I've written about how bad it is to put your heating and cooling system in an unconditioned attic, especially in a cooling climate. There's one place, though, that's even worse: an attached garage, as shown at left. From an efficiency standpoint, the garage would be a better location than an unconditioned attic. The problem here is health and safety. Garage air getting into the home can lead to poor indoor air quality.
Homes with an attached garage generally have higher levels of carbon monoxide. They also have other pollutants in their air. Think of all the stuff that gets stored in garages: lawn mowers, gasoline, paint, pesticides, and more. If you have an attached garage, you want the air in your home to remain free of the stuff out there, right?
It's possible to keep garage air out of the house, at least to a degree, but you have to do two things:
When your heating and cooling system is in the garage, as is the one shown in the photo above, it creates a number of problems. In the photo below, I've added three arrows to point to the most egregious of the problems.
Red arrow. This points to a violation of rule #1 above. Where the ducts go through the wall into this house, there's no air-sealing. Air from the garage can easily get into the house. This one's not as bad as some that I've seen.
Blue arrow. That grille you see there is a return vent. When the heating and cooling system is running, it's pulling air in. It's supposed to be pulling air in only from the house but because of air leaks, it also pulls air in from the garage. And of course, every time that door opens when the system is on, it pulls in garage air.
Imagine this is your house. You've just come home and parked in the garage. The garage door came down quickly, trapping some of the car's exhaust fumes, including carbon monoxide. You get out of the car, open the door into the house, and enter. So does a significant amount of those exhaust fumes. The weatherstripping doesn't do any good when the door's open.
Yellow arrow. Even if you had a perfect air barrier between your home and the garage, you can still get poison garage air into your home. The yellow arrow is pointing to the filter cover, which had a lot of gaps around it. The one shown at right is from a different house, but it looks similar to the one on the system above.
When the system is running, there's negative pressure in the return side of the duct system. If there are leaks in the return ducts in the garage, they'll pull in contaminated garage air—and then distribute it to you and your family through the supply vents in your home.
(This home also has a bonus problem, one that happens to be quite common. The furnace and water heater flues are tied together. See problem #3 in this article I wrote about atmospheric combustion appliances earlier.)
Sadly, the answer is yes. The building codes say you can put your HVAC system and ductwork in a garage as long as you meet certain requirements. Here are two references:
Mechanical Code 304.6: Appliances located in private garages and carports shall be installed with a minimum clearance of 6 feet above the floor. Exception: The requirements of this section shall not apply where the appliances are protected from motor vehicle impact in accordance with Section 304.3 (which addresses elevation of ignition source).
IRC 309.1.1: Ducts in the garage and ducts penetrating the walls and ceilings separating the dwelling from the garage shall be constructed of a minimum No. 26 gauge steel sheet or other approved material, and shall have no openings in the garage.
There's another section somewhere, for which I don't have the reference, that bans the use of solid fuel heating systems in garages. You can put gas furnaces, boilers, and heat pumps out there, but no wood stoves or wood pellet burners.
If you have a heating and cooling system in an attached garage, here are a few things you can do:
If you find yourself with this problem, it's definitely worth doing something about. Just because you don't get acutely ill and end up in the hospital doesn't mean that you don't have an indoor air quality problem. Remember also that children are more susceptible to pollutants than adults.
Case Closed: Get Those Air Conditioning Ducts out of the Attic
I-Joists, Attached Garages, and the Air Leaks That Poison
How to Choose a Company to Do a Home Energy Audit