The North American building industry is at the center of a national effort to curb energy consumption. Prior to “green” becoming the buzzword it is today, building design and construction changed and evolved in response to energy scarcity, public perception, and the demands of well-informed building owners. Energy crises often cause design professionals to change standard practices and drive users to value buildings that are environmentally friendly and cost-effective to operate.
The energy shortages that resulted from the Arab Oil Embargo in the 1970s led to airtight building envelopes and reduced mechanical ventilation, which decreased indoor air quality and negatively impacted building occupants.1
To correct this issue, a reevaluation of the way buildings are ventilated and how much air is necessary to maintain acceptable indoor air quality was required. ASHRAE first drafted its standards for acceptable ventilation rates in the early 1970s in response to this shift in building design. Since then, codes and standards have continued to develop as more knowledge and experimental data has become available. With this information, ventilation rates have been modified to include both space and occupant ventilation criteria.
Today, HVAC design professionals strive to provide a healthy indoor environment as efficiently as possible. However, reaching current energy standards while still complying with required ventilation rates can often prove frustrating in the design of large buildings with an active, transient population.
One strategy for efficiently delivering ventilation air is to employ demand-control ventilation (DCV), which delivers air to individual zones as required, based on real-time occupancy. Compared to traditional systems that deliver air based on maximum allowable population, DCV allows a system to actively modulate ventilation quantities. This presents a significant opportunity for energy savings in zones with transient occupancy. For instance, a 20-person conference room may only be at full occupancy for a few hours each day. With a DCV control strategy, occupancy information provided to the building automation system (BAS) would reduce unnecessary ventilation air to the corresponding zone.
There are three key aspects to employing a successful DCV system: measure outside (ventilation) air quantities, install reliable zone sensors, and select a control package capable of analyzing conditions and reacting accordingly.
Measure Ventilation Air
Ventilation air is typically treated and delivered to a building by centralized air handling units (AHUs). As the point of entry into a building, ensuring proper ventilation starts with measurement at the AHU. Although it’s not typically a direct contributor to the control sequence, this is a key feature of the DCV strategy.
Traditionally, testing and balancing contractors balance air handling units by taking an initial airflow pressure traverse of the ventilation air intake and setting the control dampers based on peak design airflow. By relying on an accurate initial reading, this passive method brings about inevitable errors and accurate readings occur only during peak operation for a fraction of the cooling season.
The solution is to actively monitor and control the volume of ventilation air being drawn into the building through an AHU by using airflow monitoring stations. Each station provides continuous pressure measurements, which are reported back to a controller that modulates the intake damper to the desired airflow. An airflow monitoring station also increases a building operator’s ability to identify and diagnose performance problems within the AHU. If an airflow station reports unexpected airflow, it can be an indication of mechanical failure in the damper components. In addition, readings from an airflow station can also be analyzed to predict energy consumption for real-time analysis, or confirm compliance with building codes. Once the design team is satisfied with the ability to deliver appropriate ventilation air to the building, the next step is to determine associated zone occupancy.
Install Reliable Zone Sensors
There are multiple ways to determine zone occupancy in a building. However, the challenge with detecting the number of occupants in a given space is that sensors have not evolved to the point of being completely accurate. Commonly implemented devices include passive-infrared motion detection, carbon dioxide (CO2) sensors, acoustic detection, and video cameras capable of occupant recognition. Each of these occupant detection methods has benefits and downfalls and must be evaluated on a project-by-project basis. Project cost and complexity will also play a large role in deciding which method—or combination of methods—a design team should employ.
The DCV control strategies are based on the fact that individual humans exhale CO2 at predictable rates. ASHRAE Standard 62.1 Ventilation for Acceptable Indoor Air Quality, Appendix C contains a chart used to determine this rate, which allows real-time determination of occupancy based on ambient CO2 levels in a space. Sensors that emit infrared light and monitor changes in wavelength can detect CO2, allowing the zone’s CO2 level to be communicated to a controller. The controller calculates occupancy based on these levels, and modulates a damper to deliver corresponding ventilation air to the space.
It’s worth noting that ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, requires automatic lighting control for buildings of more than 5,000 sq. ft., with occupant motion detection as one method of compliance. For design teams selecting motion detectors for lighting control, this means sensors can be interlocked with airflow terminal units with little extra effort and cost. Motion detectors can signal to airflow terminal units when a space becomes occupied or unoccupied, allowing the terminal unit to transition between an occupied mode and unoccupied mode. If unoccupied, it’s typical for a terminal unit to close its ventilation air damper or to turn down to minimum design airflows. Upon a motion sensor signal indicating occupancy, the damper reopens and resumes occupied airflow levels. This coordination between disciplines should be facilitated early on in order to resolve issues before installation begins in the field.
Select a Building Automation System
The third and final key aspect of implementing DCV in a building is to design and install a BAS capable of receiving, recording, and acting on the information it receives. The BAS should be capable of controlling the general conditions within the building and alerting building operation staff if irregularities surface. The ability to store operational data from mechanical equipment allows building operators to analyze system operation and develop a statistical base for building usage and function.
While DCV is implemented on a building-wide scale, the most significant impact is made by DCV sequences at the zone level. Local controllers should be empowered to make decisions regarding ventilation air rates, as well as space occupancy and temperature. Having the ability to communicate usage patterns in the space is extremely helpful.
Combine the Components
The success of DCV is dependent upon three critical components of an HVAC system. The ability to accurately measure and distribute ventilation air to the building ensures indoor environmental air quality meets the ever-increasing standards for building occupants; monitoring building occupancy with CO2 measurement devices, in conjunction with infrared motion detectors, provides an accurate, real-time evaluation of building use; and monitoring occupancy allows HVAC systems to more efficiently deliver air where it’s needed, while minimizing energy waste where ventilation air is not needed.
A strong BAS gives design professionals and owners the level of control, measurement, and verification required to keep a building operating at peak efficiency years after turnover. The ability to control this flow of information and organize it into a useful operation and evaluation tool is critical for a successful DCV.
Reference
1. Liu G., Zhang J., Dasu A. Review of Literature on Terminal Box Control, Occupancy Sensing Technology and Multi-zone Demand Control Ventilation (DCV). U.S. Department of Energy, March 2012.
Jeff Colby is a design engineer for Southland Industries, Dulles, VA. He can be reached at 703/834-5570 or by e-mail at [email protected].
Scott Winkler, PE, is managing principal engineer, Southland Industries, Dulles, VA. He can be reached at 703/834-5570 or by e-mail at [email protected].