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  1. #1
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    A/C Filter Restricts Airflow

    I have 2 new 14SEER, 12EER Rheem A/Cs that work very well. The installers recommended that I use cheap fiberglass filters, so I did. After one month, I changed the filters because they looked TOO clean (I could see right through them). I replaced them with 3M Filtrette Dust & Pollen 600 filters, which are rated at MERV = 8.

    The air flow on one of my units seemed reduced, so I checked the airflow and it is about 20% lower with the Merv8 filter than with a new fiberglass filter. 3M says the static pressure reduction is .14 on this filter.

    What is more important: keeping dust out of the evap coil? or A/C efficiency?

    Or is there a better filter choice (less restrictive)?

  2. #2
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    Get rid of those High restrictive filters,They cost to much and create high restriction,you keep using these and you will be slugging your compressor with liquid and destroy it.
    a 400 cfm per tone is pretty much required and best way to see if you are causing air flow problems is to take a pressure reading at return and supply with a megahlic gauge and total static should be around .5

  3. #3
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    Quote Originally Posted by jchunter View Post
    3M says the static pressure reduction is .14 on this filter.
    Keep in mind that those numbers are usually for air moving at 300 fpm.
    If you have just the one filter in a rack the size of the bottom of the unit, the actual are velocity is much higher, often more than twice as high, so the actual PD of the filter is often more than twice what is listed on the packaging.

    If you want more efficient filtering, and the system can handle the PD, have good quality media type filters installed, I prefer Aprilaire 2200 and 2400 air cleaners, but Air Bear, Honeywell and others also make quality products. Just don't ever use an Indigo brand replacement media in one...

    Talk to your installer about it before doing anything, and switch back to the cheep filter for now. To make the cheep filter work better, you can get a spray on product that makes the filter tacky, so it catches more stuff. It is sold under several brand names, ask for "Filter Coat".

    Depending on the equipment you have, there is a chance that the system may not be able to handle the PD of even a media type air cleaner, unless 2 are installed with parallel airflow through them.
    There are a number of furnaces out there by various manufacturers that can barely move 350-400 cfm/ton in the cooling mode with just a coil and a standard filter with no ductwork....

  4. #4
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    Quote Originally Posted by valdee View Post
    a 400 cfm per tone is pretty much required and best way to see if you are causing air flow problems is to take a pressure reading at return and supply with a megahlic gauge and total static should be around .5
    Decent as a rule of thumb, but there is a lot of equipment out there that barely cranks out 400 cfm/ton at 0.3" wc, and a lot that is still cranking out over 350 cfm/ton at 0.8" wc.
    Often that kind of range can even be found within the same model family of the same brand. Rheem/Ruud is actually one of the more notorious ones for that, heh.

  5. #5
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    .5" static is reasonable, but sacrificing clean air should be avoided.

    .5" static is a reasonable benchmark. But .5 static doesn't mean crap if your only pushing 200 or so cfm/ton.

    I'd avoid using one of those blue fiberglass filters unless you want to clean your evap coil twice a year, which is a pita.

    Think about it -- .14 static for your filter is equivalent to nearly 200 ft of ductwork at a residential standard of .08" per 100 ft -- brutal.

    I suspect poor ductwork.


    Quote Originally Posted by mark beiser View Post
    Decent as a rule of thumb, but there is a lot of equipment out there that barely cranks out 400 cfm/ton at 0.3" wc, and a lot that is still cranking out over 350 cfm/ton at 0.8" wc.
    Often that kind of range can even be found within the same model family of the same brand. Rheem/Ruud is actually one of the more notorious ones for that, heh.

  6. #6
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    Quote Originally Posted by emcoasthvacr View Post
    .5" static is a reasonable benchmark. But .5 static doesn't mean crap if your only pushing 200 or so cfm/ton.

    I'd avoid using one of those blue fiberglass filters unless you want to clean your evap coil twice a year, which is a pita.

    Think about it -- .14 static for your filter is equivalent to nearly 200 ft of ductwork at a residential standard of .08" per 100 ft -- brutal.

    I suspect poor ductwork.

    .08 per 100' is FR (friction rate) no real relation to static.Depends on the TEL of duct system.

  7. #7
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    Thread Starter
    Quote Originally Posted by mark beiser View Post
    If you want more efficient filtering, and the system can handle the PD, have good quality media type filters installed, I prefer Aprilaire 2200 and 2400 air cleaners, but Air Bear, Honeywell and others also make quality products. Just don't ever use an Indigo brand replacement media in one...

    Talk to your installer about it before doing anything, and switch back to the cheep filter for now. To make the cheep filter work better, you can get a spray on product that makes the filter tacky, so it catches more stuff. It is sold under several brand names, ask for "Filter Coat".
    The A/C has space for a 1" filter, which rules out the big electronic units.

    Does anyone have a make and model for 1" filter with less than .14 static pressure drop? Something with more pleats, maybe?

    I will check on the Filter Coat. Thanks.

  8. #8
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    Why not use a pleated filter without a high merv rating? Still getting cleaner air than the spun fiberglass, yet not restricting the air as much as what you have? Just a thought...
    Hvac Maniac

    "A negative attitude cancels out positive skills."

  9. #9
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    Thread Starter
    OK, I found a Merv 6 pleated filter at Lowes: AmericanAirFilter "Dust Shield", which definitely breaths better. It shows only half as much restriction as the 3M Merv 8 filter, in spite of having three 1" straps across the filter, to stabilize the filter against the air flow pressure. Definitely not aerodynamic. Probably overkill... But rated as 15 times better filtration than fiberglass.

    It seems to me that filter makers are focused too much on extreme air filtration, to the detriment of airflow in the efficient A/Cs. IMO, they should, instead, focus on modest filtration and low static pressure drop. If they just doubled the number of pleats, they could cut their air resistance a whole lot.

  10. #10
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    Quote Originally Posted by jchunter View Post
    OK, I found a Merv 6 pleated filter at Lowes: AmericanAirFilter "Dust Shield"
    My experience from testing the PD across many brands of pleated filters in many types of residential setups is that those particular filters are, by far, the most restrictive filters in their price range. They are even more restrictive than some of the higher end filters.

  11. #11
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    Thread Starter
    Mark,
    Please name some 1" filters (make and model) that you have found that are less restrictive and filter enough to keep the evap coil clean. I will be happy to try them.

  12. #12
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    Quote Originally Posted by jchunter View Post
    OK, I found a Merv 6 pleated filter at Lowes: AmericanAirFilter "Dust Shield", which definitely breaths better. It shows only half as much restriction as the 3M Merv 8 filter, in spite of having three 1" straps across the filter,
    Be sure the parameters are the same as the other you were comparing it too, so your not misled.
    I have a good library of test data from Air Filter Testing Lab in Kentucky, where manufacturers submit their filters for independant 3rd party test. Briefly looking at several brand filters, most of them submited a 20 x 20 x 1 filter and was tested at 1200cfm @ 300 fpm. Most acheived their highest rating at these parameters.

    So.... you must make sure you get the data from the manufacturer (most will provide you with this) so your comparison is accurate.

    In my area 9 out of 10 customers will have to remove the pleat filters due to ducting issues, compounded by our air density (elevation).

  13. #13
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    Quote Originally Posted by jchunter View Post
    The air flow on one of my units seemed reduced, so I checked the airflow and it is about 20% lower with the Merv8 filter than with a new fiberglass filter.
    Curious how you arrived at the 20% reduction?

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