UNIT DAMAGE HAZARD
Failure to follow this caution may result in minor property or
unit damage.
The furnace can operate in the high-heat mode when certain
fault conditions occur. The following precautions should be
taken:
1.Size gas piping based on the high-heat input.
2.Check the high-heat input and adjust it per the main
literature instructions.
Step 3—Furnace Location Relative to Cooling
Equipment
The cooling coil must be installed parallel with or on downstream
side of furnace to avoid condensation in heat exchanger. When
installed parallel with a furnace, dampers or other means used to
control flow of air shall be adequate to prevent chilled air from
entering furnace. If dampers are manually operated, they must be
equipped with a means to prevent operation of either unit unless
damper is in full-heat or full-cool position.
Step 4—Hazardous Locations
FIRE, EXPLOSION, INJURY OR DEATH HAZARD
Improper location or inadequate protection could result in fire
or explosion.
When furnace is installed in a residential garage, it must be
installed so that burners and ignition sources are located a
minimum of 18 in. above floor. The furnace must be located
or protected to avoid physical damage by vehicles. When
furnace is installed in a public garage, airplane hangar, or
other building having a hazardous atmosphere, unit must be
installed in accordance with requirements of National Fire
Protection Association, Inc. (See Fig. 18.)
Step 5—Furnace Location and Application
DIRECT VENT (2-PIPE) APPLICATION
Furnace may be located in a confined space without special
provisions for dilution or ventilation air.
Table 1—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
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)
40,000
20
6
13.4
5
10
4
60,000
30
7
20
6
15
5
80,000
40
8
26.7
6
20
6
100,000
50
8
33.4
7
25
6
120,000
60
9
40
8
30
7
EXAMPLES: Determining Free Area
FURNACE
WATER HEATER
TOTAL INPUT
100,000
+
40,000
=
(140,000 divided by 4,000)
=
35.0 Sq. In. for each two Vertical Ducts or Openings
60,000
+
40,000
=
(100,000 divided by 3,000)
=
33.4 Sq. In. for a Single Duct or Opening
80,000
+
30,000
=
(110,000 divided by 2,000)
=
55.0 Sq. In. for each of two Horizontal Ducts
→
Table 2—Minimum Space Volumes for 100% Combustion, Ventilation and Dilution Air from Outdoors
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
40
60
80
100
120
Space Volume (ft
3
)
0.60
1,050
1,400
1,750
1,000
1,500
2,000
2,500
3,000
0.50
1,260
1,680
2,100
1,200
1,800
2,400
3,000
3,600
0.40
1,575
2,100
2,625
1,500
2,250
3,000
3,750
4,500
0.30
2,100
2,800
3,500
2,000
3,000
4,000
5,000
6,000
0.20
3,150
4,200
5,250
3,000
4,500
6,000
7,500
9,000
0.10
6,300
8,400
10,500
6,000
9,000
12,000
15,000
18,000
0.00
NP
NP
NP
NP
NP
NP
NP
NP
16
→
→
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