View Full Version : Sizing DE filter for inground pool
Vern805
03-23-2014, 07:44 PM
I am changing out an old purex cartridge filter with DE. How do I chose the filter size correctly offer a 14,000 gallon pool?
PoolDoc
03-23-2014, 10:10 PM
DE filter flow is typically 1 - 2 GPM per square foot of filter membrane. Design flow for most residential pools is one equivalent volume every 8 hours.
Thus:
14,000 gallons / (8 hours x 60 min/hr) = 29 GPM
and
29 GPM / 2 (GPM/sft) = ~15 sft minimum size . . . or 30 sft for a more conservative rating.
With DE filters, bigger is usually better. In California, with oversize DE filtration, you might be able to run the entire season without re-coating, BUT there's a complication. Most DE filters 'drop' the DE off the membranes anytime the pump turns off. And while there you can OPERATE a DE filter with a rather low flow, you can't usually RE-COAT the membrane at that low flow. So, you need to match the DE filter with either a VS or 2-speed pump that runs ALL the time. Otherwise, you're going to have to pick a DE filter that recoats itself well. I think maybe the Hayward Perflex does so, but I'm not sure. I know that the otherwise desirable StaRite System3 DE filters do not recoat well at low flow.
By the way, ONLY the DE System3 is desirable; the sand System3 is inferior to Pentair's own TR series, and the cartridge System3 is nothing short of awful.
Vern805
03-24-2014, 02:36 PM
I just replaced the single speed pump last year so running all the time is not an option. Maybe I should stick with the cartridge system? Any suggestions?
PoolDoc
03-24-2014, 05:46 PM
My experience with DE filters is limited to commercial applications . . . but the filters I've worked with would NOT function well in a system where the pump is cycled on and off. However, on/off cycles are not a problem with sand filters, or with cartridge filters. My personal preference would be an oversized sand filter, but you have to have somewhere to backwash to . . . besides your neighbor's backyard.
Interesting.
I've been using a Hayward Pro Grid DE filter (60ft2), single speed 1hp pump which cycles on/off per timer settings, with my 20k gal pool since 2004 without any apparent issues. What kind of issues would one experience if the grids weren't recoating properly?
Once the pool is clean after spring startup, I may have to backwash and recharge once during the season, which runs early May to early Oct here (last season I didn't have to backwash until fall closing!).
PoolDoc
03-25-2014, 10:32 PM
On the commercial pools I've worked with? The grid membranes clog and have to be cleaned manually.
On a home pool? I'm not sure -- that's why I expressed some reservations, both about using a DE filter AND about my understanding of them in residential applications.
IF your filter is recoating properly, on start-up, there wouldn't be any problem. If there were problems, you'd be likely to see them as areas on the grids that were NOT coated with DE, when you open the filter for manual cleaning. Filter design matters in this regard. I know that some DE filters are designed so that the DE -- which tends to fall to the bottom of the filter when the pump is off -- is stirred up when the flow is restored. I suspect that some filters have mastered this design, but the System3 seems not to be one of those.
Sizing matters in an on/off situation, too. If the filter is oversized, then the normal flow may not be enough to 'unpack' the settled DE from the bottom of the filter.
I just compared the System3 and ProGrid designs, and the ProGrid *appears to be* setup more carefully than the System3, with respect to recoating. The System3 has an oversized exit port on the influent line, reducing flow velocity at the point where high velocity is needed to 'churn up' the settled DE. The ProGrid has either a standard, or restricted, size influent point, increasing velocity.
http://www.haywardnet.com/pdfs/replace_pdfs/128-129Hay2009.pdf
http://www.pentairpool.com/pdfs/S284COM.pdf
On the commercial pools I've worked with? The grid membranes clog and have to be cleaned manually.
On a home pool? I'm not sure -- that's why I expressed some reservations, both about using a DE filter AND about my understanding of them in residential applications.
IF your filter is recoating properly, on start-up, there wouldn't be any problem. If there were problems, you'd be likely to see them as areas on the grids that were NOT coated with DE, when you open the filter for manual cleaning. Filter design matters in this regard. I know that some DE filters are designed so that the DE -- which tends to fall to the bottom of the filter when the pump is off -- is stirred up when the flow is restored. I suspect that some filters have mastered this design, but the System3 seems not to be one of those.
Sizing matters in an on/off situation, too. If the filter is oversized, then the normal flow may not be enough to 'unpack' the settled DE from the bottom of the filter.
I just compared the System3 and ProGrid designs, and the ProGrid *appears to be* setup more carefully than the System3, with respect to recoating. The System3 has an oversized exit port on the influent line, reducing flow velocity at the point where high velocity is needed to 'churn up' the settled DE. The ProGrid has either a standard, or restricted, size influent point, increasing velocity.
http://www.haywardnet.com/pdfs/replace_pdfs/128-129Hay2009.pdf
http://www.pentairpool.com/pdfs/S284COM.pdf
Thanks Ben.
I've never opened up the filter for manual cleaning without back washing first (btw, I take it apart and manually clean it at the end of each season), so I can't say for sure if the grids recoat properly when the pump starts up. However, I've never had any problems with the water looking like it wasn't being filtered well nor any issues with the filter itself, so I'm assuming it's functioning properly without running the pump 24/7.
Overall I've been very happy with our DE filter. :)
Phillbo
03-26-2014, 03:00 PM
I have a Pentair FNS DE filter and when I've pulled it apart without back back washing first the grids were coated completely.
PoolDoc
03-26-2014, 03:14 PM
Thanks, that's helpful info.
Phillbo
03-26-2014, 07:43 PM
Even tho I stuttered :)
PoolDoc
03-26-2014, 08:06 PM
Some really dirty filters NEED to be back back washed. ;)
Vern805
03-26-2014, 10:46 PM
Thanks for the input folks! I am back to considering the DE. Can anyone give me an idea of the annual maintenance costs for either the fnsp48 or comparable Hayward model?
Thanks for the input folks! I am back to considering the DE. Can anyone give me an idea of the annual maintenance costs for either the fnsp48 or comparable Hayward model?
For my DE filter it has been next to nothing....maybe $10 a year for DE?