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  1. #1
    Join Date
    May 2004
    Posts
    31
    I have some questions regarding my residential HVAC system.

    First, the supply and return ducts are made of sheet metal and sized 7"x22". What is the CFM capacity of that size duct? Is there a max size of furnace and air conditioner that should be connected to that size duct?

    Second, my system has an Aprilaire 700 humidifier installed in the side of the indoor coil cabinet. Is this an acceptable practice? Are there any long term impacts to the coil with this type of installation?

  2. #2
    Join Date
    Apr 2004
    Location
    Niantic, Illinois
    Posts
    545
    The humidifier is a typical installation. It is better however to be on the plenum in most cases. The Aprilair model 700 is recommended for installation on the supply side of the system such as yours is. It operates best this way.


    The ductwork, are you sure it's not 8x22? This would be a more common size.

  3. #3
    Join Date
    May 2004
    Posts
    31
    The outside of the duct is 7"x22". The ducts are original to the house, 48 years old. Also the basement is very shallow, just over 7' tall.

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,914
    I calculate 600-700 CFM which would be a 1.5 ton A/C and a small furnace, maybe 50,000 BTU 80%.

  5. #5
    Join Date
    May 2004
    Posts
    31
    BaldLoonie - Thanks for the input.

    Based upon those figures, I might need to see about some possible duct work. The system has 2.5 ton A/C and a 80% variable speed furnace with a second stage of 64,000 BTU.

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Does the 7X22 ,run in only one direction from the plenum?

    If it does ,yes ,you have a problem,if it runs in two different directions ,your likely okay,as that would double the capacity.

  7. #7
    Join Date
    May 2004
    Posts
    31
    Unfortunately, the duct is a single run. The furnace is off to one end of the house with the supply and return duct running down the middle of the basement ceiling for about 40.

    Is the only option to just increase the size of the ducts?

  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by fmayberry
    Unfortunately, the duct is a single run. The furnace is off to one end of the house with the supply and return duct running down the middle of the basement ceiling for about 40.

    Is the only option to just increase the size of the ducts?

    If that trunkline is in good shape,you can run two more,one on each side of it.Disconnecting branch runs from the existing trunk and attaching them to the new one.The existing trunk would only feed the far side of the home.

    This needs to be designed by a Pro,using Manuals J and D,from ACCA.

    It may be better to just replace the ducts,have a Pro take a look.

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