Before installing boiler (continued)
freely communicate with the outdoors, as follows:
•Where directly communicating with the outdoors or where communicating to the outdoors through vertical ducts, each opening shall have a minimum free area of 1 in2/4000 Btuh (550 mm2/kW) of total input rating of all appliances in the enclosure.
•Where communicating with the outdoors through horizon- tal ducts, each opening shall have a minimum free area of 1 in2/2000 Btuh (1100 mm2/kW) of total input rating of all appliances in the enclosure.
Outside openings: ONE permanent opening method
One permanent opening, commencing within 12 inches (300 mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of at least 1 in. (25 mm) from the sides and back and 6 inches (150 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 and shall have a minimum free area of the following:
(1)1 in2/3000 Btu/hr (700 mm2 per kW) of the total input rating of all appliances located in the enclosure, and . . .
(2)Not less than the sum of the areas of all vent connectors in the space.
Combustion air — combination indoor and outdoor openings
ANSI/NFPA 54 allows combustion air to be taken from a combination of openings to outside and to interior spaces. Follow all requirements of the standard to determine the minimum volume of interior spaces and to calculate minimum sizes of openings.
system, the combustion air shall be supplied from outdoors at the minimum rate of 0.35 ft3/minute per 1000 Btuh (0.034 m3/min per kW) for all appliances located within the space.
•Where exhaust fans are installed in the building, additional air shall be provided to replace the exhausted air.
•Each of the appliances served shall be inter- locked to the mechanical air supply system to prevent main burner operation where the me- chanical air supply system is not in operation.
•Where combustion air is provided by the build- ing’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air.
Louvers, Grilles, and Screens.
The required size of openings for combustion, venti- lation, and dilution air shall be based on the net free area of each opening.
Where the free area through a design of louver or grille or screen is known, it shall be used in calculating the size opening required to provide the free area specified.
Where the louver and grille design and free area are not known, it shall be assumed that wood louvers will have 25 percent free area, and metal louvers and grilles will have 75 percent free area.
Minimum screen mesh size
Screens shall not be smaller than ¼ inch mesh.
Engineered Installations
Engineered combustion air installations shall provide an adequate sup- ply of combustion, ventilation, and dilution air and shall he approved by the authority having jurisdiction.
Mechanical Combustion Air Supply
Where all combustion air is provided by a mechanical air supply
Motorized louvers
Motorized louvers shall be interlocked with the appli- ance so they are proven in the full open position prior to main burner ignition and during main burner opera- tion. Means shall be proved to prevent the main burner from igniting should the louver fail to open during burner startup, and to shut down the main burner if the louvers close during burner operation.
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