filter sizing question

[singingpond]

We have been running for the past three years with the pump and filter listed in my signature (old Intex 2500 gph pump -- current equivalent model is 633 I believe, and size B filter -- all of 15 square feet). With careful attention to chemistry, this works, for the most part, but is hardly ideal. The pump is now failing, which gives us a chance to upgrade.

I have pretty much settled on a replacement pump -- Pentair Optiflo 347990, which is shown as 1 HP, 2 speed. (here is an Amazon link:
Amazon.com : Pentair 347990 OptiFlo Vertical Discharge Aboveground Pool Pump with 2 Speed Motor and Standard Plug, 1 HP : Swimming Pool Water Pumps : Patio, Lawn Garden). I hope to get the Optiflo pump to finish out this year's season (i.e. hook it up, make sure it runs, etc.). However, I may not be able to upgrade the filter until next year. I would plan to run the pump on low speed, to hopefully not overwhelm our dinky B size filter, during the remaining few weeks of the swim season.

My main question has to do with recommended filter size to go with the Optiflo pump. If I'm looking at the pump curves properly, I think we're looking at about 48 GPM at high speed, and about half of that at low speed for this pump (by the way, how does one decide which of the pump curves -- A, B, or C -- is most appropriate to use?). If I consider a cartridge filter, what is the minimum size that would be sensible with this pump? Do I go by the low speed flow rate (which is what we would be using most of the time for routine filtration, especially with only 5500 gallons) or by the high speed flow rate (since the filter would have to stand up to high speed flow during vacuuming, for example)?

If we go with a sand filter, how big should that be at minimum?

If anyone has any comments with regard to pump choice, I would be happy to hear that as well, of course!

 

[Melt In The Sun]

I think first you need to decide what kind of filter you want (sand, cartridge, DE). That will allow you to refine the options. You should definitely get one that is rated for the higher flow rate, but I doubt that any real filter will have a problem with that.

 

[jblizzle]

Generally the filter is sized for the pool size, season length, and debris load. If you do that, then it is generally plenty large for the pump. I would not worry about the head curves, it is not trivial to determine your pools headloss in order to estimate flow rate.

Maybe this helps you pick your filter type first: Pool School - Pool Filter Comparison

 

[singingpond]

I know I want either cartridge (which is what we have experience with, but going bigger) or sand. According to Melt in the Sun's post, I should look for something that matches the high speed of the dual speed pump. My confusion comes when I look at the filter sizing chart (in post #1 of Hydraulics 101). For pump flow rate of 50 gpm (which I think is close to the high speed rate for the Optiflo pump) it suggests a 150 sq ft cartridge filter, or 3.69 sq ft in a sand filter. I looked it up, and that's a 27" tank. Both of these options sound unnecessarily large for our pool.

I don't think the pump I've suggested is all that big, and we might want to go to a larger pool in a year or two (the typical pool creep that happens around here), and would like the equipment to potentially transfer over to a larger pool (maybe 3x the size we have now, as a rough guess).

So, what am I missing? A 150 sq ft cartridge filter, or 27" sand filter, both sound enormous for this relatively small pump, don't they? Never mind how it sounds for our 5500 gallons of pool water...

 

[jblizzle]

My guess is you will not actually have 50 GPM ... I only get about 40 GPM (I have a gauge) out of mine. EDIT: Well, maybe you could since your plumbing is likely much simpler than mine. How often would you really be running on high?

You have a small pool and a "real" pump so there is a little mismatch there. Sounds like maybe you should be sizing for a 15k pool if you are going to upgrade.

For 15k, I would suggest about a 200 sqft cartridge filter or 23" sand.
For your existing 5.5k, I would suggest a 75 sqft cartridge or 13" sand.
Or larger in either case.

I can not see the charts in Hydraulics post right now for some reason, so I am not sure how to explain the discrepancy you appear to be seeing.

 

[singingpond]

Thanks! I still wonder about the filter chart in Hydraulics 101, and hope a few more people may chime in with opinions. I can't just copy and paste the chart, so I hope people are able to look at it
Hydraulics 101 - Have you lost your head?

I have noticed that pre-packaged systems seem to have relatively smaller filters than what is suggested by the chart (e.g. 1 HP pump with 19" or even smaller sand filters seems quite common on the market). I'm a believer in bigger-is-better for filters, but was still surprised at how big the filters suggested by the Hydraulics 101 chart appeared to be.

To answer one question, I wouldn't be running the pump on high very much, certainly not for regular filtration in the current 5500 gallon pool. However, I would expect to be able to run it on high for vacuuming (without just blasting debris through an undersized filter). Also, another previous poster said that the filter should be sized to match the higher output setting of a dual speed pump.

