Basic Documentation

Fume Hoods
Siemens Industry, Inc. 39
Topic Requirement(s) Commentary
Minimum
Exhaust
National Fire Protection Association, Standard NFPA 45, 2011
A-8.4.7 In addition to maintaining proper fume hood face velocity, fume hoods that
reduce the exhaust In addition to maintaining proper fume hood face velocity, fume
hoods that reduce the exhaust volume as the sash opening is reduced should
maintain a minimum exhaust volume to ensure that contaminants are diluted and
exhausted from a hood. The chemical fume hood exhaust airflow should not be
reduced to less than the flow rate recommended in ANSI/AIHA Z9.5, Laboratory
Ventilation.
American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5-2003
3.3.1 The mechanism that controls the exhaust fan speed or damper position to
regulate the hood exhaust volume shall be designed to ensure a minimum exhaust
volume in constant volume systems equal to the larger of 50 cfm/ft of hood width, or
25 cfm/ft
2
of hood work surface area, except where a written hazard characterization
indicates otherwise, or if the hood is not in use.
The NFPA 45 stated minimum exhaust of 25
cfm per square foot of internal hood work
surface typically equates to 20% to 25% of
the required exhaust rate for a fully open sash
and 100 fpm face velocity. Frequently this
limits the “turndown” on a VAV hood to about
5:1.
An example of where a written hazard
classification would indicate that a minimum
exhaust is not required could be where the
fume hood is used only for odor control and
where hazardous fumes are never present.
Note that in ANSIAIHA Z9.5-2003, there is a
likely typo in that the term “constant volume
systems” should probably be “variable air
volume” since VAV fume hoods and not CAV
fume hoods have a mechanism to regulate
the exhaust.