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  1. #1
    Join Date
    Nov 2010
    Ontario, Canada

    Horizontal Furnace - How long for supply air plenum?

    I was looking at the possibility of mounting a high efficiency furnace horizontally in a garage as opposed to a unit heater at a customers request. I've heard guys talk about a minimum length for the supply air plenum but do not see anything in the manual for the unit I was considering.

    Is there a standard minimum and what are the reasons?

  2. #2
    Join Date
    Mar 2011
    Galveston Texas
    Depends on the number of ducts coming off of it, as well as available room. The more ducts the longer the plenum. Ours are a min 4' long, I know the pre-fabricated ones the supply houses carry are only 3'.

  3. #3
    Join Date
    Jun 2004
    We leave at least a foot at the end
    To build static and more balance output.

  4. #4
    Join Date
    Jul 2009
    Columbia, MD
    i can speak for rheem. they want at least an 18" plenum before any bends, takeoffs, etc...

    turning vanes and proper take offs are the only way to go.

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