Example: The boiler is rated at 100,000 Btuh. The water heater is rated at 30,000 Btuh. The total is 130,000 Btuh. Two grilles are needed, each
with 130 sq in. of FREE opening. Metal grilles have about 60 percent FREE area. To find the louvered area needed, multiply the FREE area
required by 1.7 (130 X 1.7 = 221.0 sq in. louvered area). In this example, 2 grilles each having an 8-in. X 30-in. (240 sq in.) louvered area would
be used. Fig. 4 shows grille locations.
B.
All Air From Outdoors
The confined space shall be provided with 2 permanent openings, 1 commencing within 12 in. of the top and 1 commencing within 12 in. of the
bottom of the enclosure. The openings shall communicate directly, or by ducts, with the outdoors or spaces (crawl or attic) that freely communicate
with the outdoors.
1. When directly communicating with the outdoors, each opening shall have a minimum free area of 1 sq in. per 4000 Btuh of total input rating
of all equipment in the enclosure.
2. When communicating with the outdoors through vertical ducts, each opening shall have a minimum free area of 1 sq in. per 4000 Btuh of
total input rating of all equipment in the enclosure.
3. When communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 sq in. per 2000 Btuh
of total input rating of all equipment in the enclosure.
4. When ducts are used, they shall be of the same cross-sectional area as the free area of the openings to which they connect. The minimum
dimension of rectangular air ducts shall not be less than 3 in.
Fig. 4—Air Openings for Closet or Utility Room Installations
A95594
VENTILATING
GRILLE
COMBUSTION
AIR GRILLE
Table 1—Fresh Air Duct Capacities for Ducts Supplying
Fresh Air to Boiler in Tightly Constructed Houses (Btuh)*
FRESH AIR
DUCT SIZE (IN.)
1/4-IN. MESH
SCREEN
WOOD
LOUVERS
METAL
LOUVERS
3 X 12
144,000
36,000
108,000
8 X 8
256,000
64,000
192,000
8 X 12
384,000
96,000
288,000
8 X 16
512,000
128,000
384,000
* Based on opening covered by 1/4-in. mesh screen, wood louvers, or metal
louvers.
Fig. 5—Fresh Air Duct for Tightly Sealed House
A95595
BOILER
FRESH AIR
DUCT
—7—
