Return Air Grate Size in Building Code for Gas Furnace Rooms

Closed-Air System

• A gas-powered, forced-air furnace uses a closed-air system to circulate heated air throughout a home. Heat generated by burning gas warms the air delivered to the furnace through a return air duct system. The supply duct system then delivers warmed air into a home’s living spaces. While multiple supply ducts and vents lead out to different areas of a home, most furnace systems only operate off of one centrally located return register. This register and duct leads directly to the return air grate inside the gas furnace room. In effect, the system reconditions the same amount of air on a repeated basis. This means the furnace unit must receive a certain amount of return air in order to operate at optimal efficiency.

Air Flow Code Requirements

• As different gas furnace units operate at different capacities, air flow code requirements dictate how much air a particular unit requires in order to function safely and properly. According to the North Carolina Department of Insurance, the 2012 International Mechanical Code requires the total area within a return air duct system to equal a minimum of 2 square inches for every 1,000 BTUs generated by a furnace per hour. A BTU, or British thermal unit, represents one unit of energy usage. This requirement ensures that the air space inside a particular unit’s return duct system is large enough to accommodate a furnace’s capacity. These measurements act as standards that should equal or exceed the furnace manufacturer’s specifications.

Furnace Air Handler Capacity

• A gas furnace air handler delivers cool air to the furnace heat exchanger for warming and sends warm air through a home’s supply duct system. Air that moves through the furnace room’s return air grate goes directly to the air handler. In effect, the size of the return air grate allows a certain amount of air to flow to the air handler at a given time. In order to meet code requirements, the return grate must allow a certain amount of air to reach the air handler device. An air handler device uses a filter to block dirt and debris from entering the furnace unit. The size of the air handler filter provides an indication of how large the furnace room’s return grate should be.

Effects

• Since forced-air systems work within a closed-air environment, a gas furnace operating off of a return air grate that’s too small has less air to work with than it puts out. These conditions leave a furnace unit starved for return air supplies. When this happens, the unit has to work harder to produce the same heating results with more air going out than coming in. One way to test the air flow to a unit involves lifting one of the covers attached to the air handler compartment and letting it drop. If the cover slams shut, this may indicate an air-starved condition exists.