Become a TFP Supporter Pool Math Forum Rules Pool School
Results 1 to 11 of 11

Thread: Need help choosing correct filter size

  1. Back To Top    #1

    Join Date
    Mar 2010
    Location
    Dallas, TX
    Posts
    61

    Need help choosing correct filter size

    Hi all,

    I need to upgrade my pools filter and possibly a new pump. I need help in figuring out what size filter to get. I want to replace my current ancient cartridge filter with a pentair FNS plus DE filter.

    The previous owner left us a folder with some papers on the pool and even the builders plans/sketches from 1985 when the pool was built.

    What info do I need to know to pick the correct size filter?

    thanks

  2. Back To Top    #2

    Join Date
    Mar 2010
    Location
    Dallas, TX
    Posts
    61

    Re: Need help choosing correct filter size

    I should say that I just bought the house and am a first time home/pool owner. I emptied the pool, power-washed it and refilled it.

    I've been reading a lot online, and also got some quotes from local pool companies to replace my filter.

    I'm a bit confused on how to pick a new filter. I want to go with a DE Pentair FNS plus filter but dont know if I can use the 24 sq ft or 36 sq ft.

    My pool is a plaster, free form 18ft x 33ft and 3 ft in the shallow end and either 8" or 8.5" feet in the deep end.
    A pool guy that came out to put chemicals in said my pool is about 15,000 gallons but using the online calculators I am getting numbers closer to 20k gallons. My pump is an American Eagle 1.5 HP and I believe I have 2 inch plumbing. I have one skimmer and one main drain. 3 jets/returns and a random hole in the side of the pool that i think is for a pressure cleaner but not sure. I dont have a spa.

    One guy I called told me that my pump is too powerful for the 24 or 36 sq ft pentair filters and that I would need to go with a 48 sq ft filter. Another guy said the 36 sq ft is more than what I need. Will my current pump cause problems/damage to the filter?


    another problem I have is that my pump has a leak and appears as if someone tried to previously repair it.


    is this something that can be fixed?

  3. Back To Top    #3

    Join Date
    May 2007
    Location
    South Central NJ
    Posts
    3,192

    Re: Need help choosing correct filter size

    1st, you have 1.5" plumbing. The pipe going in the valve shown in the picture is the give away clue. If the pipes that go in the ground are larger, replacing the 1.5" stuff will help reduce head loss.
    2nd, The 48 sq. ft is where I would steer you. It's a little larger than you might need but will not require cleaning as often, a good thing.
    3rd, The pump is a little bigger than you need. The 36 sq ft could handle it. When the time comes to finally change the pump(it is long in the tooth), drop down to 1 HP or even a 3/4 HP if your less than 50' to the pool.

    Scott
    Owner of - PoolGuyNJ LLC
    Expert Pool and Spa Repairs, Renovations, and Augmentation. Helping people decide what is the right gear for meeting their needs. Expectations Set, Expectations Met, No Surprises.

  4. Back To Top    #4
    JasonLion's Avatar
    Join Date
    May 2007
    Location
    Silver Spring, MD
    Posts
    37,879

    Re: Need help choosing correct filter size

    It is always good to get a filter a little larger than you really need. You won't need to backwash/clean it as often and it will slightly increase the overall efficiency of your system. I would suggest looking at the FNSP60, even though the FNSP48 is just fine, and even the FNSP36 would work.

    If you are getting a new pump, definitely look into getting a two speed, or perhaps even a variable speed, pump.
    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

  5. Back To Top    #5
    frustratedpoolmom's Avatar
    Join Date
    May 2007
    Location
    SWSuburban Chicago, IL
    Posts
    11,965

    Re: Need help choosing correct filter size

    I just wanted to say welcome and congrats on your new home/pool!
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
    150 Sq ft. Clearwater Cartridge Filter; Former and DISSATISFIED "Pool Frog" owner
    http://www.PerfectlyClearPoolService.com

  6. Back To Top    #6

    Join Date
    Mar 2010
    Location
    Dallas, TX
    Posts
    61

    Re: Need help choosing correct filter size

    thanks for the responses.

    so how/what determines what size filter to get? how many gallons in the pool? motor/plumbing size?

    if so, how do I determine how many gallons are in my pool? and if i get the 36 sq ft, will my motor damage the filter?

