Where should I tap in to the existing duct work?

Question

Bought a 1981 repo a year ago. The house measures around 900 SQFT. I enclosed the one car garage (10'x20') @ 200 SQFT to serve as an office. The front of the garage is insulated (10') along with the wall attached to the house (20'). I am planning on running 2 ducts off of the central air system into the room and insulating the ceiling of the garage. I do not have the cash right now to insulate the other 2 dry-walled walls with blown in insulation. One of the non-insulated walls (20') faces West and gets pretty hot in the late afternoon. The other 10' wall faces North and is shaded 24/7 by trees. I am located in Dallas so right now temps are running 95 to 105 degrees F.

I have a Goodman (Furnace: GMS80603AN/Outside Unit: VSX130361) central air 3.0 ton system (Cool is electric & Heat is natural gas) which, according to specs, does 60,000 BTU/h with a max CFM @ 0.5" ESP of 1200. The system is located in an interior closet and utilizes an up-flow method into the attic where the ducts attach. From my measurements, it appears that the ducts coming off of the supply duct measure between 8-10". One side of the supply duct is free for new ducts.

I have a few possible options that I have been kicking back and forth in my mind:

1) Take a new duct off of the supply duct to the office. A new duct run would be about 30-35' long.

I have 2 bags of 25' 12" duct that a friend gave me. Should I use this duct considering its size? Will this cause an airflow problem to the smaller ducts? I plan to divide the duct in the office into two air ducts into the room. If that size supply is safe, what size would I use for each of the two office vents?

2) I have two ducts from the supply duct that run to the kitchen and living room. They split off into two 8" ducts for each room. I could tap into one of these to run both office ducts or tap into each duct run for a single duct run into the office. The run would be about 5' long.

I hope I didn't leave any important info out. What is your opinion?

Additional Measurements: The Supply Duct measures 15" x 20" The diameter of the Kitchen and Living Room vents (2 each) measure 7". I believe but have not measured the duct before the split for each room to be either 8" or 10". I am guessing 8".

Answer 1

Basically you are trying to balance your new ducts with respect to your old ducts by sizing the new ducts. If you make a mistake or if your relative cooling loads (office vs rest of the house) ever changes, your only recourse would be to replace the ducts.

My preference is to size the ducts as large as reasonable possible (without increasing cost too much) and use a damper to tune the airflow ratios. Since you have 12" duct and a place to attach them, just get a damper like this:

http://www.grainger.com/Grainger/DAYTON-Round-Balancing-Damper-2TFY1?Pid=search

An uninsulated unshaded west wall in Texas can probably generate a very high cooling load in the afternoon.