904.10 Furnace (Upright and Horizontal)

Upright furnaces shall be permitted to be installed in an attic, furred, or under-floor space exceeding 5 feet (1524 mm) in height, provided the required listings and furnace and duct clearances

TABLE 904.3.2 CLEARANCES TO COMBUSTIBLE MATERIAL FOR UNLISTED FURNACES AND BOILERS* [NFPA 54: TABLE 10.3.3.2]
MINIMUM CLEARANCE (inches)	MINIMUM CLEARANCE (inches)	MINIMUM CLEARANCE (inches)	MINIMUM CLEARANCE (inches)	MINIMUM CLEARANCE (inches)	MINIMUM CLEARANCE (inches)	MINIMUM CLEARANCE (inches)
APPLIANCE	ABOVE AND SIDES OF FUR- NACE PLENUM	TOP OF BOILER	JACKET SIDES AND REAR	FRONT	DRAFT HOOD AND BAROMETRIC DRAFT REGULATOR	SINGLE-WALL VENT CONNECTOR
1.Automatically fired, forced air or gravity system, equipped with temperature limit control that cannot be set higher than 250°F.	6	–	6	18	6	18
2.Automatically fired heating boilers – steam boilers operating at not over 15 pounds- force per square inch (psi) and hot water boilers operating at 250°F or less.	6	6	6	18	18	18
3.Central heating boilers and furnaces, other than in 1 or 2.	18	18	18	18	18	18

For SI units: 1 inch = 25.4 mm, °C = (°F-32)/1.8, 1 pound-force per square inch = 6.8947 kPa

See Section 904.0 for additional requirements for central heating boilers and furnaces.

are observed. Horizontal furnaces shall be permitted to be installed in an attic, furred, or under-floor space, provided the required listings and furnace and duct clearances are observed.