Plenum Supply Mathematics: Volume vs Area

Discussion in 'HVAC Heating & Cooling' started by homewrecker2010, Apr 20, 2010.

  1. homewrecker2010

    homewrecker2010 New Member

    Apr 20, 2010
    Saint Louis, Missouri
    Hello Everyone,

    I am upgrading my 2-ton HVAC system, installing a new A/C condenser and coil, but keeping the existing furnace and air handler. All are made by Rheem/Ruud. My furnace / air handler is in my basement in an updraft configuration. The basement ceiling is low (about 6 ft.). In the old configuration, there was an uncased evap coil boxed in with sheet metal sitting on top of the air handler. Above that was a custom-fabricated sheet metal supply plenum chamber that measured 18" x 31" x 8" (4464 sq. in). There were 4 outlets in this chamber to which four 7" diameter steel ducts connected; each duct leads to one of the four rooms in my house (house is 107 year old)

    Problem: the new evaporator coil, which is in a factory case, is too tall to match the existing plenum chamber. I'd like to use it "cased" as opposed to un-casing it and opening a whole new can of worms (even uncased, the coil doesn't quite fit in the old ductwork, but the case fits perfectly atop my air handler in terms of height and depth).

    When I place the new cased coil on top of my air handler, I'm left with only 5.75 inches of space between the top of the cased coil and the bottom of the floor joists to distribute the airflow to the four 7" ducts.

    I can fabricate a new supply plenum chamber that will fit, but, I'm not certain about the math. (Please see the attached diagram). The new supply plenum chamber will have the same VOLUME as the old one, but, it will reduce the height and width of the plenum duct (the area of a slice of the plenum, if that makes sense) to an area that is smaller than that of the cased evap coil (the plenum would be 5.75 x 40 = 230 sq in, and the cased coil is 17 x 21 = 357 sq in.).

    I'm concerned that I may create a venturi, forcing air through a space that it too small, creating a backlash that may reduce the efficiency and lifespan of the blower motor in my air handler.

    I appreciate anyone's advice on the attached diagram. If the diagram does not show up in this posting, you can see it at


    Last edited: Apr 20, 2010
  2. Lightwave

    Lightwave New Member

    Aug 20, 2007
    Vancouver, BC
    I'm far from an expert on ducting, but I'm pretty sure a 5.75" high plenum will not work. Air velocities in the 5.75" area will be high enough to cause severe noise issues and the backpressure could be too much for the blower to handle. I'd suggest either:

    1. Reframing the floor joists to create an opening for a taller plenum.

    2. Extending the plenum parallel to the floor joists--inside a joist bay--and connect your 7" ducts to the underside of the extended plenum.

    3. Converting your AHU to horizontal mount or replacing it with a horizontally mounted AHU.

    I don't know which of these options will work best. Consulting an engineer with experience in CFD for ducting might be in order.
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  4. homewrecker2010

    homewrecker2010 New Member

    Apr 20, 2010
    Saint Louis, Missouri

    Thanks very much for your input. Option #2, placing the plenum inside the joist bay and connecting the ducts to the underside, was my next option. An HVAC tech is coming today to evaluate my project.

    Thanks again!
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    The evaporator coil of my air conditioning unit fits inside of my air handler, beneath the fan. Are you sure yours doesn't have space for that there? This would leave the entire area above the air handler for your plenum/distribution box. If the a/c unit is not the same brand as the air handler, you may have to fabricate some brackets to hold the evaporator in place, along with the drain, but it probably will fit.
  6. gator37

    gator37 Retired prof. engr.

    Nov 10, 2009
    Retired prof. engr.
    Do you have a return air plenum or is the unit on the floor with a side return duct. If on a plenum you may be able to lower the return plenum height as long as it is of adaquate size for the return giving you enough space for your supppy connection OR can your cased coil (a frame I suspect) AND furnace be placed horizontal (Coil has to be made for it with an alternated condensate drain connection and the furnace has to be made for horizontal installation.)? I would not reduce the duct system especially on a furnace.
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