Most new category IV gas furnaces are dual-rated for either a direct-vent or a non-direct-vent installation. When installing with a direct-vent configuration, the intake and exhaust pipes are supposed to terminate in the same pressure zone on the outside of the building. When installed with a non-direct-vent configuration, the exhaust is supposed to terminate outdoors, while the intake simply takes air from the furnace room. Sometimes the manufacturer advises placing an elbow on the intake opening to prevent debris from falling into the burner compartment.
Lately, I've been seeing what look like hybrid installations where the exhaust pipe goes through the roof, but the intake pipe terminates inside the attic. This is not a configuration that's described in any manufacturer's instructions that I've seen.
Clearly, the air in the attic and the air above the roof will be in different pressure zones, so this can't really be considered a proper direct-vent installation. Can it, however, be considered a proper non-direct-vent installation?
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