You cannot use the same forced air heating system for a house and garage heating purposes. I’ve explained that already in Attached Garage Firewall
posts (item #4)
The following are 2006 IRC (International Residential Code) paragraphs that refer to your questions:
309.1.1 – Ducts through a garage wall or ceiling common to a house must have minimum 26-gauge thick steel walls
309.1.1 – No air duct openings in the garage from the same heating system that is used for the house heating purposes
309.1.2 Air duct penetrations of garage – house common wall drywall surface must be sealed with approved firestop caulking (this is new in 2006 IRC)
Let me know if you have any other questions, just click “comment” below.