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Can a Furnace Be in a Closed Room?

Cassie Damewood

Furnaces are commonly surrounded by partitions or half-walls to hide them for aesthetic reasons. There is ample airflow in these enclosures, so the furnace gets appropriate ventilation and combustion air from the rest of the house or through grills in the surrounding floors or walls. Furnaces should not be housed in sealed or closed rooms for several safety and health reasons.


Carbon Monoxide

Furnaces are safest when installed in open spaces.

Non-ventilated rooms can cause a buildup of carbon monoxide from a furnace. This deadly gas is odorless and can seep into other areas of the home. According to the Uniform Building Code, which provides regulatory guidelines for building design and construction, to be safe, even a partially enclosed room with a direct-vent furnace requires a minimum of 50 cubic feet of volume for every 1,000 BTUs of fuel input. BTUs are British Thermal Units, which is how fuel input is measured. The room must also have a combustion air opening that is at least 1 square-inch for each 3,000 BTUs of incoming fuel. The opening has to be a minimum of 12 inches from the ceiling and be adequately vented to the outside of the house. Fan-assisted or gravity warm-air type furnaces must have two openings in the room at least 1 square-inch in size per 1,000 BTUs of fuel input, one installed 12 inches from the floor and one no more than 12 inches from the ceiling.

Gas Buildup

Another hazard from closed furnace rooms is the possible buildup of gas fumes. If the room is sealed, the harmful and sometimes deadly fumes cannot be smelled as they slowly penetrate the air in the house. If gas accumulates in a sealed furnace room over time, it becomes explosive before the smell is detectable and is likely to blow up when the door is opened.


Two options are available to safely operate furnaces without relying on vents inside the furnace room. Sealed combustion furnaces are factory-equipped to draw outdoor air into the furnace and expel it through the flue. No air inside the house is used, so vents do not have to compete with those used for stove or bathroom exhaust vents. Another option is the Hoyme Damper, which has a sensor that opens a motorized damper when the furnace requires air for combustion and automatically closes it when enough air has entered the furnace.

Inspections and Permits

If your house is relatively new, the building inspector should have issued permits indicating the furnace’s housing, venting and combustion systems were safe and followed codes before you occupied the residence. For older furnaces, have a certified furnace or municipal building inspector check your furnace for compliance and, if necessary, supply you with guidelines to bring it up to code.