Section 3 — Stack and Intake Vent Sizing and Installation
Part No. 750-363 3-15
D. Under no condition should the total area of the air supply
openings be less than one square foot.
Figure 3-7. Two Opening Engineered Method
E. Size the openings by using the formula:
Area in square feet = cfm/fpm
Where cfm = cubic feet per minute of air
Where fpm = feet per minute of air
2. Amount of Air Required (cfm).
A. Combustion Air = 0.25 cfm per kBtuh.
B. Ventilation Air = 0.05 cfm per kBtuh.
C. Total air = 0.3 cfm per kBtuh (up to 1000 feet elevation.
Add 3% more per 1000 feet of added elevation).
3. Acceptable air velocity in the Boiler Room (fpm).
A. From floor to 7 feet high = 250 fpm.
B. Above 7 feet above floor = 500 fpm.
Example: Determine the area of the boiler room air supply openings
for (2) CFLC 4000 boilers at 750 feet elevation. The air openings to
be 5 feet above floor level.
• Air required: 4000 x 2 = 8000 kBtuh. From 2C above,
8000 x 0.3 = 2400 cfm.
GAS
VENT
WATER
HEATER
GAS
VENT
EXTERIOR WALL
FRESH AIR OPENING
EXTERIOR WALL
FRESH AIR OPENING
Table 3-1 Boiler room air
CFLC
20 ppm
Combustion
air SCFM
Vent air
SCFM
Total
room air
SCFM
4000 1000 200 1200
5000 1250 250 1500
6000 1500 300 1800
8000 2000 400 2400
10000 2500 500 3000
12000 3000 600 3600
CFLC
9 ppm
Combustion
air SCFM
Vent air
SCFM
Total
room air
SCFM
4000 1075 200 1275
5000 1350 250 1600
6000 1625 300 1925
8000 2150 400 2550
10000 2700 500 3200
12000 3225 600 3825
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