Building codes often don't specify specific minimum clearances around a furnace within an enclosure, instead deferring to the recommendation of the furnace's manufacturer; some codes, however, specify a minimum clearance of 3 inches regardless of the manufacturer's recommendation. Generally, clearance around the furnace needs to be sufficient to allow for inspection and maintenance of the furnace without necessitating removal or disassembly of the unit. The doorway of the furnace room needs to be wide enough to allow for the removal and replacement of the furnace. Most codes require a minimum of 30 inches of clear space between the front of the furnace and any permanent obstruction.
Minimum Room Size
In order to operate effectively, a furnace needs a consistent supply of combustion air, so a furnace enclosure must contain enough air to adequately feed the furnace's flame. The UMC requires that a furnace room have a volume of at least 50 cubic feet for each 1,000 Btu/h of the furnace's input rating combined with the input ratings of any other appliances in the room. If the furnace room receives adequate ventilation from an adjacent room, the volume of the adjacent room can be counted toward the minimum volume requirement.
The UMC specifies that a furnace room should receive ventilation from an adjacent room through two separate vent openings, one of them within 12 inches of the floor and the other within 12 inches of the ceiling. The vents must provide at least 1 square inch of open space for each 1,000 Btu/h total input of all appliances in the room, with a minimum open area of 100 square inches.
Gas Valve and Disconnect Switch
Some local codes specifically require that a gas line shutoff valve and an electrical disconnect switch be located within the furnace room. These codes may require that the electrical cutoff be an actual switch and not merely a cord plugged into an electrical outlet.