Results 1 to 7 of 7
  1. #1
    Join Date
    May 2012
    Posts
    9

    Return Duct Size

    Been doing a bunch of reading on here, but still am a bit confused on return duct size.

    I have a 5 ton AC unit, and I have 3 return ducts throughout the house, the filter sizes are as follows:

    (1) 20x25
    (1) 12x12
    (1) 12x12

    Grills are slightly larger than the above filter sizes, so they fit in well.

    It seems that the return size recommendation is between 144 and 200 square inches of return per cooling ton. My calculation appears to be 157.6 sq inches of surface area per ton. (500+144+144=788 /5)

    I think my vents are somewhat loud, not howling, but very noticeable. I appear to be at the low end of the recommended size range, but they should be perfectly acceptable baring any duct problems right?

  2. #2
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,235
    At a minimum you should have 1000 sq inches of RAFG area for 5 tons. Or in other words (2) 20x25 RAFG's
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  3. #3
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    2,703
    the filter sizes you have, will whistle and plug up very quickly.

    there is a difference between Return ductwork area, and return FILTER area.

    the minimum filter area recommended by ANY system manufacturer is 200 sqin/ton. I prefer 350/ton. because this gives room for filtrete filters, and longer filter change intervals, along with quieter system operation, and longer system life.
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,452

    Hart&Cooley 'Return Air Filter' Engineering Data for 300-fpm filter Velocity

    They recommend at least 2-cfm per sq.in., I prefer at least 1.5-cfm per sq.in.

    5-Ton @ 2000-cfm / 1.5-cfm per sq.in.= 1333-sq.ins.; & that is not enough for a 'low pressure drop media type filter' for 300-fpm initial velocity...

    Actually Hart&Cooley Engineering Data for an initial 300-fpm filter velocity it takes (2) 30X24 filters (or 6.3-Ak,sf total) or 'nearly' 1440-sq.ins. for 2000-cfm. 2000-cfm/1440-sq.ins. or, 1.38-cfm per sq.in. of 'physical' filter area.

    You'll never end-up with too much RA filter area for those larger tonnages...
    Last edited by udarrell; 05-20-2012 at 10:55 PM. Reason: (or 6.3-Ak,sf total)

  5. #5
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,159

    More than the size is the location

    With the filters in the grills instead of in the unit, the chances are the resistance of the duct to those remote 12x12's is probably not allowing them to be very effective.

    The proof in the pudding is what is your negative pressure reading at the unit.
    If it is higher than the supply then trouble
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  6. #6
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    2,703
    Quote Originally Posted by udarrell View Post
    They recommend at least 2-cfm per sq.in., I prefer at least 1.5-cfm per sq.in.

    5-Ton @ 2000-cfm / 1.5-cfm per sq.in.= 1333-sq.ins.; & that is not enough for a 'low pressure drop media type filter' for 300-fpm initial velocity...

    Actually Hart&Cooley Engineering Data for an initial 300-fpm filter velocity it takes (2) 30X24 filters (or 6.3-Ak,sf total) or 'nearly' 1440-sq.ins. for 2000-cfm. 2000-cfm/1440-sq.ins. or, 1.38-cfm per sq.in. of 'physical' filter area.

    You'll never end-up with too much RA filter area for those larger tonnages...
    true information, but most homeowners are unaware of the airflow rate of their system.
    they all know "tons of cooling" that's why I posted the stats my way.

    I like big filters, placed to get the most function and least noise!
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  7. #7
    Join Date
    May 2012
    Posts
    9
    thanks for all the replies. The future may have testing and larger returns installed if the existing ducts allow for it.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event