Published in the January 2008 issue of Today’s Facility Manager
For some aspects of a facility, it is important to know the maximum possible number of occupants and fully accommodate for that figure at all times. Elevators, for example, must always be able to hold a certain weight, even if they usually carry a much lighter load.
This is not always the case with heating, ventilation, and air conditioning (HVAC) systems. When it comes to ventilation, bringing in enough fresh air for the maximum possible number of occupants in a space is unnecessary and inefficient.
Because most buildings’ actual occupancy is often less than designed limits, conditioning superfluous amounts of outside air increases energy costs. This is particularly egregious since the United States Department of Energy estimates that HVAC accounts for 40% to 60% of the energy used in this country.
Demand controlled ventilation (DCV) is a strategy that restricts ventilation rates to match the real needs of occupants.
Kent Peterson, president of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) in Atlanta, GA, explains, “Demand controlled ventilation allows the system designer to provide controls that can save heating and air conditioning consumption by reducing the outdoor air ventilation rates in periods of low occupancy—when full ventilation rates are not required.”
But how can facility managers (fm) know exactly how many people are in every space at all times? What systems do they need to implement in order to use DCV?
Before DCV is considered for a facility, basic ventilation requirements must be understood. The typical amount of ventilation needed for a space should be determined during the design of the building’s systems.
Adrienne Thomle, senior project manager for Minneapolis, MN-based Honeywell Building Control Systems, says, “Section 6 of ASHRAE 62-1-2004 offers a procedure designers can use to determine minimum ventilation rates called Ventilation Rate Procedure (VRP).”
VRP is based on typical spaces and usage. It is intended to dilute and exhaust building contaminants and bioeffluents created by occupants to satisfy 80% of occupants.
Ventilation needs can also be determined by estimating occupancy rates. Tony Lee, systems engineering expert for Trane, Inc. in Piscataway, NJ, explains, “There are a number of different methods for determining how many people are in a space. In the case of a movie theater, for example, the number of tickets sold could be used to determine the value. For a college lecture hall, a time of day schedule might be used based on the number of people registered for a class.”
Measuring ventilation needs in buildings employing DCV is a little different from these basic strategies since the entire system is dependent on current (not possible or percentages of possible) occupancy. There are a number of strategies for accurately monitoring occupancy.
Dr. Svein Otto Kanstad is the president and CEO of Comag IR Technology Inc., a company located in Hallock, MN that manufactures DCV products. He claims, “The measurement of CO2 is by far the simplest and most direct way to implement DCV.”
Carbon dioxide, as Kanstad explains, is an indicator of the concentration of odorous bioeffluents exhaled by human beings. CO2 detectors installed in facilities can be used as a measure of the number of people occupying spaces.
Lee agrees, “In spaces where the number of people fluctuates unpredictably (like a conference room or cafeteria), the CO2 level can be used as a people counter.”
According to Thomle, “Per ASHRAE 62.1-2004, a building’s outdoor damper needs to open to a position to provide ventilation for the offgassing of building components whenever the building is occupied. By using an occupancy or CO2 sensor, the outdoor damper can be opened to provide additional ventilation based on occupancy.”
Trane’s terminal unit controller, the VV550, automatically calculates the outside air setpoint relative to a space’s CO2 level. A technician must configure the controller with the CO2 level of outside air, the level inside, and the outside air requirement of the space at design occupancy; the controller does the rest.
Other options for determining occupancy include motion detectors (though Kanstad asserts they do not discriminate between one or more people, and lead to overventilation) and temperature. Of this, Kanstad states, “studies show that there is little connection between temperature and air quality.”
Implementing DCV Systems
Using this type of technology requires some changes in a facility. To begin, fms must install the necessary components.
Kanstad details, “One would ideally have a CO2 sensor in each room. Also, frequency control of ventilation fans is necessary; managers must be able to have fans run slower or faster according to demand.”
Lee’s plan for installing DCV systems (in conjunction with a ventilation reset control strategy) includes Trane’s Tracer Summit, a building automation system. Also required are direct digital control (DDC) VAV terminal controllers on a VAV system, and an experienced controls technician.
According to Kanstad, the payback time for the necessary installations in existing systems is less than three years, and is very profitable once it yields savings. “After that,” he adds, “facilities may need half their former HVAC budget.”
Lee agrees. He says, “Perhaps the most common misconception about DCV is that it is difficult and expensive to implement.”
Thomle recommends fms visit Honeywell’s Web site at http://customer.honeywell.com to take advantage of Honeywell’s annual energy savings calculator there.
Coming up, ASHRAE is considering adding requirements for systems that use DCV (which is always optional) to the organization’s ventilation standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality. “This is an area of great interest to our members,” says Peterson. “The proposed addenda to the standard regarding demand control recently went out for public review.”
Currently, the dynamic reset section of the standard allows designers to use optional controls to reset outdoor air intake flow and/or minimum airflow as conditions within the system change. For example, this section allows a ventilation system control approach that alters outdoor air intake flow based on time of day (when variations in zone population follow a predictable schedule).
According to Peterson, proposed addendum 62g provides more specific requirements for such optional system controls, in particular those historically known as DCV and those based on the use of CO2 levels as they relate to outdoor airflow rates.
DCV may provide the energy savings necessary for today’s HVAC systems. Energy efficient changes dominate current trends, and this movement shows no sign of abatement.
Peterson continues, “This is an exciting time to be involved in the changing landscape of our industry. There are several challenges we are facing as we try to advance the built environment for an increasing world population.”
Lee predicts that, as energy costs become a larger percentage of a building’s operating budget, HVAC will come under intense scrutiny. “Controls strategies,” he says, “like demand controlled ventilation and ventilation reset, will become the standard.”
In this age of rapid technological development, perhaps the key for the future might be found in learning from the past. Kanstad explains: “HVAC engineers need to learn more about natural ventilation, so the costs of mechanical systems for air transport could be reduced or even totally eliminated. Engineers should learn from the ancient desert dwellers [who devised a system of powerless ventilation using wind towers].”
Whatever the plan, it’s time to investigate new ways of using less energy in buildings. Demand controlled ventilation is one option fms may want to consider to reduce utilities costs.