Why can't I use my home's central air conditioning system to cool the garage?
Saturday, June 23, 2018
Tapping into your air conditioning system to add a couple of ducts may seem like an excellent idea if you if have a garage workshop, but it is not allowed by the building codes. The International Residential Code (IRC) and Florida Building Code (FBC - R302.5.2) both state that residential air conditioning ducts “shall have no openings in the garage.” The reason is fire safety. Garage fires tend to spread farther and cause more injuries and dollar loss per fire than fires that start in all other areas of the home, according to the U.S. Fire Administration.
So the building codes have multiple requirements intended to maintain a fire separation between a dwelling and an attached garage, particularly focused on making sure that there are no openings or penetrations through the walls and ceiling between the house and garage that would allow a fire to spread into the house. An air conditioning duct is a route that provides a direct air connection to the house if a/c vents/registers are installed, allowing quick spread of fire and fumes into the home.
There are other good reasons not to run a/c ducts out to serve a garage, including that your system was not designed for serving about 20% more square footage, garage doors leak air and are difficult to seal, and—because return air ducts would pull garage fumes into the house—the lack of return air would simply pressurize the garage and do a poor job of reducing the temperature.
Shown below is an example of a garage where the owner added two supply registers in garage to use the room as a workshop. You can see the garage door opener in the foreground. This would only be acceptable if it was a separate HVAC system with ducts serving only the garage, and if the wall between garage and living area of home extended all the way up to the underside of the roof sheathing to create a fire barrier. But that wasn’t the case here.
And here’s another example. In this case the homeowner simply had supply registers cut into each of the two main supply ducts above the two air handlers in the garage.
Cooling down your garage with a mini-split (ductless) air conditioner or wall unit is fine, just don’t use your home’s central a/c system. Also, see our article How can I make my garage cooler in the Florida heat?
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