why is pump generally before filter?

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singingpond

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Sep 15, 2013
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Connecticut
I searched a bit and didn't find an answer to a simple question - why do most pool setups push water through the filter rather than pulling it through? In other words, why is the filter downstream from the pump rather than upstream?

I recently came across the "skim filter" - a product sold for AG pools, in which the filter cartridge is in a cylindrical container just below the skimmer. Here the filter will clearly precede the pump, so it obviously can be done. Some of the smaller pump/filter combo units for smaller seasonal pools also pull the water through the filter.

Filter first would protect the pump from debris, which sounds good... So there must be good reasons why it's usually done the other way around.


Edited to add: I've uploaded the picture a second time, so it will show up directly in this thread. Not sure if there is a way to link the same picture from two different threads, but I couldn't figure out how to do that.
 

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It's easier to push water through a filter than it would be to pull it through it. As you got close to a vacuum on a pump trying to pull water through a filter, it would start cavitating and cause pump damage. The skim filter I assume is mostly for larger debris than what a regular filter would trap, and uses gravity to help carry the water through.


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Thanks, that makes sense about cavitation... The skim filter, though, is meant to hold a regular filter cartridge. I have a picture of one in this other thread:
winterizing skim filter ?

>> edited to add: the same picture is now shown in post #1, above <<

That particular one is a Waterway product, and it will hold a 50 sq ft filter cartridge. So that is meant as the main pool filter, I think. Gravity is helping it, of course. Is that kind of setup bad for the pump then?

Sorry, I don't know how to link to one of my existing attached images on the site, or I would just show the picture directly in this thread.
 
I have no idea why anyone would use one of those instead of a regular filter. From the looks of it the outlet to the pump is close to 2ft blow the water surface, so it has some pressure on it. I would think the performance would drop off fast as it got dirty.


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You could maybe put a gauge that has vacuum as well as pressure between the filter and pump. Should serve as an indication that the filter is dirty as the pressure drops vs rising after the pump in a normal configuration. Anything after the pump would just give you pressure going to the jets. Not useful necessarily, but interesting to know.


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Any opinions on just how bad/dangerous an upstream main filter is for a pool pump? In other words, is this just a really stupid idea (despite the fact that there are apparently commercial products out there that take this approach), or is it only somewhat inferior to the standard setup with the downstream filter?

If one were vacuuming a pool with an upstream filter like this, would you be able to hear if the pump is getting into trouble as the filter clogs up?
 
Under perfect circumstances, where the filter is getting cleaned frequently, you may not notice much of a difference. The biggest difference being a little less flow I would think. With a vacuum, you would probably start noticing a decrease in suction before the pump started sounding weird. It would take a while for cavitation to cause damage on a pool pump; not just instant damage. I would guess the biggest problem to be the length of time you could run between filter cleanings. I think you could push water through a filter longer than you could pull water through one. Not sure what kind of difference there would actually be. Are these skimmer/filter things super cheap, or is it all just hypothetical? One thing I just thought of was a hot tub. They have a setup just like that, with the cartridge filter before the pump. About the same 2 ft of water column above the pump inlet also. I also wonder about the weight of that thing full of water with a filter. Seems like it would stress a pool wall.


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Well, I guess my question is both hypothetical and real life...

My parents are having a ~5000 gallon pond built in their back yard, and the builder installed two of these Waterway skim filters (below ground, since the pond is an inground construction). I had never seen such a thing before, so that made me curious as to why most pools have the filter downstream from the pump -- that's why I asked the question in the subject line of the thread.


On the practical side:

As for price, I just looked online, and these particular skim filters seem to run between $135 - $200. So, two of them to get 100 sq ft of filtration for $270 - $400. So, not really tremendously cheaper than getting equivalent filtration area in a standard cartridge filter unit. I don't know what a skimmer costs by itself, but I guess that cost would have to be added into the comparison.

The pump on my parents' pond is a 3-speed Speck. Would be nice not to damage it with a weird filter setup.

Interesting to hear that hot tubs use a setup like this.

I guess we don't have to worry about water weight on the pool wall, since these things are underground in my parents' installation; however, it does cause me some concern as the manufacturer evidently didn't plan for them to be installed below grade.
 

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As an aside to the main topic of this thread, I am also trying to figure out how to vacuum with two parallel skimmers installed. No valves anywhere. Would one just put a cover of some kind over the top of the other skimmer basket, so as to get adequate suction from the skimmer where you attach your vacuum plate? Kind of weird, but I guess you would end up vacuuming through only one of the two filters.
 
If it's not in yet, definitely have them run two lines with valves closer to the pump where you could easily isolate one. Otherwise you would have to plug the hole under the filter each time.


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Unfortunately for ease of vacuuming, the lines are all installed and backfilled. I believe they Teed the two units together and ran a single pipe over to the pump. As I understand it, the reason for putting in two of the skim filters was simply to get more square feet of filter area -- I think the units don't come any larger than 50 sq ft each.

