A gas furnace is to be installed in a garage. The national standards require that the burners and ignition sources must be located ______ above the floor.

The requirement for the installation of gas furnace burners and ignition sources at a specific height above the floor is primarily to enhance safety and minimize risks associated with flammable gases or vapors that may accumulate in a garage setting. The correct answer is that the burners and ignition sources must be located 18 inches above the floor. This height is designed to mitigate the likelihood of ignition from fumes that are heavier than air and can settle near the ground.

In garages, where gasoline and other volatile substances are often stored, having the ignition sources elevated reduces the potential for these substances to ignite. The 18-inch elevation is in accordance with national safety standards, ensuring that gas appliances don't create hazardous conditions in environments where flammable vapors might be present.

This standard is critical in promoting both the safety of occupants and the proper functioning of the gas heating system in potentially hazardous environments.