matching suppy and return size

wayneman

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

Matching is just like guessing.

Manual "J" For Load Loss calculations
Manual "D" covers Duct Design

wayneman

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

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

Yes the size of warm air returns should match the supplies. 6"round 100cfm, 7"150 8"200 10"400