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Thread: Return Air

  1. #1
    DIY Member idoc4u's Avatar
    Join Date
    Sep 2009

    Question Return Air

    I apologize in advance as this is somewhat of a continuation of a previous post. I didn't know how to post a new picture in a reply.

    A few of you have replied to the previous post and I appreciate the help.

    My home is new construction; 2 story with a basement. I have a 2 ton HVAC unit. I am finishing the basement and have installed a supply branch with diffuser in the ceiling. That is one 6" branch off of the main supply to a 500 sq ft rec area. I would like to add a second and also a cold air return to the general area.

    I have 4 questions:

    1. Is there any problem installing a 6" supply with a short collar take-off from the bottom of the main supply about 80" off of the floor and about 10ft from the furnace, or would that "steal" too much volume away from the other branches being that it is right off of the main in such a way? The framing and location of pipes, etc are physically prohibitive to come off the top of the main with a take-off as I did the first supply.

    2. If this is too much of a "steal" would placing a saddle on a branch going to a main floor supply be functional or again too much of a "steal?"

    3. Regarding the cold air return; can I place a 4" pipe or a flexible pipe from the return girll up the inside of a wall and run it to an area in between the floor joists where there is existing sheet metal serving as a return from the main floor or upper level? This would sort of branch or "Y" into an existing return duct and not directly into the main return. In the photo, I'm talking about tapping in to the face of the return duct that exists as a return for another location in the house.

    4. It would be a very good location and easy to install the cold air return in my basment on a wall that backs up to and is about 3 feet from my furnace. I have a high efficiency furnance. Is there any problem installing the return on a wall this close to the furnace and connect a 6" round duct from the return grate in the wall to the main return trunk a couple of feet off of the vertical return to the furnace?

    If #4 is practical, effective and safe, and rebalancing is not a concern, this would solve my problem with return air.

    Thanks again and have a good weekend.
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    Last edited by idoc4u; 07-10-2010 at 07:03 PM.

  2. #2
    DIY Senior Member Hube's Avatar
    Join Date
    Sep 2004


    A bit of MATH may help you.
    A 6" diameter supply has a capacity of approx 28 inches of air. If you have 2 of these 6" supply pipes then you have a total of approx 56 of supply air trying to go into this 500 sq ft area. To obtain the best heating or cooling result this area will need at least the equivilent (56 ) of RETURN air from this area. (you can only put IN as much air as you can take OUT.) Note: a 4" pipe has a capacity of approx 12 in of air. A 5" pipe has a capacity of approx 19" ..... You do the MATH.

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