8
Table 2 – Minimum Free Area Required for Each Combustion Air opening of Duct to Outdoors
FUR-
NACE
INPUT
(BTUH)
TWO H OR IZONTAL DUCTS SINGLE DUCT OR OPENING TWO OPENINGS OR VERTICAL DUCTS
(1 SQ. IN./2,000 BTU H) (1,100 SQ. MM/KW) (1 SQ. IN./3,000 BTU H) (734 SQ. MM/KW) (1 SQ. IN./4,000 BTU H) (550 SQ. MM/KW)
Free Area of Opening
and Duct (Sq.. In.)
Round Duct (in.
Dia)
Free Area of Opening
and Duct (sq. In.)
Round Duct (in.
Dia)
Free Area of Opening
and Duct (Sq. In.)
Round Duct (In.
Dia)
44,000 22 6 14.7 5 11 4
66,000 33 7 22 6 16.5 5
88,000 44 8 29.3 7 22 6
110,000 55 9 36.7 7 27.5 6
132,000 66 10 44 8 33 7
154,000 77 10 51.3 9 38.5 8
EXAMPLES: Determining Free Area
FURNACE WATER HEATER TOTAL INPUT
110,000 + 30,000 = (140,000 divided by 4,000) = 35.0 Sq.. In. for each two Vertical Ducts or Open-
ings
66,000 + 40,000 = (106,000 divided by 3,000) = 35.3 Sq.. In. for a Single Duct or Opening
88,000
+ 30,000 = (118,000 divided by 2,000) = 59.0 Sq.. In. for each of two Horizontal Ducts
Table 3 – Minimum Space Volumes for 100% Combustion, Ventilation, and Dilution from Indoors
ACH
OTHER THAN FAN---ASSISTED TOTAL
(1,000s BTUH GAS INPUT RATE)
FAN---ASSISTED TOTAL
(1,000s BTUH GAS INPUT RATE)
30 40 50 44 66 88 110 132 154
SpaceVolume(ft.3)
0.60 1,050 1,400 1,750 1,100 1,650 2,200 2,750 3,300 3,850
0.50 1,260 1,680 2,100 1,320 1,980 2,640 3,300 3,960 4,620
0.40 1,575 2,100 2,625 1,650 2,475 3,300 4,125 4,950 5,775
0.30 2,100 2,800 3,500 2,200 3,300 4,400 5,500 6,600 7,700
0.20 3,150 4,200 5,250 3,300 4,950 6,600 8,250 9,900 11,550
0.10 6,300 8,400 10,500 6,600 9,900 13,200 16,500 19,800 23,100
0.00 NP NP NP NP NP NP NP NP NP
NP = Not Permitted
AIR FOR COMBUSTION AND
VENTILATION
Provisions for adequate combustion, ventilation, and dilution air
must be provided in accordance with:
S U.S. Installations: Section 9.3 of the NFGC, Air for
Combustion and Ventilation and applicable provisions
of the local building codes.
S Canadian Installations: Part 8 of the
CAN/CSA-- B149.1-- 05, Venting Systems and Air
Supply for Appliances and all authorities having
jurisdiction.
FURNACE CORROSION HAZARD
Failure to follow this caution may result in furnace damage.
Air for combustion must not be contaminated by halogen
compounds, which include fluoride, chloride, bromide, and
iodide. These elements can corrode heat exchangers and
shorten furnace life. Air contaminants are found in aerosol
sprays, deter gents, bleaches, cleaning solvents, salts, air
fresheners, and other household products.
CAUTION
!
CARBON MONOXIDE POISONING HAZARD
Failure to follow this warning could result in personal
injury or death.
The operation of exhaust fans, kitchen ventilation fans,
clothes dryers, attic exhaust fans or fireplaces could create a
NEGATIVE PRESSURE CONDITION at the furnace.
Make--up air MUST be provided for the ventilation devices,
in addition to that required by the furnace. Refer to the
Carbon Monoxide Poisoning Hazard warning in the venting
section of these instructions to determine if an adequate
amount of make-- up air is available.
!
WARNING
The requirements for combustion and ventilation air depend upon
whether or not the furnace is located in a space having a volume
of at least 50 cubic feet per 1,000 Btuh input rating for all gas
appliances installed in the space.
S Spaces having less than 50 cubic feet per 1,000 Btuh
require the OUTDOOR COMBUSTION AIR
METHOD.
315A
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