What size DE Filter?

BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
Hi, I have an 11,576 gallon pool. In the summer I try and turn it over 3x a day during the lowest air electric rate time period. I run the VSP for 16 hours at 2350rpm at a flow rate of 23 gpm and 2 more hours at 3450 rpm at a flow rate of 45 rpm (for the skimmer and pool sweep). I want to switch from a cartridge filter to a Pentair FNS Plus DE filter. What is the optimal size? FNSP 36 or 48? My pool gets a lot of leaves in it throughout the year. Thanks!
 

JamesW

TFP Expert
Mar 2, 2011
20,740
Why don't you like the cartridge?

For DE, I would go 60.

In the summer I try and turn it over 3x a day during the lowest air electric rate time period.
Turnover is mostly a myth. There's no reason to try to even get 1 turnover.
I run the VSP for 16 hours at 2350rpm at a flow rate of 23 gpm and 2 more hours at 3450 rpm at a flow rate of 45 rpm (for the skimmer and pool sweep).
How are you getting those flow rates?

For and Intelliflo at those speeds, those flow rates are way too low.

You're probably running the pump way too much. The speed is excessive and the time is unnecessary unless you turn the speed way down.
 

BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
Why don't you like the cartridge?

For DE, I would go 60.


Turnover is mostly a myth. There's no reason to try to even get 1 turnover.

How are you getting those flow rates?

For and Intelliflo at those speeds, those flow rates are way too low.

You're probably running the pump way too much. The speed is excessive and the time is unnecessary unless you turn the speed way down.

Thank you so much for your response.

We don't like the cartridge filter because it doesn't clean our pool as well as our old DE filter. We want to go back to what we had. We also find the cartridges pretty difficult to clean (we get lots of little things that fall off of our Sequoia tree into the pool and they get stuck deep in the cracks of the cartridge filter).

As to the flow rate, I installed a Flow Vis flow meter on my pool just so I could measure the flow and set my pump speeds correctly and scientifically. I am interested in your thoughts on the flow being too low for those speeds. I have Fafco rooftop solar that was fully on when I took the measurements. I could try turning it off to see if that changes the flow rate. I would be interested in any other thoughts you have on this.
 
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BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
Thank you so much for your response.

We don't like the cartridge filter because it doesn't clean our pool as well as our old DE filter. We want to go back to what we had. We also find the cartridges pretty difficult to clean (we get lots of little things that fall off of our Sequoia tree into the pool and they get stuck deep in the cracks of the cartridge filter).

As to the flow rate, I installed a Flow Vis flow meter on my pool just so I could measure the flow and set my pump speeds correctly and scientifically. I am interested in your thoughts on the flow being too low for those speeds. I have Fafco rooftop solar that was fully on when I took the measurements. I could try turning it off to see if that changes the flow rate. I would be interested in any other thoughts you have on this.
Thank you so much for your response.

We don't like the cartridge filter because it doesn't clean our pool as well as our old DE filter. We want to go back to what we had. We also find the cartridges pretty difficult to clean (we get lots of little things that fall off of our Sequoia tree into the pool and they get stuck deep in the cracks of the cartridge filter).

As to the flow rate, I installed a Flow Vis flow meter on my pool just so I could measure the flow and set my pump speeds correctly and scientifically. I am interested in your thoughts on the flow being too low for those speeds. I have Fafco rooftop solar that was fully on when I took the measurements. I could try turning it off to see if that changes the flow rate. I would be interested in any other thoughts you have on this.

As a test, I just turned the solar off . At 3450 rpm, I get 58 gpm flow and at 2350 I get 32 gpm of flow. So it looks like the solar is a significant source of drag. Does that make sense to you? Do the numbers still seem to low? Thanks!
 

JamesW

TFP Expert
Mar 2, 2011
20,740


To get the flow at a particular speed, you need to know the total head, which is the suction and filter pressure added up and multiplied by 2.31.

We don't know the suction pressure, so we will approximate it at about 1/3of the filter pressure, which may or may not be accurate. You would need a vacuum gauge to tell for sure.

For example, if the filter pressure is 15 psi, we will estimate the suction to be 5 psi for a total of 20 psi. 20 psi x 2.31 = 46 feet of head.

At 46 feet of head and 3110 rpm, you should get about 124 gpm.

You have to estimate where the curves are for the speeds between those shown.

You don't want the operating point to be at the top of the curve where it's flat or all the way to the right of the curve.

58 gpm at 3450 rpm puts you at about 93 feet of head (about 40 psi) which is too close to the flat part of the curve.

32 gpm at 2350 also puts you near the flat part of the curve, which indicates too much restriction on the suction or pressure side.

What is the filter pressure and gpm at a few different speeds including those in the graph above?
 
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JamesW

TFP Expert
Mar 2, 2011
20,740
If your system is working correctly, the operating point should be somewhere in the center third of the curve with a preference to the right side of the range.

For 3,450, you should be between 70 to 140 gpm.
For 3,110, you should be between 60 to 120 gpm.
For 2,350, you should be between 45 to 90 gpm.

How high is solar?
 
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Richard320

TFP Expert
LifeTime Supporter
Jan 6, 2010
23,930
San Dimas, CA (LA County)
We don't like the cartridge filter because it doesn't clean our pool as well as our old DE filter. We want to go back to what we had. We also find the cartridges pretty difficult to clean (we get lots of little things that fall off of our Sequoia tree into the pool and they get stuck deep in the cracks of the cartridge filter).
This jumped out at me.

