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Demand-Controlled Ventilation Controls HVAC Costs

By Dan Greenberg, E Source Associate Director, www.esource.com -- Foodservice Equipment & Supplies, 6/1/2007

Across the United States, restaurants operate for long hours every day, and although they may have relatively high occupancy during a few hours per week, at other times a lot of tables sit empty. Occupancy fluctuations like these offer restaurants an opportunity for annual energy savings that can amount to as much as $1.00.00 per square foot. Instead of continuously ventilating the space at a constant rate designed to accommodate full occupancy, building operators can implement a technique known as demand-controlled ventilation (DCV), in which the amount of outside air drawn in for ventilation depends on the actual occupancy of the building at any given time. This strategy results in energy savings because it reduces the amount of air that needs to be conditioned.

Because the average amount of carbon dioxide (CO2) a person will exhale in a fixed time period is well well-known, the concentration of CO2 in the air inside a building represents a good indicator of the number of people in a space and the dilution rate at which the air in the space is being diluted with outdoor air. For a constant volume of fresh ventilation air, the more occupants a building has at any given time, the higher the level of CO2 in the air. DCV exploits this fact to adjust the amount of fresh ventilation air brought into a building based on the measured concentration of CO2.

DCV is not a new concept, but has grown tremendously in popularity in recent years as the cost of implementing it has declined substantially, while at the same time sensor reliability has improved. The main cost reduction has come from CO2 sensors, some of which operators can purchase for less than $200, compared to more than $500 a decade ago. Today’s sensors can also self-calibrate, so they need far less maintenance than their predecessors. Also, several HVAC equipment manufacturers now offer DCV-ready rooftop units and variable air volume (VAV) boxes. This equipment comes with terminals for the CO2 sensor wires and preprogrammed controls that implement a DCV strategy. By limiting installation costs to the cost of mounting the sensor and running wires to the rooftop unit or VAV box (wireless models are available), DCV DCV-ready HVAC equipment substantially reduces the cost of implementing DCV.

DCV provides multiple benefits to building operators and occupants. It can:

• Reduce energy consumption. DCV systems save energy by reducing the need to heat or cool outside air. The only system change is the ratio of recirculated air to outside air — usually this does not affect fan power. DCV systems can save from US$0.05 to $1.00 per square foot, depending on the occupancy schedule and climate. Modeling studies indicate that due to their highly variable occupancy and the fact that they’re designed for very high occupant densities, foodservice operations represent some of the best applications for DCV, with economic paybacks of less than two years in most climates, and less than one year in many. For example, a 2003 Purdue University study shows favorable paybacks for DCV in a variety of buildings. The study investigated four types of buildings — a restaurant, a retail store, a school, and an office — in each of two cities in California and three cities outside the state. The restaurants and retail stores showed the most opportunity for DCV, with savings estimated at as much as 50 percent of the total energy operating cost in some cities.
  
  

  Percentage Annual Energy Cost Savings for DCV
  Although savings will depend heavily on actual occupancy patterns, the relatively larger percentage of savings in restaurants and retail stores highlights these types of facilities as particularly good candidates for demand-controlled ventilation (DCV).
  	Office				Restaurant
  	energy	demand	gas	total	energy	demand	gas	total
  Oakland, CA	2.3	8.4	56.8	7.5	-5.3	9.2	100	17.9
  El Centro, CA	8.4	11.8	54.5	9	18.4	25.4	100	20.2
  Phoenix, AZ	7.4	5	53.7	6.4	16.4	12.5	99.7	17.3
  Charleston, SC	12.3	10.5	46.5	12.3	16.6	23.4	98.1	27
  Fargo, ND	6.2	11	37.3	19	7.2	20.8	82.3	49.8
  	Retail Store				School
  	energy	demand	gas	total	energy	demand	gas	total
  Oakland, CA	-2.2	19	100	16.6	-0.3	8.2	99	6.5
  El Centro, CA	22.6	35.7	100	24.3	13.1	17.4	100	13.7
  Phoenix, AZ	20.1	19.5	100	20.9	11.8	9.5	95.5	11
  Charleston, SC	21.9	35.7	99.5	32.6	13.8	13.5	91.4	15.5
  Fargo, ND	10.5	31.8	90.7	53.4	6.8	12.2	61.4	28.2

• Provide proper ventilation. If a building is not drawing in enough outside air, a DCV system may actually increase energy use, but it will also bring the building into compliance with ventilation codes and do so more efficiently than a simple increase in the constant ventilation rate. Because DCV provides the proper amount of ventilation for building occupants, it prevents underventilation, which can make buildings seem stuffy and increase the chances that occupants will get sick.
• Show that buildings are in compliance with building codes. It is relatively easy to prove that buildings are properly ventilated when you can simply check to see that CO2 sensors read at or below the maximum allowable CO2 concentration. If the DCV system is working properly, this will always be the case.

  Most restaurants can benefit from DCV. The best candidates have long operating hours and highly variable occupancy, and are located in climates with moderate to extreme heating and/or cooling loads. Several free computer simulation tools can help foodservice professionals evaluate the cost-effectiveness of DCV for any particular facility, including the following:

• Hourly Analysis Program from Carrier Corp.
• Savings Estimator from Honeywell
• CO22 Ventilation Control & Energy Analysis from AirTest

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