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  1. #1

    When is air velocity low enough to use a restrictive filter?

    Hello,

    I am trying to find out when it would be acceptable to use restrictive air filters, like the Filtrete red/purple/blue label filters sold in the big box stores.

    My understanding is that with sufficiently low air velocity (i.e. sufficiently large filter area), the static pressure can be held to acceptable levels.

    My upstairs unit has a 2.5 ton air handler and a 20x25 filter grill. The downstairs one is 3.5 ton with two 20x20 filters (heat pump split system). If I simplify by assuming the entire filter area is usable, I get 288 fpm upstairs, and 252 fpm downstairs. At those velocities, what kind of air filters can I use w/o risk of damaging my equipment?

    Thanks.

  2. #2
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    The rating and size of the filter will not tell you if the duct and blower can overcome the pressure drop. If the system was designed to use a .2" drop filter it will be fine, if it's designed to use a see through filter then there will be problems. Sorry but there is no fits all answer.

  3. #3
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    Chances are your duct was not designed for this type of filter. The fc40 Honeywell filter has less pressure drop and has a higher merv rating. As long as the metal box that the filter grill is in is large enough to accept a 5" filter I would choose it. I am against 3m filtete 1" filters, they are good at marketing and ruining compressors that's about it

  4. #4
    Join Date
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    Quote Originally Posted by BuddyBoy View Post
    Hello,

    I am trying to find out when it would be acceptable to use restrictive air filters, like the Filtrete red/purple/blue label filters sold in the big box stores.

    My understanding is that with sufficiently low air velocity (i.e. sufficiently large filter area), the static pressure can be held to acceptable levels.

    My upstairs unit has a 2.5 ton air handler and a 20x25 filter grill. The downstairs one is 3.5 ton with two 20x20 filters (heat pump split system). If I simplify by assuming the entire filter area is usable, I get 288 fpm upstairs, and 252 fpm downstairs. At those velocities, what kind of air filters can I use w/o risk of damaging my equipment? Thanks.
    Well, you want the initial velocity through a clean fikter to be within 300-fpm.

    One chart lists a 25x20 @ (Ak (sf free-area) 2.24) up to only 898-cfm at 400-FPM velocity. At 300-fpm 700-CFM.

    The initial velocity on a pleated filter is listed at 500-fpm; however, as
    martyinlincoln posted:

    The rating and size of the filter will not tell you if the duct and blower can overcome the pressure drop.

    If the system was designed to use a .2" drop filter it will be fine, if it's designed to use a see through filter then there will be problems. Sorry but there is no one size fits all answer.

  5. #5
    Quote Originally Posted by jtrammel View Post
    Chances are your duct was not designed for this type of filter. The fc40 Honeywell filter has less pressure drop and has a higher merv rating. As long as the metal box that the filter grill is in is large enough to accept a 5" filter I would choose it. I am against 3m filtete 1" filters, they are good at marketing and ruining compressors that's about it
    Thanks for the responses everyone, they've been very helpful.

    I took a look at the cavity behind the 20"x25" filter and it's just a flat box with a total depth around 6". In the center is a round opening for a 14" diameter duct. Is this ok for mounting an FC40? I'm just wondering if there is some minimum distance required between the back of the filter and the opening for the duct. In my case it would be 1", assuming the filter measures 5" deep.

  6. #6
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    Quote Originally Posted by BuddyBoy View Post
    Thanks for the responses everyone, they've been very helpful.

    I took a look at the cavity behind the 20"x25" filter and it's just a flat box with a total depth around 6". In the center is a round opening for a 14" diameter duct. Is this ok for mounting an FC40? I'm just wondering if there is some minimum distance required between the back of the filter and the opening for the duct. In my case it would be 1", assuming the filter measures 5" deep.
    No, that filter box is not deep enough to use a 5" filter

  7. #7
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    Quote Originally Posted by jtrammel View Post
    No, that filter box is not deep enough to use a 5" filter
    Why would it not be deep enough?

  8. #8
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    Massachusetts
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    If you install a filter having a 5-inch depth into a box of 6-inches and having a round 14-inch return attached to the back of the 'box', then the majority of air will be pulled only through the center of the filter material rather than the entire filter. This is a function of airflow and turning corners. Since the air outside of the 14-inch diameter must change direction as it nears the trunk, it will simply choose to not turn and will just aerodynamically stall and 'plug up' the corners, allowing only the straight ahead airflow to get through. When trying to change airflow directions, you need a distance of 6 to 7 pipe diameters of space to allow for the direction change. You don't have anywhere near that dimension with the 6-inch 'box'. That's why it's no deep enough.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  9. #9
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    Quote Originally Posted by skippedover View Post
    If you install a filter having a 5-inch depth into a box of 6-inches and having a round 14-inch return attached to the back of the 'box', then the majority of air will be pulled only through the center of the filter material rather than the entire filter. This is a function of airflow and turning corners. Since the air outside of the 14-inch diameter must change direction as it nears the trunk, it will simply choose to not turn and will just aerodynamically stall and 'plug up' the corners, allowing only the straight ahead airflow to get through. When trying to change airflow directions, you need a distance of 6 to 7 pipe diameters of space to allow for the direction change. You don't have anywhere near that dimension with the 6-inch 'box'. That's why it's not deep enough.
    The proper inlet duct length with a connection that is entering, say, the fan, is 2 Xs the diameter of the inlet duct after a 90-EL turn- to allow equal airflow & velocity across the entire diameter of the duct.

    With only one inch of space you'd have a radical situation that is untenable.

    To Summarize these important efficiency design points:
    This is a critical air flow design that ought to be dealt with but is rarely considered by far too many...it was very important that jtrammel, martyinlincoln & skippedover covered these points.

    jtrammel
    Chances are your duct was not designed for this type of filter. The fc40 Honeywell filter has less pressure drop and has a higher merv rating. As long as the metal box that the filter grill is in is large enough to accept a 5" filter I would choose it. I am against 3m filtrate 1" filters, they are good at marketing and ruining compressors that's about it.
    ALSO, VERY IMPORTANT:
    martyinlincoln
    The rating and size of the filter will not tell you if the duct and blower can overcome the pressure drop. (ONLY)If the system was designed to use a .2" drop filter it will be fine, if it's designed to use a see through filter then there will be problems. Sorry but there is no fits all answer.
    Last edited by udarrell; 08-21-2012 at 09:22 AM. Reason: after a 90-EL...

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