ASHRAE Journal - February 2013 - 49

• Sensing exhaust temperature (measured in the duct collar or in the hood reservoir) and/or temperature rise between
kitchen and exhaust temperature;
• Sensing smoke or steam produced by cooking process using an infrared beam, combined with temperature sensing;
• Monitoring cooking surface temperature or activity using
infrared beams, combined with temperature sensing; and
• Direct communication from cooking equipment controls
to the DCV processor.

Types of DCV
Which one to choose: temperature-sensing-only DCV versus optics/cooking-activity-sensing combined with temperature-sensing DCV? This is an escalating debate within the
industry as DCV systems gain traction in the design of CKV
systems and as more systems are introduced. First, the authors
believe that any type of DCV is better than no DCV (although
there are projects where DCV is not going to be cost effective
for either system).
Second, we believe that the potential turndown (average
cfm reduction) for the more sophisticated DCV systems can
be greater than the temperature-only systems. This is based
on fan speed reduction during no-load (no cooking) periods
and can be greater for optics-based DCV systems because of a
quicker response time. However, if the cookline has high-heat
producing, non-thermostatic appliances such as a charbroiler,
conveyor oven or wok, the response time of the system may
not be a significant factor and the performance of the temperature-only systems may more closely match the more complex
optics-based systems.
The response time of a temperature-sensing system should
be recognized as a factor in the design, particularly when thermostatic appliances such as fryers and griddles are specified.
This is because the effluent produced during cooking may not
be “seen” as quickly by the temperature probe in a duct collar
that is providing a measure of the average exhaust temperature.
The economic return on a DCV package generally increases with the size of the project (i.e., larger exhaust systems
in hospitals, hotels and casinos). It is also a fact the temperature-only DCV systems are less expensive than the more
sophisticated optics/temperature based systems. However,
the value proposition for investing in DCV is often based on
rule-of-thumb estimates using a $/cfm ($/[L·s]) index (i.e.,
annual energy cost to operate the CKV system divided by the
average exhaust ventilation rate), typically ranging between
$1 and $3 per cfm ($2 and $6 per L/s) per year. If the $/cfm
($/(L·s) indicator has been derived from a computer simulation of a similar project in a similar location (e.g., from a
LEED project), its application may be appropriate and relatively accurate. However, if the index has been casually selected, the resulting estimate of the system operating cost
may under or overstate the savings.

The magnitude of the energy consumption and cost of a
CKV system (or the DCV saving) is a function of the actual
exhaust ventilation rate, geographic location, operating hours
of the system, static pressure and fan efficiencies, makeup air
heating setpoint, makeup air cooling setpoint and level of dehumidification, efficiency of heating and cooling systems, level of interaction with kitchen HVAC system, appliances under
the hood and associated heat gain to space, and applied utility rates. While stating the obvious to the ASHRAE engineer,
makeup air (MUA) heating and cooling loads vary dramatically across the continent. The MUA heating load in Minneapolis and Chicago can be a significant cost component, while
in San Diego and Miami it may not exist at all. The reciprocal
is true for cooling. And the latent energy component in Miami
quickly differentiates itself from the desert climates.
Outdoor Air Load Calculator (OALC). The need for an
easy-to-use tool that would accurately determine the heating
and cooling load for a given amount of outdoor (makeup) air
led to the development of a no-cost, publicly available software tool, the Outdoor Air Load Calculator (OALC).1 Since
this tool does not model a complete building in detail, the
minimal required input parameters are geographic location,
outdoor airflow, operating hours, and the heating and cooling
setpoints. With these basic inputs, the OALC is able to calculate monthly and annual heating and cooling loads, as well
as design loads (the maximum heating and cooling load that
occurred during the year). Through a “Details” menu it is possible to further customize the calculation setup for dehumidification, equipment lockout during parts of the year, and fan
characteristics for estimating exhaust and makeup air fan energy consumption. The versatility of the OALC allows simulation of a variety of scenarios, but it also places responsibility
on the user to carefully choose the parameters. Casual selection of user inputs may result in unrealistic results. This tool
was used as the foundation for an ASHRAE Journal article.2

DCV and Codes
A significant obstacle to the adoption of DCV in commercial kitchens in the past was the minimum 1,500 fpm (8 m/s)
duct velocity requirement dictated by National Fire Protection
Association Standard 96, Standard for Ventilation Control and
Fire Protection of Commercial Cooking Operations. Since
1,500 fpm (8 m/s) was a reasonable velocity for sizing ductwork, CKV systems typically were designed within the range of
1,500 to 1,800 fpm (8 m/s to 9 m/s). Therefore, if a DCV control
strategy was selected that could potentially reduce the exhaust
airflow below 1,500 fpm (8 m/s) during periods of light cooking, this fire-safety code was at risk of being violated.
In response to this issue, a 2000 ASHRAE research project,
RP-1033, Effects of Air Velocity on Grease Deposition in Exhaust
Ductwork, concluded that duct velocity was not a significant
driver of grease deposition in Type I ductwork. As a result, NFPA

*This article reports only performance data for the strategy that comprises both sensing temperature and sensing smoke/steam using an infrared beam. With the
recent availability of various temperature-based-only systems and ongoing field monitoring, the authors anticipate a future Journal article presenting data for such
DCV systems.

February 2013

ASHRAE Journal

49
ASHRAE Journal - February 2013

Table of Contents for the Digital Edition of ASHRAE Journal - February 2013
