Exhaust
Fans:
Suggested Air Changes for Good Ventilation
Properly
sizing exhaust fans has become more important in buildings with the new
heightened awareness of mold and moisture control along with occupant comfort
and wellness.
Sections 1203.1 & 1203.4 of the 2007 California Building Code (CBC) stipulates that buildings shall be provided with natural
ventilation through operable windows, doors, louvers or other openings to the
outdoors, or mechanically ventilated in accordance with the California Mechanical Code (CMC).
Section 402.3 of the 2007
“. .
. shall be designed, constructed and
installed to provide a method of supply air and exhaust air.”
CMC
Section 504.0 gives
additional requirements for Environmental Air Duct systems.
Two
methods are available for properly determining the required exhaust rates:
1.
Calculations based on the airflow per
floor area (cfm/sqft) per CMC Table 4-4; or,
2.
The Minimum Air Changes per space usage (as listed below).
To use
the “Minimum Air Change” method, simply determine the airflow required to adequately
ventilate an area by dividing the room volume (in cubic-ft.) by the appropriate
“Minutes
per Air Change” as shown in the chart below.
|
Type of Space |
Minutes per Air
Change |
Type of Space |
Minutes per Air
Change |
|
Assembly
Halls |
3-10 |
Heat
Treat Rooms |
1-2 |
|
Attic |
2-4 |
Hospital |
4-6 |
|
Auditoriums |
4-15 |
Kitchens |
2-4 |
|
Bakeries |
1-3 |
Laboratory |
2-5 |
|
Banks |
3-10 |
Laundries |
2-5 |
|
Banquet
Halls |
3-4 |
Locker
Rooms |
2-5 |
|
Bar/Lounge |
2-5 |
Lodge
Rooms |
3-5 |
|
Barns |
10-15 |
Machine
Shops |
3-5 |
|
Beauty
Parlors |
2-5 |
Meeting
Rooms |
4-6 |
|
Boiler
Rooms |
2-5 |
Mill
(General) |
3-8 |
|
Bowling
Alleys |
2-8 |
Mill
(Paper) |
2-3 |
|
Cafeteria |
3-5 |
Mills
(Textile) |
5-15 |
|
Church |
4-10 |
Office |
2-8 |
|
Classrooms/Schools |
4-6 |
Packing
Houses |
2-5 |
|
Club
Room |
3-7 |
Plating
Rooms |
1-5 |
|
Corridors/Halls |
6-20 |
Printing
Plants |
3-8 |
|
Dairies/Creameries |
2-5 |
Projection
room |
1-2 |
|
Dance
Hall |
3-7 |
Recreation
Rooms |
2-8 |
|
Dining
Rooms |
3-6 |
Residences |
3-6 |
|
Dormitories |
5-8 |
Restaurants/Dining |
3-7 |
|
Dry
Cleaners |
2-5 |
Restrooms |
3-6 |
|
Engine
Rooms |
1-2 |
Retail
Stores |
3-8 |
|
Factories
(Light) |
5-10 |
Ship
Holds |
8-10 |
|
Factories
(Heavy) |
2-6 |
Shops
(General) |
3-10 |
|
Forge
Shops |
1-3 |
Theaters |
3-8 |
|
Foundries |
1-4 |
Transfer
Room |
1-5 |
|
Garages |
2-10 |
Transformer
Rooms |
1-5 |
|
Generator
Room |
2-5 |
Tunnels |
6-10 |
|
Glass Plants |
1-2 |
Turbine Rooms |
2-6 |
|
Gymnasiums |
3-8 |
Warehouses |
3-10 |
Additional
considerations when determining the number or air changes include:
q Local code requirements on air changes.
q Specific use of the space.
q The type of climate in the area.
In the
most severe conditions, select the lower number (in the range shown) to change
the air more frequently.
For
moderate conditions, select the mid range.
For less
severe conditions in cool climates, the higher number will provide adequate
ventilation.
Example:
A typical single-occupant, Office
Restroom is 7’-6” wide x 7’-0”deep wide with an 8’-0” ceiling.
·
The volume of the Restroom is 420
cubic-ft.
·
The range of minutes per
air-change is 3-6.
·
Using a mid-range value of 5
minutes per air-change, the total ventilation required is:
(420 cubic-ft / 5 min) = 84
cubic-ft/min (CFM)
By properly
sizing exhaust fans, especially in rooms which generate a substantial amount of
moisture (Restrooms, Kitchens, etc), mold and moisture related problems
resulting from poor ventilation can be greatly reduced – resulting in fewer
health issues for the occupants and less property damage.
Acknowledgements: Some of the in formation
contained in this article, including the “Minutes per Air Change” tables, has
been obtained from the Greenheck Engineering Update Newsletter, Volume IV, Issue 1 (January – March 2002).