A slight change to our filter suggestions

[duraleigh]

As many of you know the forum generally is a proponent of "bigger is better" when it comes to filters. We have promoted this without exception. A month or so ago someone mentioned on here about a sand or DE filter being able to maintain sufficient pressure to do an adequate backwash and it sorta' dawned on me that perhaps we were being just a tad too exuberant in our promotion of giant filters.

I am fortunate to know Brice Johnson, a Pentair filter engineer, so I wrote him this note. I hope you find his response as interesting as I did.

Hey, Brice,
On the forum, we constantly preach "bigger is better" when it comes to filters but someone brought up a good point that I'd like your opinion (and I'd like to post it on the forum).

He stated that a filter can be so big so as not to produce sufficient pressure to get the desired backwash you need. That makes sense to me and I had never thought about it. Do y'all (Pentair)think the same? certainly you need some limitation so it will backwash properly but is it almost impossible to get that big unless you GROSSLY undersize the pump?

Also, I assume that for a cart filter, you really can't get too big, correct?

Dave

Johnson, Brice
8:41 AM (12 hours ago)

Most definitely. For our sand filters, we typically recommend 20 GPM/sq ft for proper backwash.

DE filters, it all depends. Realistically, you won’t get a fully cleaned filter with backwash alone on most DE filters, particularly the really large ones (like the Quad 100). It depends a lot on the filter design, but you will typically see better backwash performance out of a grid style DE filter than a cartridge style DE filter. Its not an official spec or anything, but I would recommend at least 1 GPM/sq ft backwash for DE, ideally 2, although I understand on larger filters, that’s not really achievable. The saving grace with DE is that if you don’t have enough pump to fully backwash, you can always take it apart and hose it off.

For cartridge, you are correct. Bigger is almost always better. The only caveat there is you have to be mindful that some manufacturers will pack so much filter media in such a small area that dirt holding capacity can go down. For example, we offer a private label System3 filter with 600 sq ft of area instead of the typical 400 or 500. It actually holds less dirt per given pressure rise than the 500 b/c the interpleat spaces fill up so quickly and bridge with dirt.

 

[Bama Rambler]

That is interesting and I understand that you can get too large a sand filter if you have a small pump. Most people wind up with a 24" filter which is about 3 sqft which requires 60 gpm to backwash. Even my little Dynamo pump can outdo that when opened into the backwash line, because it has a lot less head loss than the pool return. It'll do about 80 GPM at 20 feet of head.