Homewrecker2010
New Member
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 http://www.masterkeyhost.com/will/plenum_diagram.gif
Thanks!
Will
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 http://www.masterkeyhost.com/will/plenum_diagram.gif
Thanks!
Will
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