Bard Bayrd Furnace 403293A - Manual (Page 14)

5. if combustion air ducts will run horizontally, ducts
and each opening must have a minimum free
area of one square inch per 2,000 Btuh total
combined input rating. You must allow for all
gas appliances within the furnace space. See
Figure 14.
CHIMNEY OR CAS vent
NOTE: EACH AIR DUCT
OPENINO SHALL HAVE A
FREE AREA OF NOT LESS
THAN ONE SQUARE INCH
PER 2.000 BTU PER HOUR
OF THE TOTAL INPUT RATI
n C
OF ALL EQUIPMENT IN THE
ENCLOSURE
IF THE EQUIPMENT ROOM IS LOCATED AGAINST AND OUTSIDE WALL ANO THE AIR OPENINGS
COMMUNICATE OIRECTLY WITH THE OUTDOORS. EACH OPENING SHALL HAVE A FREE AfffiA
OF NOT less than one SQUARE INCH PER 4,000 BTU PER HOUR OF THE TOTAL INPUT
RATING OF ALL EQUIPMENT IN THE ENCLOSURE.
EQUIPMENT LOCATED IN CONFINED SPACES; ALL AIR FROM OUTDOORS.
FIGURE 14
For example:
Total input
Btuh
Free Area per Opening
(square Inches)
Round Pipe
(Inches dia.)
40,000 20 6
60,000 30 7
80,000 40 8
100,000 50 8
120,000 60 9
140,000 70 10
to 25% free area. Assume metal louvers and grilles
have 60 to 75% free area. Refer to current National
Fuel Gas Code ANSI Z223.1/NFPA 54.
Example: Furnace is 100,000 Btuh input and is to be
installed in a confined space that contains no other
gas appliances. Rectangular combustion and ventila-
tion air ducts will run horizontally from outdoors to
furnace space.
a. Calculate free area required.
Because combustion and ventilation air ducts run
horizontally, allow 2,000 Btuh. See 2b. above.
Furnace Input
Btuh
2,Q0Q Btuh
per square inch
100,000
2,000
Free Area Required
50 square inches
b.
Each opening must have a free area of 50 square
inches.
Calculate overall area of openings to give needed
free area once you cover them to keep out rain
and other outside elements.
Example: If you will use wood louvers to cover
combustion and ventilation air openings and you
do not know free area of wood louvers, use 20%,
as suggested in ANSI Z223.1/NFPA 54.
100 x Free area
from a) above)
Louver free area
openings expressed
as a percentage
overall area of
openings with wood
louvers installed
6. Ducts which introduce combustion and ventila­
tion air from outside structure into furnace space,
must have the same cross sectional area as the
free area of openings to which they connect.
The minimum dimension of rectangular air ducts
shall not be less than 3 inches.
2. Louvers, grilles, and screens.
Sometimes, louvers, grilles, or screens cover com­
bustion and ventilation air openings. If so, you must
provide larger openings than those calculated above.
This is necessary because louvers, grilles and
screens block and reduce an opening's free area.
Louver, grille and screen manufacturers supply tech­
nical data on their products, which usually includes
the Tree area." Sizing the openings with louvers,
grille or screen in place will provide minimum free
area to furnace space as calculated above. Do not
use screen smaller than 1/4" mesh. If you do not
know free area, assume that wood louvers have 20
100 x 50
250 square inches
20
Each of the two combustion and ventilation air
openings must have a total area of 250 square
inches. This is when wood louvers cover openings
for a 100,000 Btuh input furnace and combustion
and ventilation air ducts run horizontally.
c. Calculate minimum rectangular duct size needed.
If you choose a rectangular duct with the mini­
mum allowed dimension of 3", the other duct
dimension must be at least 16-2/3". This is cal­
culated by dividing the 50 square inches of free
area from a) by 3", with equals 16-2/3".
WARNINGi You must fix combustion and ventila­
tion air louvers and grilles In open position or Inter­
lock them with furnace operation. Furnace operation
12