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Can a grille covering a hole in the drywall ceiling be used for the required make up (combustion supply) air for a gas water heater or furnace in a garage?
Saturday, July 27, 2019
The walls and ceiling between an attached garage and the house living area are required to be minimum 1/2” drywall or equivalent. Most garages have a ceiling secured to the bottom chord of the roof trusses or rafters. If there is no ceiling and the roof structure is exposed in the garage, then the drywall separating the garage from the dwelling must extend up to the underside of the roof sheathing.
There is a high incidence of fires started in garages due to the gasoline and other flammable materials often stored there and the intent is to create a drywall barrier between the garage and house for fire protection. So, any holes in the ceiling, including one to supply make up air for gas appliances, are not allowed.
While this is intended to create a fire barrier between garage and house, it is not specified as “one-hour rating” or “fire wall” by the Residential Editon of the Florida Building Code (FBC) or the International Residential Code (IRC). It is called a “separation” in section R302 of the code entitled “Fire-Resistant Construction."
Here’s the citation at R302.5 and Table 302.6 of the FBC that spells out these requirements. The International Residential Code is similar.
We still occasionally see installations done incorrectly, like the one shown at the top of this page. When we asked the building code expert Jerry Peck why this continued to be done incorrectly until a few years ago, he had an interesting comment: "The old days of 'making a hole through the garage ceiling into the attic for combustion air’ hasn't been allowed for quite some time, and maybe never was allowed. It was done and accepted for a long time, then as inspectors and contractors began to understand what the code was saying, and as codes began saying things more clearly—which is one reason code books are so much larger today than 30-50 years ago—the 'hole through the garage ceiling' began being recognized as not allowed."
The correct way to provide make up air for a water heater or furnace in a garage is to add an air connector (an uninsulated duct) that complies with R302.5.2 of the FBC (minimum No. 26 gage (0.48 mm) sheet steel, 1 inch (25.4 mm) minimum rigid nonmetallic Class 0 or Class 1 duct board), like in the photo below, viewed from the attic.
A grille and weather-resistant exterior hood for make up air can also be installed through an exterior wall, as shown below.
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