My parents have no previous pool experience, so these sorts of questions would not occur to them while watching the construction. I guess we will figure out how to make vacuuming work as is.

Thank you very much for all your comments thus far. Very helpful! Did you have concerns about winterizing (as you asked that question earlier)?
 
Built-in filtration | Desjoyaux
This manufacturer uses a suction side filter bag. There are commercial vacuum de filters. They can be ok in some circumstances. one key is to keep the filters very clean to reduce suction side head loss. Overall, I don't think that they are a good choice in most cases.

Most spas/hot tubs use a suction side cartridge filter.
 
Thanks for the comments! Both of you have pointed to the greater importance of keeping an upstream filter clean.

On the practical side, since these units are installed in a pond (the intent is to try to maintain chemistry as in a swimming pool, not to run wild) I now have increased misgivings about this upstream filter approach. There is no reason to think there will be less debris than in a typical small pool. Most likely, given the relatively large surface area of the water, the presence of underwater rocks, and the already frequent visits by birds, there will be more debris. Also, at the moment there is a lot of cement dust that has settled and needs to be vacuumed.

The builder is telling my parents that cleaning the filters once per year will be enough -- I suppose that we will have to be much more cautious about filter cleaning.


I wonder if the suction side cartridge filters are practical on hot tubs, simply because there is usually less stuff to filter than in an outdoor swimming pool?
 
I was just curious about winterizing as far as what creatures where going to be in the pond. I was thinking koi, but didn't remember if you had said whether it would be inhabited. I was also thinking that you could put a heater like for livestock waterers near the skimmers to keep water moving instead of actually winterizing. Don't know if that's feasible up there. My mind was just wandering. I do that sometimes.


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My parents' current hope is not to have the water inhabited by creatures, except for the occasional visit by grandchildren. We will see how that goes, as to whether we are actually able to maintain the water to pool standards (in another thread about the pond construction itself, I was cautioned that this may be very difficult or impossible, given the underwater rocks, and resulting nooks and crannies for living things and debris to hide out). Currently, the pond is being inhabited by algae, LOL, according to the latest report by my mom.
 
Waterco and Baker Hydro HydroPak Skim Filter Parts and Accessories- PartsWarehouse, there was a builder in Tucson years ago that used this filter all the time. Generally they were a pain to clean and if not maintained properly you could damage pump. If you wanted to vacuum there was a control plate in the skimmer to allow you to hookup hose too. If you had multiple skimmers you could plug one off by putting plug in one of the control plates and use the other. At the pump discharge there was a tee and two valves . One going back to pool and the other to waste, so when you pull the filter out to clean you could send dirty water and debri to waste. Some pumps would have a vac guage on trap drain plug and a pressure guage on discharge line, once the difference became higher than at startup you knew it was time to clean
 
I just found (in a thread on cruisersforum.com) an explanation that seems to lay out the problems of suction-side filtration pretty clearly. Does the reasoning seem sound?

QUOTE
At sea level the absolute maximum suction pressure available is 14.7psia. The pump (any pump) has an NPSH (net positive suction head) requirement that has to be subtracted from that. Leaving, at very best about 10psia for all the losses on the suction side of the pump. Depends on the pump, a positive displacement pump (like a diaphragm pressure pump) may have a low NPSH requirement, a straight centrifugal pump generally has a high NPSH.

After your NPSH requirement you only have the remainder of your 14.7psia to lift the water to the pump inlet and pass it through the whatever strainer or filter you have on the suction side. The net result is you need very low losses on the suction side of the pump in order for the pump to even get water.

On the discharge side, however, you have available whatever pressure the pump can make, which is entirely dependent on pump design and available power. Normal water supply pumps might put out 30psig, and losing 5psi of that in a filter is no big deal. Lose the same 5psi on the suction side and the pump will likely cavitate and destroy itself.

Anything that might result in high head losses (a fine filter) belongs on the pressure side of the pump. Only items necessary for the protection of the pump (a coarse strainer) belong on the suction side.
END QUOTE

- - - Updated - - -

kadavis said:
Waterco and Baker Hydro HydroPak Skim Filter Parts and Accessories- PartsWarehouse, there was a builder in Tucson years ago that used this filter all the time. Generally they were a pain to clean and if not maintained properly you could damage pump. If you wanted to vacuum there was a control plate in the skimmer to allow you to hookup hose too. If you had multiple skimmers you could plug one off by putting plug in one of the control plates and use the other. At the pump discharge there was a tee and two valves . One going back to pool and the other to waste, so when you pull the filter out to clean you could send dirty water and debri to waste. Some pumps would have a vac guage on trap drain plug and a pressure guage on discharge line, once the difference became higher than at startup you knew it was time to clean
Click to expand...
Interesting to hear how these units worked in practice. Interesting. Not very reassuring.
 

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