I was recently asked to look over the utility closet of a friend, and several things I saw looked very concerning, from an energy-efficiency standpoint. This closet is approximate 3' x 6', and is in the (unheated) garage. It shares a wall with the house, but also has an external wall. Through the external wall are two 4" x 10" vents, one near the floor, the other near the ceiling. The large garage-facing wall has a 48" double bi-fold closet door.
A sketch (arrows show exterior vents):
During the winter, outside temperatures can reach -20 F. During the summer, they can reach 110 F. This is a gas furnace, so I understand it needs clean air for combustion and free exhaust. (It does appear to have a chimney.) But I'm concerned that it is doing far more work than it should need to, when it's environmental air can easily drop below freezing. Is there anything that can be done to make it's life happier?