A return air register located in the same room as the furnace can disrupt the flow of air for combustion to the burners, and even suck combustion gases into the return air duct. So it is not a good idea. The problems caused by air being sucked away from the furnace burners are so serious that the access panel to the blower of a furnace (typically at the the bottom of the unit and just below the burners) even has a safety switch that will shut off the system if the panel is open, eliminating the possibility of the blower sucking combustion gases from the burners into the home. 

    But the International Residential Code (IRC M1602.2) includes an exception, which allows a sole return air register to be in the same room as the furnace when it meets all of these three conditions: 

1. If supply air is provided (room not sealed), and
2. If the return is located a minimum of 10 feet away from equipment, and
3. If the volume of air in the room is a minimum of 100 cubic feet per 1,000 BTU of output rating. A 40,000 BTU/Hr system, for example, would require a room with a volume of 4,000 cubic feet.

    The photo below shows an example of a furnace that is not compliant with this rule. The return air and filter is on the left side of the base of the unit at left arrow, and combustion air is supplied through the grille slots at the front of the furnace at the right arrow. Not even close to acceptable. This system had multiple other defects and was a classic, unsafe amateur installation.

    Also, see our blog post Is a furnace allowed in a bedroom, bathroom, or an adjoining closet?

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I find out the age of my air conditioner or furnace?

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning?

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

    Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.