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gjones 12-06-2011 03:39 PM

Proper trunk/duct size?
I have a house that was built in the late 50's and then (I believe) had air conditioning added later. It has baseboard registers in all of the rooms.

My question is what is the proper trunk size for my house? It has an 8" x 12" trunk the entire length of the house, and it does not reduce in size anywhere along the length of the trunk. One leg is approximately 30' and the other is about 12'. All of the ducting to the registers is 6" regardless of the length of the run.

I would think that you would want to reduce the size of the trunk along the 30' run in order to increase pressure at the end of the trunk.

The reason I am asking this question is that the trunk runs about 2' from the bottom stair in my basement. The basement was finished by the previous owner and they framed around the trunk and the steel beam. If you have ANY bounce in your step and you're taller than 5'9", your head is crashing into this!

The second reason I am asking this is that my friends house is 20 years newer, roughly the same size, and it only has an 8" x 8" trunk with the same size ducting to the registers.

It would be great to be able to put smaller or flatter ducting in the basement for some more head clearance and to clean things up a bit. Thanks for your insight!


beenthere 12-06-2011 05:16 PM

Without knowing the size of the furnace, no way to know what size the trunk line should be. It could be too small.

A 30 foot long trunk line should reduce.

gjones 12-06-2011 05:20 PM

I have a Bryant furnace. I looked but could not find the model number. Is there somewhere in particular I should look for it? I will look again.

gjones 12-06-2011 05:28 PM

Found it. The furnace is as follows:

Model 383KAV042091
Series F
Output 71,000 BTU/HR
MAX Supply PSI 13.6
MIN Supply PSI 4.5
Temp Rise Low 40-70

Hope this helps

beenthere 12-06-2011 06:54 PM

The 12X8 is small for the air that furnace should move.

gjones 12-06-2011 07:04 PM

Is there a size that is flatter and wider that would work better?

beenthere 12-06-2011 07:14 PM


gjones 12-06-2011 07:18 PM

Awesome thank you!

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