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or by ducts, with the outdoors or spaces (crawl or
attic) that freely communicate with the outdoors.
1. When communicating with the outdoors
directly or 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.
2. 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.
3. 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.
FRESH AIR DUCT CAPACITIES
The following table shows fresh air duct capacities (in
Bthu) for ducts supplying fresh air to boilers in tightly
APPLICABLE FEDERAL CODES
NFPA 54/ANSI Z223.1, National Fuel Gas Code (Part
7) and NFPA/ANSI211, Chimneys, Fireplaces, Vents,
and Solid Fuel Burning Appliances. These codes
contain information on special gas vents for Category
II, III and IV appliances, vent sizing, location, air space
clearances to combustibles and safe installation
practices. The gas vent installer should be familiar
with these Federal Codes as well as Local Codes and
Regulations.
GENERAL INFORMATION -
GAS VENTS & APPLIANCES
By federal codes, gas appliances are categorized by
the pressure and temperature of the flue gas vented
from the appliance. Category I and II appliances are
natural draft (draft hood) vented, with high flue gas
temperatures (Category I) and low flue gas temperatures
(Category II). Category III and IV appliances are fan
forced vents with high temperature (Category III) and
low temperature (Category IV) flue gasses. Appliance
efficiency is directly related to flue gas temperature.
Higher efficiency appliances remove more heat from
the gas, so they will have lower temperature flue
constructed houses (based on openings covered by ¼"
mesh screen, wood louvers, or metal louvers):
Fresh Air
Duct Size
3½" x 12"
8" x 8"
8" x 12"
8" x 16"
FIG. 2 – FRESH AIR DUCTS FOR TIGHTLY SEALED
HOUSES
FLUE CONNECTIONS
products. When flue gas temperatures are lowered,
corrosive condensates may form in Category II, III, IV
appliance vents, so special, corrosive resistant venting
systems are required for higher efficiency appliances.
Vents for Category I appliances may not
be suitable for use with Category II, III, or
IV appliances because condensate may
corrode the vent.
Vents for Category III appliances may not be
suitable for use with Category I appliances
because flue gas temperatures may be too
high.
Proper operation of the vent system and
appliance is dependent upon the use of all
parts specified by the manufacturer for use
in the particular installation. Appliance and
vent system performance may be affected by
improper assembly.
See this manual for vent pipe installation
4
Duct Covering
¼" Mesh
Wood
Screen
Louver
144,000
36,000
256,000
64,000
384,000
96,000
512,000
128,000
!
!
WARNING
and components.
Metal
Louver
108,000
192,000
288,000
384,000
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