# Thread: is there a minimum backpressure for filtering?

1. ## is there a minimum backpressure for filtering?

I just got my new Intellifo VS hooked up and programmed. I had to increase the default priming time from 20 seconds to 2 minutes, since it wasn't pulling enough water into the pipes before dropping down to low speed.

Now that I have that solved, I am running 2x a day for 4 hours @ 1000 rpm b/c I was told that is the most efficient setting. The pump reports it is consuming 125 Watts of power at the speed. The pressure gage on my sand filter shows 0 psi. There is water moving, I can feel suction in the skimmer and weak flow out of some of the return jets. When I run high speed for vacuuming I set 20 psi across the filter. So is there a minimum pressure for filtering, or is lower always better, as long as you are moving water?

2. ## Re: is there a minimum backpressure for filtering?

No minimum. As long as water is moving you are good ... And saving money!

3. ## Re: is there a minimum backpressure for filtering?

if there is flow you should have some kind of pressure in my book...
how long has it been since you installed a new \$7 gauge?
you might be by-passing the filter due to a leak in the backwash valve with a bad
I had mine get stuck to the valve this year when I opened my pool up.

4. ## Re: is there a minimum backpressure for filtering?

With a VS pump it is fairly easy to get the filter pressure below the lowest level the gauge can measure. There still is some pressure, the gauge simply can't distinguish it from no pressure. Those gauges are often cheaply made and frequently don't notice to 1 or 2 psi.

5. ## Re: is there a minimum backpressure for filtering?

well, I agree in theory there must be some finite pressure in order to drive flow
the question is whether it is high enough to be measured by a 0-60 psi gage
The gage, filter, and pump are all ~1 month old
The fact that the gage reads 20 psi in high-speed pump mode indicates that I am not bypassing the filter, right?

6. ## Re: is there a minimum backpressure for filtering?

Obviously it appears that it is not high enough to be read by your gauge. If you got a better 0-30PSI gauge it might.

7. ## Re: is there a minimum backpressure for filtering?

With your psi that low, you may not be pushing enough water through the filter to clean your pool effectively. I understand about efficiency (but it would be more "efficient" if you just shut it off) but there is a point where you don't cycle enough water to do much good.

I would calculate how much GPM you are getting at that setting and see if it will turn your pool over in a reasonable time....say 12 hours.

8. ## Re: is there a minimum backpressure for filtering?

how do you recommend estimating the flow rate? I don't have a flow meter. As an engineer, I know that the number of assumptions required to get a number can produce wildly divergent results, especially since my plumbing is complex and buried

9. ## Re: is there a minimum backpressure for filtering?

It is completely normal for a VS pump running at 1000 RPM to show 0 PSI on a pressure gauge.

Assuming you have 2" plumbing, the theoretical pressure of a pressure gauge 3' above the water line and the pump running at 1000 RPM would be 0.7 PSI and the operating point would be 28 GPM @ 6.5' of head. Since most gauges cannot read above 2-3 PSI, it appears as 0 pressure. Remember that a filters pressure gauge above water level reads a little more than 1 PSI below what it would at pump level.

As for estimating flow rates, there are a couple ways but I have a spreadsheet in my signature "Pump Spreadsheets" that can help you do that. You just need a pressure reading at a fairly high RPM with a description of the suction side plumbing and the spreadsheet will estimate the flow rate and plumbing curve. With the calculated plumbing curve, you can then determine flow rate for any other RPM. There is also a spreadsheet that can do the same with a watts and pressure reading although it isn't as accurate as the Pump Tools spreadsheet.

However, I really don't think you need to determine flow rate. If the water is circulating in the pool and the skimmers are working well, that is all you need to know.

10. ## Re: is there a minimum backpressure for filtering?

I just switched to an intelliflo today (man is that thing quiet) and had the same thing happen to me except my pressure gauge is reading 2psi. I know I'm getting good flow because the SWG requires a minimum of 20 g/min before it turns on (the returns are also circulating the water well) so I imagine we are both all right.

One question- with the reduced pressure do I need to run longer to get the correct amount of filtration and if so how much longer. I currently am running 12 hours

Thanks
P

11. ## Re: is there a minimum backpressure for filtering?

Originally Posted by kupermanp
One question- with the reduced pressure do I need to run longer to get the correct amount of filtration and if so how much longer. I currently am running 12 hours

Thanks
P
Run long enough that your pool looks clean enough for you:
pool-school/pump_run_time

12. ## Re: is there a minimum backpressure for filtering?

Cool beans. Thanks

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