Model 4
1500 - 6000 MBTU/hr
Rev. 03-08
Section B2-18
room is neutral or slightly positive pressure), or with a direct vent to outside the
boiler room where air is drawn directly from the exterior of the building. Regardless
of the method, all installations must comply with NFPA 54 (National Fuel Gas Code
- NFGC) for U.S. installations and CAN/CSA B149/.1 and B149.2 for Canadian
installations.
Engineered Design
- When determining boiler room air requirements for the boiler,
the "Engineered Design" method may be used. Following this method, consideration
must be given to the size of the boiler room, airflow, and air velocity as follows:
A. Two permanent air supply openings in the outer walls of the boiler room are
recommended. Locate one at each end of the boiler room, preferably below a height
of 7 feet. This allows air to sweep the length of the boiler. Refer to Figure B2-3.
B. Air supply openings can be louvered for weather protection, but they should not
be covered with a fine mesh wire, as this type of covering has poor air flow qualities
and is subject to clogging with dirt and dust.
C. A vent fan in the boiler room is not recommended as it could create a slight
vacuum under certain conditions and cause variations in the quantity of combustion
air. This can result in unsafe burner performance.
D. It is forbidden to have the total area of the air supply openings at less than one
square foot.
E. Size the openings by using the formula (Area in ft
2
= cfm
a
/fpm
a
), where cfm
a
=
cubic feet per minute of air; fpm
a
= feet per minute of air.
F. Amount of air required (cfm):
1. Combustion Air = Maximum boiler horsepower (bhp) times 8 cfm.
2. Ventilation Air = Maximum boiler horsepower (bhp) times 2 cfm.
3. Total Air = 10 cfm per bhp (up to 1000 feet elevation, add 3% more per 1000
feet of added elevation).
G. Acceptable air velocity in the boiler room (fpm):
1. From floor to 7 feet high = 250 fpm.
2. Above 7 feet from boiler room floor = 500 fpm.
Example of required air openings (Engineered Method):
Determine the area of the boiler room air supply openings for [2] size 4500 Model
4 boilers at 750 feet elevation; each have a rating of 107 boiler horsepower. The
air openings will be 5 feet above the floor level.
The total boiler horsepower (bhp): 107 x 2 = 214 bhp.
From F.3 above, total air required = 214 bhp x 10 = 2140 cfm.
Air Velocity: From G.1 above = 250 fpm.
Area required: From the formula in E above, 2140 cfm/250 fpm = 8.56 square
feet total.
Area opening: 8.56 divided by 2 = 4.28 ft
2
per opening (2 required).
Notice
Consult local codes, which may supersede these requirements.
See Table B2-15 for combustion air and flue gas flow.
