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Thread: Sand Filter Backwash pressure

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    Sand Filter Backwash pressure

    Is there a minimum backwash pressure for a sand filter to ensure efficient cleaning? I've never questioned this until this evening when I noticed a filter gauge reading of 2 psi during backwash (installed new filter with Quikrete Pool filter sand). Wondered if it should be higher? Startup filtering pressure reads 10 psi.

    Much thanks.
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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    Re: Sand Filter Backwash pressure

    Yeah, something's not right. What psi do you get in the "rinse" position?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Sand Filter Backwash pressure

    Needle doesn't deflect on the rinse or backwash setting. It remains on the first index marking which I assume is "0" but may also be "2" by the way it is indexed on the gauge. This is a brand new multivalve that came with the filter. The OLD filter/multivalve had force when backwashing: this new one doesn't. I put in the required amount of sand....200 lbs. I just removed the multivalve and checked the stand pipe...it's clear. I used the garden hose to flush the sand (which is level)...this didn't rectify the problem. Needle still doesn't deflect in rinse or backwash setting. Hmmm....
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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    Re: Sand Filter Backwash pressure

    Well, it is certainly unusual on a new part but I wonder if somehow your spider gasket inside the multiport is damaged/misaligned. It's reasonably quick to unscrew the head of the multiport and visually inspect that rubber "wagonwheel" or "spider" gasket to see if it is seated properly. Frankly, I don't think that's the issue but I am stumped as to what else it might be.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Sand Filter Backwash pressure

    Thanks duraleigh. I'm on my way right now to see if I can dismantle the multiport and check the gasket.
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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    Re: Sand Filter Backwash pressure

    Checked the spider gasket...it is aligned and completely glued down.
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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    Re: Sand Filter Backwash pressure

    I emailed Hayward and here is the response I got:

    "Wayne,
    The only position the filter pressure will register a
    normal pressure is in the 'filter' mode. Make a note of the clean
    starting pressure. When the pressure rises 8-10 psi above the
    starting pressure, that is your indication to backwash the filter.
    Regards,
    ******** "

    The filter is working properly according to Hayward. Just doesn't seem the sand is getting enough force to be washed thoroughly.

    Thanks for the help. It's greatly appreciated.
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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    Re: Sand Filter Backwash pressure

    Hayward gave you answer #6 straight from the experts in Pakistan. They don't understand your issue.

    With a clean pressure of 10, your backwash and rinse pressure should be somewhere reasonably close to 10....not 0 or 2.

    When you backwash, do you get what appears to be normal flow from the backwash pipe?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Sand Filter Backwash pressure

    As soon as I spin the handle to "backwash" the water flows from the backwash port as usual or "normal". When I press the pump "HI" switch, there is very little backwash pressure...as before...the needle does not deflect on the pressure gauge...it remains on the first mark. The sight glass doesn't even completely fill with water.

    When installing the new filter, I did change out the return hose (the old one was 9' and leaked). The closest pool store only had 6' hoses so I connected two of them with a 1" PVC coupling and two worm clamps. They do not leak at all. (Side note: the pool has a 6' deck around it. The return hose has to be longer since the filter cannot fit under the deck.)

    Could that be the problem: the return hose is too long?
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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    Re: Sand Filter Backwash pressure

    With a sand filter, there is quite a bit of head loss in the sand itself and the pressure gauge reading includes the pressure loss of the sand in filter mode (gauge is pre-sand) while it excludes the pressure loss in backwash mode (gauge is post-sand). So I would expect a pressure reading difference between filter mode and backwash mode. Plus in filter mode, there is extra plumbing due to the return line and eyeballs. How much depends on several factors but it is quite plausible that the return head loss would change by 18' of head (10-2 PSI * 2.31) when in backwash mode.
    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: Sand Filter Backwash pressure

    With a sand filter, there is quite a bit of head loss in the sand itself and the pressure gauge reading includes the pressure loss of the sand in filter mode (gauge is pre-sand) while it excludes the pressure loss in backwash mode (gauge is post-sand).
    I can't quite follow that. I'm having trouble envisioning how the direction of water through the sand makes much difference. My gauge seems to agree in that my clean psi is 10.5 and my backwash psi is about 11.5.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Sand Filter Backwash pressure

    I will attempt a crude picture.

    Filter Mode:

    Pump -> Backwash Valve -> Gauge -> Sand -> Backwash Valve -> Return plumbing


    Backwash Mode:

    Pump -> Backwash Valve -> Sand (reversed) -> Gauge -> Backwash Valve -> Backwash plumbing


    So in backwash mode, the pressure gauge is only measuring the pressure loss of the backwash valve and backwash plumbing. In filter mode, the gauge is measuring the pressure loss of the sand, backwash valve and return side plumbing. Different plumbing results in different pressure.

    Also, the pressure loss is dependent on the filter size and I believe in your case, you have a very large filter while the OP has a smaller filter. The pressure differential will also depend on the plumbing down stream from the filter so pipe/fitting size and number eyeballs and eyeball size will have an affect. Most AG pools have only one eyeball and usually 3/4" so the head loss is quite high just for that. Your plumbing is probably much different than the OPs plumbing so there will be a big difference in pressure experienced in backwash vs filter modes.
    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: Sand Filter Backwash pressure

    I don't want to hijack the thread.....I need to study some more.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Sand Filter Backwash pressure

    Thanks for all the great info. I misunderstood the flow of water when backwashing...thought somehow the valve was making the pump pull the water from the return line, through the sand, and out the drain pipe. I see that isn't the case so the length of the return line has nothing to do with backwash pressure.

    I appreciate the help. I'll just remove the multipart and garden hose the sand instead of backwashing....at least that will save on chemicals.
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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    Re: Sand Filter Backwash pressure

    Okay....I'm stupid. I connected the pump hose to the return port on the multivalve and the return hose to the pump port on the multivalve. Everything works fine now. The backwash PSI is 8. Thanks.
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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    Re: Sand Filter Backwash pressure

    If you discovered the problem and fixed it, "Stupid" is not the correct word. Nice work and glad it was fairly simple to fix!
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Sand Filter Backwash pressure

    Okay duraleigh.....I WAS stupid....now I SMART.

    Have a good one.
    30 ft. Round AG Esther Williams Classic (21k gal.)...installed in 2001.
    Hayward 1.5 hp 2 speed Power-Flo Matrix pump
    Hayward 21" Sand Filter
    Aquabot AG Pool Rover

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