  7. Back To Top    #7
    JasonLion's Avatar
    Join Date
    May 2007
    Location
    Silver Spring, MD
    Posts
    37,879

    Re: Need help choosing correct filter size

    Normally you look at the size of the pool and estimate the number of GPM you want to aim for. Then you pick a pump that is close to that number of GPM for the kind of plumbing setup you have. Your pump is larger than I would recommend for a pool without a spa or any water features. Further you don't normally want to pair a 1 1/2 HP pump with 1.5" plumbing. It will work, but it puts more stain on everything in the system and wastes a substantial amount of electricity.

    The filter must be large enough to handle the GPM coming from the pump. Unfortunately, while it is fairly easy to guess at GPM, it can be quite tricky to figure it out with any precision. I would guess that your current pump might be putting out 80 GPM (very approximate). The FNSP36 is rated at 72 GPM, so it would be marginal in that situation. It would very likely work, they always leave some margin on the ratings, but it would be slightly inefficient and could be prone to tearing the grids if the pump is actually moving more water than that. (All assuming I'm not way off on the GPM.)

    Second, it is a good idea to get a filter that is larger than the minimum required. Larger filters mean longer times between backwashing/cleaning the filter, slightly better filtering, and a higher overall efficiency.
    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

  8. Back To Top    #8

    Join Date
    Mar 2010
    Location
    Dallas, TX
    Posts
    61

    Re: Need help choosing correct filter size

    Thanks jason. I am now leaning towards the 48 sq ft vs the 36 sq ft. If I do go with the 48 thou, in the future when I get a new pump can I go down to a 1HP pump? and if I do, can it have a negative effect on water pressure or filter operation?

  9. Back To Top    #9

    Join Date
    Mar 2010
    Location
    Dallas, TX
    Posts
    61

    Re: Need help choosing correct filter size

    another question, If I decide to replace the pump now with a pentair 1HP whisperflo or superflo, will the DE pentair fns plus 36 sq ft filter be ok?

    I am trying to figure out what the best thing to do is. The filter has to be replaced no matter what. The pump works but has that leak (pic posted earlier) and I think the builder oversized the pump for my pool. What do you guys recommend? Im afraid of getting the big 48 filter only because I have a 1.5 hp pump when I dont know how long the pump is going to last.

  10. Back To Top    #10
    frustratedpoolmom's Avatar
    Join Date
    May 2007
    Location
    SWSuburban Chicago, IL
    Posts
    11,965

    Re: Need help choosing correct filter size

    Bigger is ALWAYS better (filter-wise). Don't be afraid - even if you get the 48 now and go down in pump size in the future, you'll be extremely satisfied with the outcome, less cleaning, backwashing etc.
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
    150 Sq ft. Clearwater Cartridge Filter; Former and DISSATISFIED "Pool Frog" owner
    http://www.PerfectlyClearPoolService.com

  11. Back To Top    #11
    mas985's Avatar
    Join Date
    May 2007
    Location
    Pleasanton, CA
    Posts
    11,482

    Re: Need help choosing correct filter size

    Quote Originally Posted by heyitsme06
    Thanks jason. I am now leaning towards the 48 sq ft vs the 36 sq ft. If I do go with the 48 thou, in the future when I get a new pump can I go down to a 1HP pump? and if I do, can it have a negative effect on water pressure or filter operation?
    Filters tend to work better with lower flow rates and lower pressure so a smaller pump is better for at least that. And unless you need high flow rates for a spa, waterfall, etc, I would go with as small a pump as you can. I now have a 1/2 HP on a 20k pool and use it for a suction cleaner and even a decent spa spillover. Skimming is reduced slightly but you will find that you really don't lose that much flow with a lower HP pump. I use to have a full rated 1 HP pump and the 1/2 HP flow rate is only about 20% less than the 1 HP but the energy use of the 1/2 HP is close to 1/2 of the 1 HP.
    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

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •