How can I run added duct work from my house furnace to heat my attached garage?
Get a separate heater for your garage. The vent size that you ran is big enough to carry 70 cfm of air which is enough for a small bathroom. In some areas you may be violating codes. Most areas do not allow ducts to vent into a garage from your house duct. The reason being is you have the ability to get fumes from you garage directly into your house. If you have exhaust fumes, gas fumes and etc. they may easily get into you duct system. I wish you luck! They make regular small heaters for your garage.