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View Full Version : Location of A/C Evaporator Coils At Downdraft Furnace


Bruce Breedlove
11-10-2009, 11:18 PM
The evaporator coils are located above this downdraft furnace which places them in the return plenum rather than in the supply plenum. Please confirm that this location is incorrect. (Yes, that is a second downdraft furnace beside the one in question. The evaporator coils are located below - downstream from - this furnace.)
Scott Patterson
11-11-2009, 09:07 AM
The evaporator coils are located above this downdraft furnace which places them in the return plenum rather than in the supply plenum. Please confirm that this location is incorrect. (Yes, that is a second downdraft furnace beside the one in question. The evaporator coils are located below - downstream from - this furnace.)

Yes it is wrong. In that setup the cool moist air from the evaporator coils will cause moisture to form on the heat exchanger and this will cause premature rusting and failure.
Bruce Breedlove
11-11-2009, 10:30 AM
Thanks, Scott. I knew that but I just wanted some confirmation.
David Bell
11-14-2009, 10:48 AM
It is a legal install ,actually the cooling coil dehumidifies the air
Jim Luttrall
11-14-2009, 11:07 AM
It is a legal install ,actually the cooling coil dehumidifies the air


IRC 2003 M1411.2 Refrigeration coils in warm air furnaces. Where a cooling coil is located in the supply plenum of a warm-air furnace... Cooling coils shall not be located upstream from heat exchangers unless listed and labled for such use. ...Not allowed unless it can be proven by the manufacturer's instructions.

The problem with installing cooling coils on the inlet of the furnace is not the conditioned air stream being wet or dry but rather the chilling of the heat exchanger which is now below the dew point of the combustion and dilution air which continues to flow through the heat exchanger causing condensation, rust and corrosion on the inside of the heat exchanger. If it has a coil on the inlet side and is not a unit designed and built by the manufacturer (one piece), I would call it out. It is dumb even if you can find supporting documentation to allow it. Just my 2 cents.
Scott Patterson
11-14-2009, 07:19 PM
It is a legal install ,actually the cooling coil dehumidifies the air

Have you ever measured the RH of the supply air with a wet bulb hygrometer?