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  1. #40
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    the 3M "purple" filter which I think is MERV 11 has something like 3 pleats per inch, while the MERV 7 has only 1 pleat per inch. But the MERV 11 filter has a much more restrictive filter material.

    Why would you expect all different MERV filters to have the same pressure drop? or to have the pressure drop increase with MERV ratings. You have to consider the pressure dorp of a "1" square of their filter material, then know the total surface area of material used. So the MERV 11 might have 3X the pressure dorp of the MERV 7, but has 3.5X the total surface area, so it has a lower overall pressure drop.

  2. #41
    Join Date
    Feb 2001
    Posts
    447
    Quote Originally Posted by motoguy128 View Post
    Why would you expect all different MERV filters to have the same pressure drop?
    I don't "expect" anything. I'm simply trying to understand how to choose the best filter, which will be the one with the highest MERV and lowest pressure drop over its service life, at the lowest cost.

  3. #42
    Charles2
    Your quest could be like searching for the Holy Grail. Your odds on finding the "perfect" filter using manufacturers printed test results are pretty low. The problem is that there is a considerable amount of variation in testing results. For example read the article on air filter testing in Indoor Environment Connections here:

    http://www.ieconnections.com/pdfs/ne...EC-01-2009.pdf

    It starts on page 34. Look at the table on page 37. The same filters made in the same factory with the same materials were sent to 8 different laboratories for testing. All tests were done with approved ASHRAE 52.2 testing devices. The initial pressure drops ranged from 0.27" wg to 0.58" wg. The MERV's were either MERV 7 or MERV 8. Welcome to the world of testing variation.

    Here is my advice. Install a 4" filter box that holds a 4" commercial filter (3 5/8" deep). Use the best filter your system will allow - preferably a MERV 11.

    P.S. - Filter testing is done at air velocities not air volume. It doesn't matter what filter surface area is tested as long as the ratio of surface area to media area is the same.

  4. #43
    Join Date
    Feb 2001
    Posts
    447
    Quote Originally Posted by breathe easy View Post
    The initial pressure drops ranged from 0.27" wg to 0.58" wg.
    Thanks for the link. Average pressure drop reported by most mfrs on MERV 8 is about 0.2. What's really shocking is that some filter mfrs recommended that their filter be changed when the pressure drop hits 0.5, so it's DOA.

    Still, I think it would be helpful for a single lab to test all of the disposable pleated filters on the market. You could get a pretty good idea of which is the best value.

  5. #44
    Join Date
    Sep 2007
    Location
    TX
    Posts
    211
    for what it's worth, parts peddler said a 2" pleated is around 40% more area than a pleated 1" And was told nobody makes 2' filter grill! I still dont buy that one.........

  6. #45
    Join Date
    Feb 2001
    Posts
    447
    Camfil-Farr tells me they know somebody who makes a 2" filter grille, but I haven't seen it and it may be a commercial style, not residential.

  7. #46
    EZ Filter Base makes filter boxes that hold either a 1" or 2" filter. Some models also hold a 4" filter.

    Charles2
    Your comment on pressure drop from the test results misses the point. It is not that the manufacturer is producing a product with a much higher pressure drop. The problem is that test results vary substantially. These differing pressure drops from 0.27" to over 0.50" were for the same filter using the same materials produced at the same time.

    The reality is that many filters from different manufacturers use the same raw materials with the same media, same frames, same wire and same glue. One would anticipate that they would all produce about the same test results. There is just not that much difference in performance, length of life or pressure drop to matter. In fact, I cannot think of a reason why a MERV 8 filter would be preferable to a MERV 7 filter. They will pull out approximately the same levels of particulate.

    That is why I say this quest is meaningless. Focus on the bigger issues like a MERV 8 vs. a MERV 11. Or the depth of the filter. It makes a substantial difference whether the filter is a 1" or a 4". If testing is done by Consumer Reports, I would prefer they focus on these types of things.

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