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Thread: Return Duct Filters OK with Whole House Filter?

  1. #1
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    Return Duct Filters OK with Whole House Filter?

    Hello -

    I just had a new Bryant 3-Ton, 15 SEER Preferred Series Heat Pump and Variable Speed Air Handler installed in my 1750 sq feet, 3 story town house in NC. I had TechPure whole house filter and UV light put in at air handler. Installer says OK to use non pleated high flow 30-day register filters only at my 4 returns. (2 are 12", 2 are 14"). Never had whole house filtration but was good about changing mid level pleated filters on my previous (Goodman 11 SEER) system so my 10 year old ducts are pretty clean I think. Want the best performance and longest life out of the new system and want to preserve warranty 100% but hate to start dumping dust and hair into my clean ducts (pet owner). What do I do? I still run portable HEPA filters in rooms - can I leave return filters in just 2 of highest dust generating rooms to limit strain on the system?

    Thanks all-
    CK

  2. #2
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    We don't normally double filter paths to the return air chamber as it can cause too much pressure drop in the system.

    An ACCA manual D ought to be performed to see if there is enough Available Static Pressure (ASP) remaining for proper performance.

    If the blower calls for .5" of static pressure & a simple static pressure shows it is much higher; then steps need to be taken to correct the problem.

    I understand your desire to keep the ducts reasonably clean, especially with pets in the house.

    The installer may be right; however, I would do some testing to verify if it is a workable situation.

    Determine if leaving filters in the two highest dust generating rooms imbalances the airflow delivery too much.

    You need to realize that those room returns have a pressure drop for the filter plus the grille; therefore those areas need to be large enough to result in a low 300-fpm velocity or lower through the grille & filter. That calls for oversized areas as compared to what usually exists for the required airflow of each room; if those filter grilles were sized just for the return area of the individual room they probably won't be big enough.

    Looking at the sizing it looks like they were sized for the entire return air; I won't take time to try to figure it because I don't know the requirements of the rooms, etc.
    Last edited by udarrell; 01-19-2015 at 08:51 AM.

  3. #3
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    Thread Starter

    Thanks for the response udarrell

    Quote Originally Posted by udarrell View Post
    We don't normally double filter paths to the return air chamber as it can cause too much pressure drop in the system.

    An ACCA manual D ought to be performed to see if there is enough Available Static Pressure (ASP) remaining for proper performance.

    If the blower calls for .5" of static pressure & a simple static pressure shows it is much higher; then steps need to be taken to correct the problem.

    I understand your desire to keep the ducts reasonably clean, especially with pets in the house.

    The installer may be right; however, I would do some testing to verify if it is a workable situation.

    Determine if leaving filters in the two highest dust generating rooms imbalances the airflow delivery too much.

    You need to realize that those room returns have a pressure drop for the filter plus the grille; therefore those areas need to be large enough to result in a low 300-fpm velocity or lower through the grille & filter. That calls for oversized areas as compared to what usually exists for the required airflow of each room; if those filter grilles were sized just for the return area of the individual room they probably won't be big enough.

    Looking at the sizing it looks like they were sized for the entire return air; I won't take time to try to figure it because I don't know the requirements of the rooms, etc.

    I decided to play it safe and go without any filters at the return vents and go with the portable air cleaners in the rooms to help out.

  4. #4
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    If you're using a low-cost-throwaway 1 inch deep filter; for 3-Ton 1200-CFM airflow you would need a 30 x30 filter or, (2) 25x20 RA filter grilles in the conditioned area & none in the furnace.

    Hart&Cooley engineering data; 30x30 RA grille yields 4.363-Ak & 1309-CFM at 300-fpm velocity. the filter may have a lower Ak (sf) of open-area than the grille.

    To also keep the duct system clean; You would need to turn all of the return air grilles into properly sized filter grilles. You cannot use 1" deep pleated filters; too much pressure drop.

  5. #5
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    Thread Starter
    Got it - two more questions since you appear to really know this stuff. My new install looks good to me and the system seems to be working well but:

    Why is the sheet metal 90 degree turn coming out of the air handler (and others out in the original duct system in the attic installed when the place was built in 2006) totally square boxes? Wouldn't the air flow smoother through a rounded corner piece similar to what you see in exposed industrial ductwork? And:

    Is there a way to get an impartial technician to check my new system and tell me how much of the 15 SEER I'm really getting with the new setup?

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