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Discussion Starter #1
I have a tri-level with a sub basement. The lower level has no HVAC return and the cold air just collects at this level. The HVAC registers for the lower level are on the ceiling. I can add a HVAC return on the partial shared wall (due to tri-level layout) between lower level and sub basement. The new HVAC return would have to cross below several basement joists then two 90s to turn and get up into a joist bay to travel towards the HVAC trunk. Once at HVAC trunk I have two choices: A) topside takeoff but would have cut a hole in the bottom of the HVAC return to cut out for the takeoff on top of the HVAC return and then repair the trunk OR B) pass over the HVAC trunk return and use two 90s into the available side of the HVAC trunk. I know each 90 impacts airflow. While I am adding this new HVAC return should I also add a HVAC return for the basement. Thoughts on what option to use.
 

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I'd cut the access hole and dump a 90 in the top. What's the largest size you can get in there?
 

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Discussion Starter #3
All existing topside return takeoff are 6". So I can do a 6". A 7" might be snug.

I might have a another option. C) take the new return which is perpendicular to existing basement supply & return trunks over both trunk line and then use 2 90s to swing back into the return side of the furnace above the humidifier. Option C would require 2 90s but go directly into furnace vs. into an existing return trunk heading to the furnace.

Regarding the approach (A, B or C), should I add tee on this new return and add a return from the basement floor area as I pass over basement wall that has drywall on one side.
 

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Regarding the approach (A, B or C), should I add tee on this new return and add a return from the basement floor area as I pass over basement wall that has drywall on one side.
I like option C better and wouldn't bother with the basement tee. Run 7" if possible, a squeeze here and there will still be better than a 6".
 
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