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Old 08-28-2009, 09:53 AM   #1
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Adding Flex Duct to Bonus Room


I am currently finishing a "bonus" room (attic space above my garage). The house was built two years ago and the space was left unfinished, but the second floor HVAC system was supposedly sized to handle the bonus room. Gas furnace located in the attic above the second floor, about 5 feet from the room to be finished.

All framing is complete, now I need to add the ductwork, and would like to do it myself to save some money. The finished space will be approximately 450 square feet with 9' ceilings. Return air vent for the second floor is in the hallway immediately outside this room.

From my research it appears I should use 4 registers with 6" insulated flex duct for this space. The problem - from what I can tell - is that I do not have enough space on the main trunk to add 4 new supply lines. Can I instead use two larger (i.e. 8") supply lines with only 2 registers? Or should I use two 8" supply lines and "split" them into 4 registers?

Also, will it be necessary to add a return line?

Any help is greatly appreciated. I'm confident I can do the work, but want to ensure I provide appropriate supply for the room.

Also if it helps- we have about 16" of insulation below the floor, and the ceiling will be R38 (at the top flat part) and R30 (on the slanted part). Knee walls will be R13, are completely sealed and have house wrap on the back to prevent air washing.

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Old 08-28-2009, 11:06 AM   #2
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Adding Flex Duct to Bonus Room


What research did you do that indicated 4 6" supplies are needed? That would be taking alot of air from the existing system. Where has this air been going up to now? Its hard to give you direction as to how to run the ducts without seeing and sizing the room.

Historically thes bonus rooms are very hard to heat and cool. I am glad to see you are taking steps to seal up the envelope.

I would add return air if possible.

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Old 08-28-2009, 11:21 AM   #3
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Adding Flex Duct to Bonus Room


Various internet research, which is why I've been skeptical. I also spoke with a local contractor who indicated that he thought it would require 4 supplies, but didn't give a size and said he would need to see the space to confirm. What do they look at when visiting the space? I may be able to describe or provide pictures.

Up to now this air has been heating and cooling the rest of the second floor. I've attempted to attach the second floor plan below (space in question is called "Unfinished"). The other three bedrooms each have three supply lines (all 6" flex duct), which I also thought was a little much, but I'm no pro.

Thanks for your help.

Adding Flex Duct to Bonus Room-2nd-floor-reich-7-11-07.jpg
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Old 08-28-2009, 10:13 PM   #4
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Adding Flex Duct to Bonus Room


The only way to tell just what is needed is to run a manual J load calculation on the second floor and see how much load each room requires. HVAC-Calc is a program where you can purchace it for a one time use pretty cheap. Once you have that info you can size the ducts and have the info you need to balance the system so the bonus room will be conditioned properly.

Anything else would just be a guess. The contractor most likely would have wanted to measure the room, insulation, windows, direction etc. so they could run the calculation program for sizing. Just going by square footage is not the answer.
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Old 08-28-2009, 11:16 PM   #5
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Adding Flex Duct to Bonus Room


How big was the second floor before you finished the new area? And how big is your furnace/AC? If they are big enough, 4x6" ducts would permit more even air distribution that 2x8" ducts. Won't be perfect without a formal ductulator calculation.
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