One permanent opening, commencing within 12 inches (305 mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of not less than 1 inch (25.4 mm) from the sides and back and 6 inches (152 mm) from the front of the appliance. The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 701.6.2) and shall have a free area not less than the following:
- One square inch per 3000 Btu/h (0.0007 m2/kW) of the total input rating of appliances located in the enclosure.
- Not less than the sum of the areas of vent connectors in the space. [NFPA 54:220.127.116.11]