We have a heated/air conditioned garage attached to our home. The furnace that heats the garage also heats other parts of the house which were added on when the garage addition was done in 1984. We had an issue with carbon monoxide when my husband ran one of his cars. It makes me wonder if having ducts in a garage that also connects to a house is up to code. We have a furnace with a humidifier as well as a sink in the garage, so not heating it would cause pipes to freeze. We live in the Philly area and have some very cold winters here.
We do have a CO detector in the garage. The car set it off and our alarm monitoring company couldn't get through to us about it because the alarm was sending info to them over our phone line. They called in the situation and we had at least half a dozen police/fire vehicles here. It was all because it took my husband a few tries to get his car aligned in the garage so he can walk around it. The highest CO level in the house was 10 which isn't dangerous, but it makes me wonder if having the house and garage connected by ducts is allowed by code.