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Fresh Air for Combustion - Chimney Vented Boilers
8
WARNING
Be sure to provide enough fresh air for combustion.
Enough air ensures proper combustion and assures that
no hazard will develop due to the lack of oxygen.
You must provide for enough fresh air to assure proper
combustion. The fire in the boiler uses oxygen. It must have a
continuous supply. The air in a house contains only enough
oxygen to supply the burner for a short time. Outside air must
enter the house to replace that used by the burner. Study following
examples 1 and 2 to determine your fresh air requirements.
EXAMPLE 1: Boiler Located in Unconfined Space
If your boiler is in an open area (unpartitioned basement) in a
conventional house, the air that leaks through the cracks around
doors and windows will usually be adequate to provide air for
combustion. The doors should not fit tightly. Do not caulk the
cracks around the windows.
An unconfined space is defined as a space whose volume is not
less than 50 cubic feet per 1,000 Btu per hour of the total input
rating of all appliances installed in the space.
EXAMPLE 2: Boiler Located in Confined Space
A. All Air from Inside the Building: The confined space shall
be provided with two permanent openings communicating
directly with an additional room(s) of sufficient volume so that
the combined volume of all spaces meets the criteria for an
unconfined space. The total input of all combustion equipment
installed in the combined space shall be considered in making
this determination. Each opening shall have a minimum free
area of one square inch per 1,000 Btu per hour of the total
input rating of all combustion equipment in the confined space,
but not less than 100 square inches. One opening shall be
within 12 inches of the top and one within 12 inches of the
bottom of the enclosure. See Figure 5.
Example: Your boiler is rated at 100,000 Btu per hour. The
water heater is rated at 30,000 Btu per hour. The total is
130,000 Btu per hour. You need two grilles, each with 130
square inches of FREE opening. Metal grilles have about 60%
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 two grilles, each having an 8" x 30"
(240 sq. in.) louvered area would be used. Fig. 5 shows the
grille locations.
B. All Air from Outdoors: The confined space shall be provided
with two permanent openings, one commencing within 12
inches of the top and one commencing within 12 inches 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 one square
inch per 4,000 Btu per hour 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
one square inch per 4,000 Btu per hour of total input rating
of all equipment in the enclosure.
NOTE
If you use a fireplace or a kitchen or bathroom exhaust
fan, you should install an outside air intake. These devices
will rob the boiler and water heater of combustion air.
3. When communicating with the outdoors through horizontal
ducts, each opening shall have a minimum free area of
one square inch per 2,000 Btu per hour 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 be not less than three inches.
¼" Mesh
Screen Btuh
Wood
Louvers Btuh
Metal
Louvers Btuh
3½" 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
Fresh Air
Duct Size
FIG. 5 - AIR OPENINGS FOR CLOSET OR
UTILITY ROOM INSTALLATIONS
Fresh Air Duct Capacities for Ducts Supplying Fresh Air to
boiler in tightly constructed houses. British Thermal Units
per hour input (Btuh)*.
FIG. 6 - FRESH AIR DUCT CAPACITIES
* Based on opening covered by 1/4 mesh screen, wood louvers, or metal louvers
FIG. 7
FRESH AIR DUCT FOR TIGHTLY SEALED HOUSE