59CU5A: Installation, Start-up, Operating and Service and Maintenance Instructions
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
10
2. An attic or crawlspace may be considered a space that freely
communicates with the outdoors provided there are adequate
permanent ventilation openings directly to outdoors having free
area of at least 1-in.2/4,000 Btuh of total input rating for all gas
appliances in the space.
3. In spaces that use the Indoor Combustion Air Method, infiltration
should be adequate to provide air for combustion, permanent
ventilation and dilution of flue gases. However, in buildings with
unusually tight construction, additional air MUST be provided
using the methods described in the Outdoor Combustion Air
Method section.
4. Unusually tight construction is defined as Construction with:
a. Walls and ceilings exposed to the outdoors have a continuous,
sealed vapor barrier. Openings are gasketed or sealed and
b. Doors and openable windows are weatherstripped and
c. Other openings are caulked or sealed. These include joints
around window and door frames, between sole plates and floors,
between wall-ceiling joints, between wall panels, at penetrations
for plumbing, electrical and gas lines, etc.
Combination of Indoor and Outdoor Air
1. Indoor openings shall comply with the Indoor Combustion Air
Method below and,
2. Outdoor openings shall be located as required in the Outdoor
Combustion Air Method mentioned previously and,
3. Outdoor openings shall be sized as follows:
a. Calculate the Ratio of all Indoor Space volume divided by
required volume for Indoor Combustion Air Method below.
b. Outdoor opening size reduction Factor is 1 minus the Ratio in a.
above.
c. Minimum size of Outdoor openings shall be the size required in
Outdoor Combustion Air Method above multiplied by reduction
Factor in b. above. The minimum dimension of air openings shall
be not less than 3 in. (80 mm).
Table 3 – Minimum Free Area Required for Each Combustion Air Opening or Duct to Outdoors
EXAMPLES: Determining Free Area
Table 4 – Minimum Space Volumes for 100% Combustion, Ventilation and Dilution Air from Outdoors
NP = Not Permitted
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 (Sq. mm)
Round Duct
In. (mm) Dia
Free Area of
Opening 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)
FURNACE
WATER HEATER
TOTAL INPUT
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
100,000
+
30,000
=
(130,000 divided by 4,000)
=
32.5 Sq. In. for each two Vertical Ducts or Openings
AIR
CHANGES
PER HOUR
(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
26
40
60
80
100
120
140
Space Volume Ft3 (M3)
0.60
1,050 (29.7) 1,400 (39.6)
1,750
(49.5)
910 (25.8) 1,400 (39.6) 1,500 (42.5) 2,000 (56.6) 2,500 (70.8) 3,000 (84.9) 3,500 (99.1)
0.50
1,260 (35.6) 1,680 (47.5)
2,100
(59.4)
1092 (30.9) 1,680 (47.5) 1,800 (51.0) 2,400 (67.9) 3,000 (84.9)
3,600
(101.9)
4,200
(118.9)
0.40
1,575 (44.5) 2,100 (59.4)
2,625
(74.3)
1365
(38.7)
2,100 (59.4) 2,250 (63.7) 3,000 (84.9)
3,750
(106.1)
4,500
(127.3)
5,250
(148.6)
0.30
2,100 (59.4) 2,800 (79.2)
3,500
(99.1)
1820
(51.5)
2,800 (79.2) 3,000 (84.9)
4,000
(113.2)
5,000
(141.5)
6,000
(169.8)
7,000
(198.1)
0.20
3,150 (89.1)
4,200
(118.9)
5,250
(148.6)
2730
(77.3)
4,200
(118.9)
4,500
(127.3)
6,000
(169.8)
7,500
(212.2)
9,000
(254.6)
10,500
(297.1)
0.10
6,300
(178.0)
8,400
(237.8)
10,500
(297.3)
5460
(154.6)
8,400
(237.8)
9,000
(254.6)
12,000
(339.5)
15,000
(424.4)
18,000
(509.2)
21,000
(594.1)
0.00
NP
NP
NP
NP
NP
NP
NP
NP
NP
NP
