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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): sketch of the just-described utility closet

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?

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Is there a chance that this is a high efficiency unit that has PVC inlet and exhaust? –  Jack Dec 7 '13 at 0:45
    
No, this is not a high efficiency unit. The exhaust is metal, and connects with the exhaust for the nearby water heater, which is also gas. –  Scivitri Dec 9 '13 at 8:53
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Without major retrofits, probably not a great deal. Not blowing up the house trumps insulating and especially air-sealing with gas appliances - someone already made the decision to park these out in the cold rather than in the house, and adequate ventiliation is a must. There may also be a deliberate intent to bleed some heat to the garage without actually heating it, as such. You certainly can't effectively insulate the closet with those doors, and given the needed vents, it wouldn't do a great deal anyway.

There is no real concern about the air feeding the combustion being cold - it makes little to no difference. Being sure that the air ducts to and from the house are well-insulated where exposed would be good, and likewise the water lines for the water heater.

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So we can wrap the air ducts with insulation? Does it take special insulation, for fire safety? I expect probably the simplest thing would be to grab a couple of water heater wrap sheets, and wrap around the ducts. –  Scivitri Dec 9 '13 at 8:56
    
Should not need anything too special - air ducts should not be running at extremely elevated temperatures - water heater wraps should be fine. Just keep clear of the exhaust (the water heater wrap instructions will probably mention a specific distance - it may not come into play on a furnace, depending where the ducts and exhaust are located.) Remember to insulate the "cold air return" as well - from this point of view, it's warm house air. –  Ecnerwal Dec 10 '13 at 0:40
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