I'd like to add a vent from the house to the attached garage to make the garage more comfortable in the winters (I'm in the Kansas City area). My garage is 26' x 30' (3 car with an extra 4' length) and is insulated (marginally with insulation in the walls and styrofoam insulation in the garge doors. My thought is to attach an insulated run to a manual damper I'll add to the main trunk so I can close the damper in the summer. How will this affect the heating efficiency of the rest of the house? What other issues should I consider before doing this?
You will lose some air that is currently going to your house and will now go to your garage. It may result in your unit running slightly longer between cycles to satisfy the existing room thermostat. You will be trying to heat a space that has minimum insulation, so it will lose more heat than the same size room inside your house. I would put the manual damper close to the takeoff from the existing duct and make it easily accessible so that I could open it only when I wanted heat there, and close it the rest of the time. Get a damper that seals tightly when closed. Locate the new register near where you desire the heat also. Insulate and tightly seal the new duct!