There's an easy solution. Stretch a hairnet or a knee-high nylon over your skimmer basket. It'll catch some pretty fine stuffso your filter can concentrate on just filtering microscopic stuff. Pictures Skimmer Sock/Nylon Illustrated
 
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BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
If your system is working correctly, the operating point should be somewhere in the center third of the curve with a preference to the right side of the range.

For 3,450, you should be between 70 to 140 gpm.
For 3,110, you should be between 60 to 120 gpm.
For 2,350, you should be between 45 to 90 gpm.

How high is solar?
Thank you James,

The solar is a 7 panel 4 x 10 array on top of a one story house. The top of the panels is 108" above ground (where the pump is located.) The pump is about 8" feet above the grade of the pool and is 30' away from the skimmer.

I dont have anywhere near those numbers. With the solar OFF, I have:

RPM. Filter Pressure. GPM

750. 0. 0
1500. 5. 8
2350. 12. 32
3110. 20 50
3450. 25. 58

Something is not right here. What do you think? Thanks for your help!
 

JamesW

TFP Expert
Mar 2, 2011
20,740
Assuming the numbers are accurate, the suction seems to have a problem. The head loss on the suction seems to be about equal to what you would get with 100 feet of 1.5” pvc pipe, which is way too small for the suction of the pump.

What is the actual plumbing size and length of the suction plumbing?

Are you pulling from 1 or two skimmers?

Are you pulling from the main drains at the same time?

You might have a clog in the suction plumbing or baskets or maybe a clog in the impeller.

You might need to get a vacuum gauge to check the suction pressure. You can screw it into the drain plug port of the pump. It’s 1/4” NPT thread. Just don’t over tighten the gauge or it will crack the drain plug hole.

What is the actual model number (part number) from the front barcode sticker on the pump?

The silver sticker should have a barcode, a date and a P/N (part number).
 

BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
Assuming the numbers are accurate, the suction seems to have a problem. The head loss on the suction seems to be about equal to what you would get with 100 feet of 1.5” pvc pipe, which is way too small for the suction of the pump.

What is the actual plumbing size and length of the suction plumbing?

Are you pulling from 1 or two skimmers?

Are you pulling from the main drains at the same time?

You might have a clog in the suction plumbing or baskets or maybe a clog in the impeller.

You might need to get a vacuum gauge to check the suction pressure. You can screw it into the drain plug port of the pump. It’s 1/4” NPT thread. Just don’t over tighten the gauge or it will crack the drain plug hole.

What is the actual model number (part number) from the front barcode sticker on the pump?

The silver sticker should have a barcode, a date and a P/N (part number).
Thanks for staying with me here!

The plumbing is 2". The pool is about 30 years old. My best guess is that the suction plumbing from the pool to the pump is about 40 feet.

The pool has a hot tub built into it. All of my calculations were taken with the pump pulling from the skimmer in the pool (there is only one) and the drain at the bottom of the pool. The pull on the drain at the bottom of the pool appears to be pretty faint.

The pump is a pentair Intelliflo Variable Speed 354605. The motor appears to be Model 353114 Rev C. I do not see a barcode or anything with a date.

I have also attached a couple of pictures of the plumbing if that helps.

Thanks so much!!
 

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JamesW

TFP Expert
Mar 2, 2011
20,740
It looks like you have a check valve in front of the pump.

Can you verify that it is a check valve?

Do you need a check valve?

The pump is about 8" feet above the grade of the pool and is 30' away from the skimmer.
Is the pump 8 inches or 8 feet above the pool?

Does the main drain have a separate line all the way to the pump or does it tie into the bottom of the skimmer?
 
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BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
It looks like you have a check valve in front of the pump.

Can you verify that it is a check valve?

Do you need a check valve?


Is the pump 8 inches or 8 feet above the pool?

Does the main drain have a separate line all the way to the pump or does it tie into the bottom of the skimmer?
Thank you.

That IS a check valve in front of the pump. It has always been there. Not sure if it is needed.

The pump is 8 inches above the pool.

I dont know if the main drain ties into the skimmer. I don't have a good way to determine that. From what I can see above ground, there is one return line from the hot tub and one return line from the pool. I dont know where the drain line and the skimmer line for the pool are tied together.
 

JamesW

TFP Expert
Mar 2, 2011
20,740
The available information is indicating that the suction side has excessive restriction.

I would get rid of the check valve. I don't see any reason to have it there and it might be clogged with debris.

Does the pump get really loud at full speed like it's cavitating?

You can put a Jandy check valve between the pump and filter if you want, which I would recommend if you switch to a DE filter.

Remove the check valve and recheck the readings for flow to see if there is any improvement.
 

JamesW

TFP Expert
Mar 2, 2011
20,740
If the hot tub is elevated above the pool, you will need a check valve on the spa return line if you do a spillover mode.
 

BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
Thank you for all of this help. I am going to measure the suction with a pressure gauge. My pump has two drain plugs on the side, one under the basket and one a few inches behind it. Both plugs are on the side. Does it matter which plug I test from? Thanks!
 

BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
Thank you. I am going to get rid of the check valve between the skimmer return line and the pump as you suggested. It will take me a few days to get it done, but I will report back.

Also, you asked whether the pump gets loud and sounds like it is cavitating. The answer is no.

Thanks so much for your help.
 

BlueBaron

Bronze Supporter
Aug 27, 2020
47
Orinda, California
The one under the basket has the suction.

I would definitely get rid of the check valve.
James, I am attempting to measure the suction on the pump using a vacuum gauge. I have a Robinair Digital Micron Vacuum Gauge. Should that type of gauge work? I can send you a picture of not if that would help. Thank you very much for helping me!