Finishing remaining 430 sq. ft of 1st floor townhouse - duct question


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Old 02-01-13, 11:50 AM
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Finishing remaining 430 sq. ft of 1st floor townhouse - duct question

We have a 3 level center townhouse. The first level consists of a garage, a finished (by builder) 200 sq. ft entryway/open stairwell and an unfinished 430 sq. ft. "basement" area.

The new area will consist of a 10x10 bedroom, 6 x 5.5 bathroom, 5 x 5.5 laundry room, and 14.5 x 12 living area.

The finished entryway has a 12 x 12 return grill with 7" return duct and 1 - 6 x 12 register for heating/cooling on a 7" supply duct.

The main supply trunk (8 x 14) is easily accessible.

Questions - What size duct should we use, and how many registers should we use(I was thinking 1 in the bedroom and 2 in the living room)?

Is the current return duct adequate for the entire first level space?
 
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Old 02-01-13, 07:14 PM
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Only way to know is to do a heat load
 
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Old 02-02-13, 03:37 AM
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Airman is correct, the HVAC guys can tell you if your furnace is sized to handle more and then how much of that will be needed in the new area. Your return path is as important as the supply. There are on-line heat load programs so if you want to DIY it you can, but your heat guy does it all the time.

Bud
 
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Old 02-03-13, 07:13 AM
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We are doing the whole project (minus electric) DIY. I know the current furnace is sized appropriately for the whole house as the basement was originally supposed to be finished when the designs were done. I just did a heat load calculation for the new finished area, and got 8447 btuhs. The existing finished area (with return and supply) has a load of 4224 btuhs.

What would the next step be? Thanks for your help
 
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Old 02-03-13, 06:53 PM
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Next step would be size the duct to the btu,s needed in each room
 
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Old 02-04-13, 07:53 AM
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So I have determined that the bedroom needs 106 cfm and the main living room (including laundry and bath) needs 175 cfm.

I was not going to put a supply in the laundry and bathroom, as they are small and are not on exterior walls.

I am finding a lot of mixed information regarding what size duct is adequate for this airflow.

For now I am using this chart:
http://www.mechanicaledge.com/sitebu...CALCULATOR.pdf

Is there a tried and true calculation for this?

Also, I realized that the 8 x 14" supply line is maxed out with the number of takeoffs on it.

The main plenum (I think this is what it is called - the part that comes right of the unit) is 20 x 20 x 26. It currently supplies a 12" round duct for the 3rd floor and the 8 x 14" duct for the 2nd floor and the one 6" duct on the main floor.

Can I tap an additional supply on this main plenum? I was thinking a 10" line teed off to a 6" line for the bedroom and an 8" line for the living area (maybe split the 8" into 2 - 6" lines)? Should I also put a small 4" line for the bathroom (it shares a wall with the furnace room)?

Thank you so much for your help... Just trying to get this project done with our limited funds before the baby is here in May!
 
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Old 02-04-13, 08:42 AM
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Ancillary question - do you run a dehumidifier in the summer? If so, in what room? The reason I ask is that if you run one and it's in the laundry room, I would put a vent in that room to help circulate the dehumidified air throughout the house.
 
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Old 02-04-13, 10:39 AM
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yes 10 inch bedroom 7in main room 8in

7in good for 145 cfm in sold pipe
8in good for 205 cfm in sold pipe

Flex will lower the CFM

Bath id but a 6in in
 
 

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