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Thread: Backwashing frequency - pressure is 0 at filter.

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    Backwashing frequency - pressure is 0 at filter.

    Hi All,

    I built my pool as an owner/builder with a lot of research and assumptions (hopefully good ones) so if you notice that my setup as I describe it below is not common, now you know why.

    As for my question, I want to know when to backwash. I know about the increase of pressure by 10 but my pressure while running the pump for filtration is 0. Yes you read it correctly, it is zero at the filter and has been for the past 2 months (new pool). This is due to my setup. As you can see from my signature I have the Pentair variable speed pump which I run on 1100 rpms all day long. Consumes 110 Watts... virtually nothing. This according to specs should provide up to 40 GPM depending on TDH. Due to low (maybe moderate) TDH, I know I am getting at least 20 GPMs (large pipes, closeness to pool and not too many curves in the pipes) which is more than enough for turning over the 14,000 gals. My filter (Hayward S310T2) is over-sided and is loaded with zeosand which as you know will keep pressure lower than regular sand due to its low density. Additionally, not sure if this is a direct contributor to the filter's pressure (I know it helps the pump work consume less energy), but I have larger pipes, 3 inches comes in (skimmer + drains) and 2 inches for 5 returns. The inlet 3 inch size is there to support running the pump at high speeds (3000+ RPMs) when feeding my water feature (also 3 inch pipe) which is a grotto / slide 7' tall. The pressure at the filter btw will rise up to 20 when I push the pump to feed the waterfall. Anyway, I have a feeling that the pressure will stay at 0 for EVER during filtration.

    Also, as I was writing this post, I was wondering if I should compare the pressure while running the pump at higher speeds. But even then, I have a feeling that the pressure may not increase (didn't notice a change yet) as time goes by due to the other variables. So, I can convert the question to, what if the pressure never rises?

    Regards,
    Allen
    Size: 14,000 Gal Freeform
    Pump: Pentair VS-SVRS
    Filter: Hayward S310T2 with ZeoSand
    Type: IG - Stonescapes

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    Re: Backwashing frequency - pressure is 0 at filter.

    Anyway, I have a feeling that the pressure will stay at 0 for EVER during filtration
    Interesting. This same subject came up in another thread. You do not have 0 psi if you have flow......you do have psi low enough that your gauge cannot register it. If you need to know what that psi is, you will have to purchase a more sensitive gauge.

    The pressure will inevitably rise if your filter is catching dirt. Increased resistance in the filter (which the dirt provides) results in increased psi to overcome that resistance and be able to maintain the same gpm.

    I don't believe 10 psi to backwash should be a hard, unchangeable rule. I think it should be a percentage increase of your normal psi.

    i.e. If your normal psi is 20, then a percentage increase of, say, 50% would indicate you should backwash at 30 psi. Likewise, if you choose to run @ 1 psi, then an increase to 1.5 psi would indicate the need to backwash. However, just like the 10psi rule, there are some other factors involved that make it not quite so simple. In a nutshell, you eventually "learn" your pool and will be able to simply "know" when your system needs backwash/cleaning......but you will get an increase in pressure because of the increase in resistance that the debris caught in the filter provides.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Mod Squad JohnT's Avatar
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    Re: Backwashing frequency - pressure is 0 at filter.

    Your pressure is more than zero, although probably only a few psi. It may be that you need a pressure gauge with a lower range or a larger face to let you see your pressure. Is yours 0-60psi or 0-30psi? I like to get agricultural gauges with larger faces and lower ranges. Here's a link to a 0-30 gauge http://www.intheswim.com/shopping/produ ... word=E2435
    TFP Moderator
    20K Gallon 20X36 Vinyl Inground
    Hayward S244T Sand Filter with 1HP Whisperflo Pump. Liquidator C-201 and Solar Heat

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    Mod Squad Bama Rambler's Avatar
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    Re: Backwashing frequency - pressure is 0 at filter.

    I would use the waterfall pressure to determine when to backwash.

    If you don't get a lot of debris in the pool it may well be a long time between backwashes. IIRC Dave (duraleigh) only backwashes his monster once a year or less. I only backwash about twice a year and I get tons of stuff in the pool. It's entirely possible to run a pool all season without the pressure rising.
    Dave J. TFP Moderator
    24' x 52" Round AGP. 2hp/¼hp SPL Power-Flo 2-speed pump. 200sqft Waterway Cartridge Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
    Pool School ----- Pool Math ----- TF-Test Kit

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    Re: Backwashing frequency - pressure is 0 at filter.

    Thank you all for the replies.

    @JohnT: I am not 100% sure about the range of the gauge as I am not there right now but I am sure it is not 30 and from memory looks like it could be 60. Thank you for the link.

    My final question is: what if I do not experience a significant rise in pressure in say 6 months or even 1 year. Should I still backwash and if so why? Is there a need to "clean" the filter or is backwashing usually only intended to reduce the workload on the pump? If there a time limit ?
    Size: 14,000 Gal Freeform
    Pump: Pentair VS-SVRS
    Filter: Hayward S310T2 with ZeoSand
    Type: IG - Stonescapes

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    JasonLion's Avatar
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    Re: Backwashing frequency - pressure is 0 at filter.

    It is always a good idea to clean the filter at least once a year. If debris is left in the filter for too long (usually years) it can become impossible to clean off.
    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: Backwashing frequency - pressure is 0 at filter.

    Thank you
    Size: 14,000 Gal Freeform
    Pump: Pentair VS-SVRS
    Filter: Hayward S310T2 with ZeoSand
    Type: IG - Stonescapes

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    Re: Backwashing frequency - pressure is 0 at filter.

    I have the same issue on my pump/filter; in low speed, the pressure doesn't register- on high speed I read 5-6 psi after backwash.
    As Jason said, an occassional backwash is desirable to ensure nothing gets permanently "glued" in you filter-
    But as long as the filter is working, and you have good flow, no reason to backflush! as debris "wedges" in the sand, it actually increases the filter efficiency!
    I backflush normally for 30 seconds (maybe!) once a week- just to gross out my son when he sees the gunk coming out!
    24ft x 52in AGP 18000 gal
    sand filter, 200#,1.5 hp single speed pump
    Wood deck (still in progress...)

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    Re: Backwashing frequency - pressure is 0 at filter.

    Cool thanks madwil. I originally planned all this so I don't have to backwash often. Never thought it would work so well. Either way, since gunk can get lodged in there I should probably backwash every once in a while just to keep stuff loose. But now I am starting to wonder... how long does it take for stuff get hard and lodged? I guess since I am running the filter 24/7, maybe not too easy for it to settle ?! I assume that gunk will get hard if water is not moving through?
    Size: 14,000 Gal Freeform
    Pump: Pentair VS-SVRS
    Filter: Hayward S310T2 with ZeoSand
    Type: IG - Stonescapes

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    Re: Backwashing frequency - pressure is 0 at filter.

    with the water flowing, it probably will take a long while- so just a periodice backwash to get rid of bulk, not even necessarily til water is clear in discharge, is what I recommend...
    The stagnant water would definitely be more concern to me!
    24ft x 52in AGP 18000 gal
    sand filter, 200#,1.5 hp single speed pump
    Wood deck (still in progress...)

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    Re: Backwashing frequency - pressure is 0 at filter.

    yep, sounds good to me. Thank you all.
    Size: 14,000 Gal Freeform
    Pump: Pentair VS-SVRS
    Filter: Hayward S310T2 with ZeoSand
    Type: IG - Stonescapes

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