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  1. #1
    Join Date
    Jun 2010
    Posts
    16

    Return Air Grill

    I have a 3 ton evaporator unit that is rated at 1200cfm airflow, but my return air grill is only 14" x 24". Am I correct in thinking that this is too small? I have read many places that I should expect to get about 2cfm/in² from my return air grill, and following this, I expect only to get about

    2cfm/in² x 14in x 24in = 672cfm

    which is barely over ½ the rated 1200cfm of my evaporator unit. What other considerations should I be making? I have a house with about 1400-1500 ft² living space, and this fairly new unit does WILL NO cool my house in 90° heat (it can only maintain the inside temperature and sometimes not even).

    I am wondering if my return air is a bottleneck for my system.

    Thanks in advance!

  2. #2
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,062
    Quote Originally Posted by tlhiv View Post
    I have a 3 ton evaporator unit that is rated at 1200cfm airflow, but my return air grill is only 14" x 24". Am I correct in thinking that this is too small? I have read many places that I should expect to get about 2cfm/in² from my return air grill, and following this, I expect only to get about

    2cfm/in² x 14in x 24in = 672cfm

    which is barely over ½ the rated 1200cfm of my evaporator unit. What other considerations should I be making? I have a house with about 1400-1500 ft² living space, and this fairly new unit does WILL NO cool my house in 90° heat (it can only maintain the inside temperature and sometimes not even).

    I am wondering if my return air is a bottleneck for my system.

    Thanks in advance!
    You would be correct

  3. #3
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,198
    second opinion is right, the face velocity is high BUT the real issue is what the duct connection looks like between the grill and the filter.
    If the connection is short and sweet the extra pressure drop with the grill won't be noticed, the extra grill noise might be another story
    So IMO it,s the pressure drop on the return/ intake side that really makes the difference
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  4. #4
    Join Date
    Jun 2010
    Posts
    16
    Quote Originally Posted by second opinion View Post
    You would be correct
    Should I add the square area of the supply registers and try to match that with the return air grill size? I assume that I am not going to be able to get 1200cfm of airflow total out of my supply registers even if I add another 14x24 return grill. Is this correct?

  5. #5
    Join Date
    Jun 2010
    Posts
    16
    Quote Originally Posted by genduct View Post
    second opinion is right, the face velocity is high BUT the real issue is what the duct connection looks like between the grill and the filter.
    There is no duct work connecting the evaporator unit to the return air grill and filter. The evaporator unit is in a mini-closet in my hall which is elevated about 2ft above the floor. A portion of the bottom of this elevated closet is open to a cavity in the wall below it which is (of course) about 2ft high and is probably 3ft x3ft square. The existing return air grill is on one of the walls of this cavity, namely the hall wall. Thus, the sheet rock itself is serving as the "return duct" (if you will). It would be relatively easy to add an additional 14" x 24" return grate on the adjacent wall. Is this something that I should consider?

  6. #6
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    1600 CFM / 500 FPM = 3.2 SF

    24*14 = 2.3 SF * 50% free area = 1.2 SF. Yep... bigger return grille... atleast 3.2 SF of free area.
    Last edited by hcong; 07-23-2012 at 09:04 PM. Reason: Mis-read return grille... thought it said return duct.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  7. #7
    Join Date
    Jun 2010
    Posts
    16
    Quote Originally Posted by hcong View Post
    24X14 = 1800 CFM @ 0.05 w.c. /100 ft.

    http://www.hydro-temp.com/help/drawi...ct%20chart.pdf

    Have someone take some static pressure measurements to see if it is air flow.
    Am I misunderstanding something? I cannot see how there should be any noticable pressure drop between the unit and the return air grill/filter since there is no duct and the unit sits directly above the cavity in which the return air grill is on one wall of this cavity.

  8. #8
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Quote Originally Posted by tlhiv View Post
    Am I misunderstanding something? I cannot see how there should be any noticable pressure drop between the unit and the return air grill/filter since there is no duct and the unit sits directly above the cavity in which the return air grill is on one wall of this cavity.
    Sorry.. I misread... I edited my post.. see above

    There is static pressure drop over the grille face... if you go to the manufacturer specs... it will have how much for a your grille. I would say about 0.016 for Return.. Yours is higher since you don't have are much free area.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  9. #9
    Join Date
    Jun 2010
    Posts
    16
    Quote Originally Posted by hcong View Post
    1600 CFM / 500 FPM = 3.2 SF

    24*14 = 2.3 SF * 50% free area = 1.2 SF. Yep... bigger return grille... atleast 3.2 SF of free area.
    Am I correct in assuming that my evaporator unit is "choking"? That is, does it seem like the return air is a bottleneck for the system?

  10. #10
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Quote Originally Posted by tlhiv View Post
    Am I correct in assuming that my evaporator unit is "choking"? That is, does it seem like the return air is a bottleneck for the system?
    How is your airflow in your supply registers? Are you getting around 55 F coming out of your supply registers? You are probably correct in that there is too much static pressure drop for your blower fan to handle, but there could be system issues too. Have you checked that your coils aren't dirty? Wall cavity returns are never as good as ducted.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  11. #11
    Join Date
    Jun 2010
    Posts
    16
    Quote Originally Posted by hcong View Post
    How is your airflow in your supply registers? Are you getting around 55 F coming out of your supply registers? You are probably correct in that there is too much static pressure drop for your blower fan to handle, but there could be system issues too. Have you checked that your coils aren't dirty? Wall cavity returns are never as good as ducted.
    I am getting anywhere between 52F to 57F from my supply registers depending on the current temperature in the house (typically there is about a 18F difference between the return air and supply registers). My condensor coils are not dirty, and I cannot easily access the evaporator coils to check them, but I change my filters pretty regularly, so I assume (perhaps incorrectly) that they are not dirty.

  12. #12
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,198

    What you describe is a unit

    with no real return duct so the grill size seems to be not an issue since the air is getting back to the unit through all the nooks and crannies.
    You can add another grill but I doubt if you'll notice any more air flow
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  13. #13
    Join Date
    Jun 2010
    Posts
    16
    I guess I would like to know if adding another return grill and filter (same size) on an adjacent wall if I could expect more cool air to enter through the supply registers and thus cool the rooms faster. As it stands now, my unit does not power cycle at all throughout the day if the temperature is more than about 85F - 90F. I have been told by more than one professional that a 3 ton unit should be sufficient for my house, but my friend (who is also a professional) suggested that my return air may be too small. I'm wondering if I increased it whether I could supply a larger volume of the same temperature (cold) air into the house (thus cooling the house better than it currently does).

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