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• If providing openings into adjacent rooms is undesirable, combustion and ventilation air can be brought into
the boiler room from outdoors. See the instructions under “For Buildings of Unusually Tight Construction”.
B. For Buildings Of Unusually Tight Construction
1) Provide outdoor air through two permanent openings which communicate directly or by duct with the outdoors or
spaces (crawl or attic) freely exchanging air with the outdoors. The top edge of the upper opening must be within
12” of the ceiling and the bottom edge of the lower opening must be within 12” of the floor. The minimum
dimension of each air opening is 3 inches. Size each opening per the following instructions:
a. Direct communication with outdoors (Figure 18) require a minimum free area of 1 square inch per 4,000 Btu per
hour input based on all fuel burning appliances in the boiler room space but not less than 100 square inches. The
minimum opening size is 3 inches.
b. Vertical ducts or Openings (Figure 16 and Figure 17) require a minimum free area of 1 square inch per 4,000 Btu
per hour input of all fuel burning appliances in the boiler room space but not less than 100 square inches. The cross-
sectional area of the duct shall be the same as the required free area opening. The minimum opening size is 3 inches.
c. Horizontal ducts to the outdoors (Figure 19) require a minimum free area of 1 square inch per 2,000 Btu per hour
input of all fuel burning appliances in the boiler room space but not less than 100 square inches. The cross-sectional
area of the duct shall be the same as the required free area opening. The minimum opening size is 3 inches.
d. Louvers and Grilles of Ventilation Ducts - All outside openings should be screened or louvered. Louvers will
prevent the entrance of rain and snow. Louvers and grilles must be fixed in the open position. If motorized louvers
are used, they must be interlocked to prevent boiler operation in the event the louvers do not fully open.
Figure 15: Boiler Installed In Confined Space, All Air From Inside
Important
When determining the “free area” of an opening the blocking effects of mesh screens,
grills, louvers and the like should be taken into account. Where screens are used, they
must be no finer than 1/4” (4 x 4) mesh.