A 200 sq ft cartridge filter is probably more than we want to spend, especially adding in the occasional cost of replacement cartridges (not cheap at that size), so that would lead me to look more closely at sand filters. Arguing in the other direction, we have iron in our well water, so the water always turns brownish on chlorinating a new fill as the iron precipitates out. The small cartridge filters we have been using filter all that stuff out by the end of the first few days of the swimming season (very convenient for us). I have some concern that a sand filter (with larger particle size going through) might not work as well for that function. No way of knowing without trying it, I guess.

 

[jblizzle]

You could certainly PM Mark and ask him about the chart. I do read that it is based on a fairly high turn over rate that is important for public pools, but we now consider the turn over rate to be very unimportant in residential pools.

So, as long as the filter is rated at least 50 GPM (just a guess of your pump/head), it would be fine, but as also noted in Hydraulics 101, bigger is better as far as filter cleaning is concerned.

 

[mas985]

The text above the table is important to read:

As for sizing the filter, the following table is derived from the APSP-15 and ANSI/NSF 50 Specifications and shows the minimum recommended filter size based upon the specifications maximum GPM/Sq-Ft for each filter type. These requirements are shown in the APSP-15 standard and are now being adopted as regulations by many states for both public and residential pools.

It is your choice if you want to follow those or not. However, if you downsize by more than 25% from the sizes in the table , then you will likely exceed the maximum flow rate of the filter which is never a good idea.

 

[singingpond]

The text above the table is important to read:
It is your choice if you want to follow those or not. However, if you downsize by more than 25% from the sizes in the table , then you will likely exceed the maximum flow rate of the filter which is never a good idea.

Thanks for the response! Am I correct, then, in how I am reading the information in Hydraulics 101... i.e. that the Pentair Optiflo 347990 (1 hp dual speed pump) is rated to produce about 48 gpm at high flow (depending on plumbing losses), and that that in turn suggests the seemingly large filter sizes (150 sq ft cartridge, or 3.69 sq ft sand filter) that are shown in the filter table for that flow rate? I guess I wondered if I was reading something wrong, since those filter sizes seem quite a bit larger than what most people are running with smallish pumps (and also larger than the filters in commercially packaged systems).

I understand what you said about 25% less filter probably being OK -- thanks for that ballpark number; I wanted to be sure, though, that I had the initial values correct.

All this does make me wonder how my current setup is working at all for filtration -- the Intex pump is described as 2500 gph, which would be roughly 40 gpm; and that is all going through a 15 sq ft cartridge filter. I suppose the 2500 number is probably pretty meaningless in reality, but it still seems amazing that some filtration is, in fact, taking place with this arrangement.

 

[mas985]

No, you are reading it correctly and yes it is bigger than most people use in their pools but that is the point. Most people use filters that are far too small and one the reasons they have issues with their filters. The table is to help prevent that from happening.


Also, I think the Intex pumps are rated at the end of the head curve (i.e. little to no head loss) so not very meaningful.

 

[singingpond]

Thank you very much!

I note that the equipment in your signature is itself an illustration of the concept -- 1/2 HP dual speed pump with a 420 sq ft cartridge filter. That also helps assure me that I'm heading in the right direction as I contemplate filter choices over the coming winter. Gotta say, a 27" sand filter next to our modest pop up pool would be a conversation starter.

 

[mas985]

One option is a combo filter pump system. They are usually matched well, especially for sand filters. This is a good one:

Pentair: PNSD0060DO2260

 

[singingpond]

So, as long as the filter is rated at least 50 GPM (just a guess of your pump/head), it would be fine, but as also noted in Hydraulics 101, bigger is better as far as filter cleaning is concerned.

Thank you! A useful summary of what to keep in mind during the filter search.

 

[singingpond]

Interesting chart from an ebay listing for a Hayward Star Clear cartridge filter -- note the difference in recommendation for residential versus public pools. Exactly the same filter is specified for only about one-third the flow rate for a public pool versus a residential pool. So that has to be a filtration quality / regulatory difference in the public versus residential settings (since the filter part numbers are identical in the two charts). I thought this was an interesting confirmation of some of the discussion earlier in this thread, and perhaps useful to anyone reading through this thread in the future.

 

[mas985]

0.375 GPM/sq-ft comes from the NSF 50 document. But I would not follow the residential recommendations as that is way too much flow rate for those filters. The cartridges will not last very long under that stress.

 

[singingpond]

So... the manufacturers basically are making up different numbers for the residential market (and putting them out to the public in charts like the one above), even though the cartridges will actually be over-stressed by those residential "design flow rates" (to quote from the authoritative-sounding column heading in the chart).

Not sure what adjective to use for this sort thing -- marketing trumps reality, I suppose.

The chart was interesting to me because of the identical model numbers appearing in the two parts of the chart. Clearly the same physical object cannot simultaneously be designed to work best at 120 GPM and at 45 GPM... but that is what the chart is implying.