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matching suppy and return size

wayneman Member Posts: 29
I have 1 unit apt. with forced air heat.Furance is in basement. Some rooms have 2 supply lines with floor vents and some have one. The return is one floor return in a room without supply line( kitchen). I would like to add cold air returns in as many rooms as possible. I have read using floor joists is not the right way to go because hard to get good seals.Should returns pipe sizing match supply size?? example. if room feed with 2- 6" duct it looks like 1-8" return would handle about the same volume of air. any thoughts??
sealed Joist??
round vs. Rectangular piping
tin vs. flexible insulated tubing??


  • pecmsg
    pecmsg Member Posts: 2,217
    Matching is just like guessing.

    Manual "J" For Load Loss calculations
    Manual "D" covers Duct Design
  • wayneman
    wayneman Member Posts: 29
    so I have to Hire someone to calculate those?? duct work air volume in- matching duct work air volume return does not work?? 6" duct air flow 120cfm x 2 ducts means 8" duct air flow 300cfm would handle return.
  • wayneman
    wayneman Member Posts: 29
    I have ordered a big Mouth vent and want to install correctly. The attached pictures show where it would be located. can I directly put 90 in end of 2" main pipe or do I need 8-10" vertical extension then the 90 degree elbow? what difference does it really make?

    EBEBRATT-Ed Member Posts: 9,708
    edited January 2020
    Yes the size of warm air returns should match the supplies. 6"round 100cfm, 7"150 8"200 10"400