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  1. #1
    Join Date
    Nov 2007
    Posts
    10

    return air..is it enough?

    Hi,

    Is it totally out of the question for two split sytems to share a return air? I'm looking at replacing two furnaces (3 ton & 90k BTU 80% AFUE maybe 95%) and two condensers (16 SEER). The furnaces are located to the left and the right of return air duct in the attic basically in the form of a "T". The openings for the return air are on the corner of the upstairs hallway and the stairwell. The opening on the stairwell is 20x25 and the opening in the hallway is 16x25. Both openings are for the same return air duct. The return air duct is about 20x20. If both furnances or condensors are running at the same time would the return air be large enough? How much would it impact efficiency (all units would be matched appropriately to get 16 SEER)? I've received various answers from different contractors. Thanks in advanced.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,900
    If both units are running at the same time, it will work, but you'll be low on return.

    The real problem comes when only one is running.
    The other blower could run backwards. If the blower tries to start while spinning backwards, the motor won't last long.
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  3. #3
    Join Date
    Nov 2007
    Posts
    10
    Thanks for the response. So, what happens when your low on the return?

  4. #4
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    a/c coil can freeze up, heat exchanger can crack
    You can't fix stupid

  5. #5
    Join Date
    May 2002
    Location
    Houston Texas
    Posts
    6,321
    You definitely need separate chases for each system. Low return air capacity will shorten the life of the equipment as well a reduce systems capacity and efficiency.

    In short you will not get the performance you are paying for with either the furnaces or the A/C.

  6. #6
    Join Date
    Jul 2006
    Location
    MN
    Posts
    173
    You can move a lot of air through a 20x20 duct. 2400 cfms shouldnt be a problem. Might be a little noisy at the returns and verify statics.

    You would have to twin the units so that both blowers start simultaneously, its called twinning to keep one blower from starting backwards as mentioned.

    Most units have a twinning capability built into the control board or simply wire the fan controls as one. Consult the manual.

    Have not done this yet on the ECM motors so cant help there.

  7. #7
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060

    Hmm

    Quote Originally Posted by airon View Post
    You can move a lot of air through a 20x20 duct. 2400 cfms shouldnt be a problem. Might be a little noisy at the returns and verify statics.

    You would have to twin the units so that both blowers start simultaneously, its called twinning to keep one blower from starting backwards as mentioned.

    Most units have a twinning capability built into the control board or simply wire the fan controls as one. Consult the manual.

    Have not done this yet on the ECM motors so cant help there.
    Any "ROCK SALT" available?

  8. #8
    Join Date
    May 2002
    Location
    Houston Texas
    Posts
    6,321
    Quote Originally Posted by airon View Post
    You can move a lot of air through a 20x20 duct. 2400 cfms shouldnt be a problem. Might be a little noisy at the returns and verify statics.

    You would have to twin the units so that both blowers start simultaneously, its called twinning to keep one blower from starting backwards as mentioned.

    Most units have a twinning capability built into the control board or simply wire the fan controls as one. Consult the manual.

    Have not done this yet on the ECM motors so cant help there.
    You may be able to move 2400 CFM through a 20"X20" duct but not through a grill of that size or even a grill double that size.

    Currently no manufacturer that I know of has an approved system for twinning ECM motors.

  9. #9
    Join Date
    Nov 2007
    Posts
    10
    Would it be feasible to to add one grill (return) in the ceiling to feed one of the furnances? The furnace would be right above it the attic. The other two grills would be at floor level below the return that would be added.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,900
    Without seeing the set up, can't say for sure.

    A contractor would have to look at it.
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