avatar
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    Re: is there a minimum backpressure for filtering?

    No minimum. As long as water is moving you are good ... And saving money!
    Jason, TFP Moderator
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    Re: is there a minimum backpressure for filtering?

    if there is flow you should have some kind of pressure in my book...
    how long has it been since you installed a new $7 gauge?
    you might be by-passing the filter due to a leak in the backwash valve with a bad
    or misplaced gasket.
    I had mine get stuck to the valve this year when I opened my pool up.
    18X36 Grecian 24400 est., pacfab sand Triton II, 1.5hp AOSmith, clormatic 2

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    JasonLion's Avatar
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    Re: is there a minimum backpressure for filtering?

    With a VS pump it is fairly easy to get the filter pressure below the lowest level the gauge can measure. There still is some pressure, the gauge simply can't distinguish it from no pressure. Those gauges are often cheaply made and frequently don't notice to 1 or 2 psi.
    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: is there a minimum backpressure for filtering?

    well, I agree in theory there must be some finite pressure in order to drive flow
    the question is whether it is high enough to be measured by a 0-60 psi gage
    The gage, filter, and pump are all ~1 month old
    The fact that the gage reads 20 psi in high-speed pump mode indicates that I am not bypassing the filter, right?
    25K gal IG plaster pool w/ spillover jacuzzi, 12 Gulf Solar coils (leaking/currently bypassed), ancient Raypak propane heater (rusted/currently bypassed), Pentair 001018 VS pump, 4.9 sq ft Triton II sand filter, Taylor K-2006 FAS-DPD kit

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    Mod Squad jblizzle's Avatar
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    Re: is there a minimum backpressure for filtering?

    Obviously it appears that it is not high enough to be read by your gauge. If you got a better 0-30PSI gauge it might.
    Jason, TFP Moderator
    18k IG pebblesheen pool, Hayward ProLogic P4 w/ T-15 SWG, Pentair 1HP 2-speed Superflo, Hayward 6020 DE filter
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    Re: is there a minimum backpressure for filtering?

    With your psi that low, you may not be pushing enough water through the filter to clean your pool effectively. I understand about efficiency (but it would be more "efficient" if you just shut it off) but there is a point where you don't cycle enough water to do much good.

    I would calculate how much GPM you are getting at that setting and see if it will turn your pool over in a reasonable time....say 12 hours.
    Dave S.
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    Re: is there a minimum backpressure for filtering?

    how do you recommend estimating the flow rate? I don't have a flow meter. As an engineer, I know that the number of assumptions required to get a number can produce wildly divergent results, especially since my plumbing is complex and buried
    25K gal IG plaster pool w/ spillover jacuzzi, 12 Gulf Solar coils (leaking/currently bypassed), ancient Raypak propane heater (rusted/currently bypassed), Pentair 001018 VS pump, 4.9 sq ft Triton II sand filter, Taylor K-2006 FAS-DPD kit

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    Re: is there a minimum backpressure for filtering?

    It is completely normal for a VS pump running at 1000 RPM to show 0 PSI on a pressure gauge.

    Assuming you have 2" plumbing, the theoretical pressure of a pressure gauge 3' above the water line and the pump running at 1000 RPM would be 0.7 PSI and the operating point would be 28 GPM @ 6.5' of head. Since most gauges cannot read above 2-3 PSI, it appears as 0 pressure. Remember that a filters pressure gauge above water level reads a little more than 1 PSI below what it would at pump level.

    As for estimating flow rates, there are a couple ways but I have a spreadsheet in my signature "Pump Spreadsheets" that can help you do that. You just need a pressure reading at a fairly high RPM with a description of the suction side plumbing and the spreadsheet will estimate the flow rate and plumbing curve. With the calculated plumbing curve, you can then determine flow rate for any other RPM. There is also a spreadsheet that can do the same with a watts and pressure reading although it isn't as accurate as the Pump Tools spreadsheet.

    However, I really don't think you need to determine flow rate. If the water is circulating in the pool and the skimmers are working well, that is all you need to know.
    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: is there a minimum backpressure for filtering?

    I just switched to an intelliflo today (man is that thing quiet) and had the same thing happen to me except my pressure gauge is reading 2psi. I know I'm getting good flow because the SWG requires a minimum of 20 g/min before it turns on (the returns are also circulating the water well) so I imagine we are both all right.

    One question- with the reduced pressure do I need to run longer to get the correct amount of filtration and if so how much longer. I currently am running 12 hours

    Thanks
    P
    20K ,in-ground cement (pebble tek) with attached spa, Sand filter (model triton Tr-100, area-4.9 sqft, flow 100 gal/min), Pentair WhisperFlo Pump 1.5 HP Model WFE-26, Total control SWG, pool is old (around 30 years)

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    Mod Squad jblizzle's Avatar
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    Re: is there a minimum backpressure for filtering?

    Quote Originally Posted by kupermanp
    One question- with the reduced pressure do I need to run longer to get the correct amount of filtration and if so how much longer. I currently am running 12 hours

    Thanks
    P
    Run long enough that your pool looks clean enough for you:
    pool-school/pump_run_time
    Jason, TFP Moderator
    18k IG pebblesheen pool, Hayward ProLogic P4 w/ T-15 SWG, Pentair 1HP 2-speed Superflo, Hayward 6020 DE filter
    500 sqft Heliocol solar panels, ThePoolCleaner, TF-100 test kit w/ SpeedStir
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    Re: is there a minimum backpressure for filtering?

    Cool beans. Thanks
    20K ,in-ground cement (pebble tek) with attached spa, Sand filter (model triton Tr-100, area-4.9 sqft, flow 100 gal/min), Pentair WhisperFlo Pump 1.5 HP Model WFE-26, Total control SWG, pool is old (around 30 years)

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