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CO2 “Demand-Controlled
Ventilation” (DCV) and Intake Rate
Control
By Len Damiano
EBTRON, Inc.
What does the
term “demand controlled ventilation” (DCV) mean?. Very
simply, it is “any” method used to control ventilation
that modifies intake rates based on changing “demand”.
The intention is to control ventilation rates based on occupancy
within a predefined space (assuming it varies over time). Because
ventilation rates are normally associated with occupancy levels,
we conclude that the “demand” for ventilation is due to
a measured change in the occupancy level for the space.
DCV is a method
of measurement that approximates the number of people that occupy
a space, and thereby allow the intake rates to be reset based on
the indicated occupancy. It is only with this data that one can
optimize the rate of outside air intake, to something less than
maximum capacity. It is this method that most CO2 sensor makers
and ASHRAE 62 refers to.
However, many
readers misinterpret technical articles on CO2 measurement touting
the energy benefits of DCV, but the authors rarely provide sufficient
information allowing the reader to apply the methodology. Seldom
do they provide supporting details on how it allows the user to
comply with ventilation requirements. Can we conclude that even
the authors are unsure how to justify it?
Problems occur
when methods of control and their terminology are used interchangeably.
“Demand Controlled Ventilation” is not synonymous with
“CO2 measurement for control”.
Let’s unravel
some of the persistent confusion that surrounds the application
of Demand Controlled Ventilation.
CO2 Measurement
and Ventilation Rates
First, let’s
get one simple truth out of the way. There is no direct relationship
between interior CO2 levels and intake rates. At best, an indirect
relationship exists that relies on numerous assumptions, most of
which are not valid in most dynamic commercial building environments.
Because ASHRAE
initially used odor control and comfort as the minimum criteria
for ventilation effectiveness, many have assumed that the use of
CO2 measurement is supported by the Standard. This conclusion is
not valid.
Andy Persily
best explained the usage and relationships of CO2, in his paper
for the Indoor Air ’96 conference. Now the current Chairman
of ASHRAE SSPC 62.1, Mr. Persily presented his conclusions for:
The Relationship Between Indoor Air Quality and Carbon Dioxide.
The
relationship between CO2 and outdoor air ventilation rates is
well understood and is based on the consideration of CO2 as a
tracer gas. ….However, to make quantitative estimates of
ventilation parameters based on measured CO2 concentrations one
must employ a specific tracer gas technique [e.g. ASTM]
that is appropriate to the conditions that exist in the building.
………The
Ventilation Rate Procedure in the standard is based on outdoor
air ventilation rates requirements, not on the maintenance of
indoor CO2
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