Pentair filter 420 vs 520 initial pressure differences?

AB

Well-known member
Jan 18, 2011
75
Houston, TX
Hi,
I had a Pentair cartridge filter and used 420's for many years. It died and I got a slightly larger of a similar Pentair model, the salesman said it could take 420's or 520's.
I installed a new set of 4 520's yesterday and the initial pressure reading seemed higher. With the 420's a new clean filter would start at 12 PSI, with the 520 it seems to start at 16 PSI.

Am I right in thinking that since the total square area of the 520 is ~ 25% greater than the 420, the initial pressure reading would be 25% higher as well?
 

Jimrahbe

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Jul 7, 2014
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Bedford, TX
the salesman said it could take 420's or 520's.
AB,

What does that mean? It does not make sense to me that the 520 could be run with 420 sized cartridges... :scratch: Not sure it can't, just does not make sense that it could???

I have a VS pump, so my filter pressure on the 520 is only about 1 lb. at 1200 RPM..

I agree with your assumption that a 520 would have less filter pressure than a 420... When new, not sure it would be a lot, but I can't see why it would be 4 lbs more??

Were both measurements taken in the same pool/spa/waterfall/heater configuration???

Thanks,

Jim R.
 
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duraleigh

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Am I right in thinking that since the total square area of the 520 is ~ 25% greater than the 420, the initial pressure reading would be 25% higher as well
No. Bigger filter = lower pressure.
 
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Jimrahbe

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Am I right in thinking that since the total square area of the 520 is ~ 25% greater than the 420, the initial pressure reading would be 25% higher as well?
AB,

Well, I read that sentence backwards in my original post above.. Duh!!!

As Dave says, the larger the filter the less the back pressure..

Filter pressure is really the amount of effort it takes for your pump to force water through the filter and the rest of the rest of the return plumbing.. The lower the pressure the better.

Thanks,

Jim R.
 

AB

Well-known member
Jan 18, 2011
75
Houston, TX
Great info! thanks!
AB,

Well, I read that sentence backwards in my original post above.. Duh!!!

As Dave says, the larger the filter the less the back pressure..

Filter pressure is really the amount of effort it takes for your pump to force water through the filter and the rest of the rest of the return plumbing.. The lower the pressure the better.

Thanks,

Jim R.
Thanks for the info! I was not completely clear in my filter upgrade. My old filter used 420's. When I upgraded to a new one, it was larger, and was designed for 520's but could run 420's (because I am cheap and wanted to reuse my old 420's that were in ok shape). Finally it came time to replace the filters so I debated 420 vs 520 and went with the 520. One of the reasons was that 4 520's had come down in price and were around $180 shipped...vs $150 for the 4 420's....so I figured I would get a bit of an upgrade as well using 520's.
 

Jimrahbe

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AB,

Are the 420 and 520 cartridges the same physical size, but the 420's just have less filter material??? I am just curious how the 420 cartridges fit into the 520 filter??

Thanks,

Jim R.
 
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AB

Well-known member
Jan 18, 2011
75
Houston, TX
AB,

Are the 420 and 520 cartridges the same physical size, but the 420's just have less filter material??? I am just curious how the 420 cartridges fit into the 520 filter??

Thanks,

Jim R.
The 520 is about 8-10 inches taller than the 420. The other dimensions are the same. So basically when I was using 420's in my new filter (designed for a 520) the top 1/3 of the filter dome was not holding filter. The 520 takes up the entire room within the filter.
I am hoping it will give me another month or so before I have to change them. In Houston there are tons of trees dropping junk in my pool so the cartridges get pretty green.
 

Jimrahbe

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Jul 7, 2014
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Bedford, TX
AB,

What keeps the 420 filters from just moving up?? I thought that the filters were held in place by the upper lid???

When you say "change them" are you talking about installing new cartridges or just swapping the dirty ones with already cleaned ones?

My 520 cartridges are over 6 years old and seem to be working just fine..

Thanks,

Jim R.
 

JamesW

TFP Expert
Mar 2, 2011
22,013
The 520 cartridges are 6" taller than the 420 cartridges. The spring is supposed to touch the top to keep the filters secure.

The cartridges can stay in place if the spring isn't in contact with the top, but they are much less secure.
 
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AB

Well-known member
Jan 18, 2011
75
Houston, TX
AB,

What keeps the 420 filters from just moving up?? I thought that the filters were held in place by the upper lid???

When you say "change them" are you talking about installing new cartridges or just swapping the dirty ones with already cleaned ones?

My 520 cartridges are over 6 years old and seem to be working just fine..

Thanks,

Jim R.
I was referring to swapping the dirty ones with brand new 520's.
Over the years by power washing the 420's the bands have mostly busted. I havent found a quick way to clean between the pleats. But at $180 for a new set...if they hold up a while, I'm seriously rethinking the labor to clean vs. buy new (especially since my son, who usually cleans them, is now in college!)