User Manual

19
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/K W)
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 Du ct (sq In.)
Round Duct (in.
Dia)
Free Area of Opening
and Du ct (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---AS SISTED TOTAL
(1,000’S BTUH GAS INPUT R ATE)
FAN---ASSISTED TOTAL
(1000’S BTUH GAS INPUT R ATE)
30 40 50 40 60 80 100 120
SpaceVolume(ft3)
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
b. Combining spaces on same floor level. Each opening
shall have free area of at least 1 in.
2
/1,000 Btuh (2,000
mm
2
/kW) of the total input rating of all gas appliances
in the space, but not less than 100 in.
2
(0.06 m
2
). One
opening shall commence within 12” (300 mm) of the
ceiling and the second opening shall commence within
12” (300 mm) of the floor. The minimum dimension of
air openings shall be at least 3 in. (80 mm). (See Fig.
20.)
c. Combining space on different floor levels. The volumes
of spaces on dif ferent floor levels shall be considered as
communicating spaces if connected by one or more
permanent openings in doors or floors having free area
of at least 2 in.
2
/1,000 Btuh (4,400 mm
2
/kW) of total
input rating of all gas appliances.
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.
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.
NOTE: In determining the free area of an opening, the blocking
effect of the louvers, grilles, and screens must be considered. If the
free area of a louver or grille design is unknown, it may be
assumed that wood louvers have a 20 percent free area, and metal
louvers or grilles have a 60 percent free area. Screens, when used,
must not be smaller than 1/4 --in. (6 mm) mesh. Louvers and grilles
must be constructed so they cannot be closed.
355A