Return air taken from a garage or a mechanical closet containing fuel-burning appliances is a concern because:

A return opening is the suction side of the air handler, and whatever air it can reach it will pull. In a garage that means vehicle exhaust, stored fuels, solvents and pesticides; in a mechanical closet it means the products of combustion from a furnace or water heater. Two mechanisms deliver them. The return draws directly from the space if an opening is located there, and it depressurises the space enough to reverse the natural draft of any atmospheric appliance, so flue gases spill instead of rising. Either way the contaminants enter the return and are distributed through every supply register in the house. This is why return air from garages and from rooms containing fuel-burning appliances is prohibited, and why leaky return ducts passing through those spaces produce the same result even without a deliberate opening.

The return draws air from the space and depressurises it, which can reverse an atmospheric appliance's draft. Combustion products and garage contaminants then enter the return and are distributed throughout the house.

Return air must not be taken from garages, or from rooms and closets containing fuel-burning appliances, because blower-induced depressurisation can reverse appliance draft and because contaminants enter the distribution system directly.

The Return Drinks the Room. Whatever is in that space goes into the ducts and out every register in the house.

Leaky return ducts crossing a garage or furnace closet produce the same outcome without any grille. Check duct integrity in those spaces, not just grille locations.

• •Framing a prohibited return location as an efficiency issue
• •Checking only for grilles while ignoring leaky return ducts in the same spaces
• •Overlooking depressurisation as the mechanism that reverses draft