7
Table 2 – Minimum Free Area Required for Each Combustion Air Opening or Duct to Outdoors
FURNACE
INPUT
(BTUH)*
TWO HORIZONTAL DUCTS
(1 SQ. IN./2,000 BTUH)
(1,100 SQ. MM/KW)
SINGLE DUCT OR OPENING
(1 SQ. IN./3,000 BTUH)
(734 SQ. MM/KW)
TWO OPENINGS OR
VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH)
(550 SQ. MM/KW)
Free Area of Open
ing and Duct
Sq. In (Sq. mm)
Round Duct
In. (mm)
Dia
Free Area of Open
ing and Duct
Sq. In (Sq. mm)
Round
Duct
In. (mm)
Dia
Free Area of Opening
and Duct
Sq. In (mm)
Round Duct
In. (mm) Dia.
40,000
20 (12904) 5 (127) 14 (8696) 5 (127) 10 (6452) 4 (102)
60,000
30 (19355) 6 (152) 20 (13043) 5 (127) 15 (9678) 5 (127)
80,000
40 (25807) 7 (178) 27 (17391) 6 (152) 20 (12904) 5 (127)
100,000
50 (32258) 8 (203) 34 (21739) 7 (178) 25 (16130) 6 (152)
120,000
60 (38709) 9 (229) 40 (26087) 7 (178) 30 (19355) 6 (152)
* Not all input sizes are available.
EXAMPLE: Determining Free Area
FURNACE WATER HEATER TOTAL INPUT
100,000 + 30,000 = (130,000 divided by 4,000) =
32.5 Sq. In. for each two Vertical Ducts or
Openings
60,000 + 40,000 = (100,000 divided by 3,000) = 33.3 Sq. In. for each Single Duct or Opening
80,000 + 30,000 = (110,000 divided by 2,000) = 55.0 Sq. In. for each two Horizontal Ducts
Table 3 – Minimum Space volumes for 100% combustion, Ventilation, and Dilution from Indoors
ACH*
OTHER THAN FAN-ASSISTED TOTAL
(1,000'S BTUH GAS INPUT RATE
FAN-ASSISTED TOTAL
(1,000'S BTUH GAS INPUT RATE)
30 40 50 44 66 88 110 132
Space Volume (ft.
3
)
0.60 1,050 1,400 1,750 1,100 1,650 2,200 2,750 3,300
0.50 1,260 1,680 2,100 1,320 1,980 2,640 3,300 3,960
0.40 1,575 2,100 2,625 1,650 2,475 3,300 4,125 4,950
0.30 2,100 2,800 3,500 2,200 3,300 4,400 5,500 6,600
0.20 3,150 4,200 5,250 3,300 4,950 6,600 8,250 9,900
0.10 6,300 8,400 10,500 6,600 9,900 13,200 16,500 19,800
0.00 NP NP NP NP NP NP NP NP
*Air Changes/Hour
FIRE HAZARD
Failure to follow this warning could result in personal
injury, death and/or property damage.
Do not install the furnace on its back or hang furnace with
control compartment facing downward. Safety control
operation will be adversely affected. Never connect
return−air ducts to the back of the furnace. (See Fig. 6)
!
WARNING
A02054
Fig. 6 − Prohibit Installation on Back
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, detergents, bleaches, cleaning solvents, salts, air
fresheners, and other household products.
CAUTION
!
LOCATION RELATIVE TO COOLING EQUIPMENT
The cooling coil must be installed parallel with, or on the
downstream side of the unit to avoid condensation in the heat
exchangers. When installed parallel with the furnace, dampers or
other flow control must prevent chilled air from entering the
furnace. If the dampers are manually operated, they must be
equipped with means to prevent operation of either unit unless
the damper is in the full−heat or full−cool position.
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 current edition of
NFGC NFPA54/ANSI Z223.1, Air for Combustion
and Ventilation, and applicable provisions of the local
building codes.
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