Energy efficiency is becoming increasingly important for both newly designed buildings and existing ones, as the average commercial building in the U.S. wastes 30% of the energy it consumes. Whether the push for efficiency is incentive-based or a push to meet the more stringent energy code requirements, buildings are now implementing strategies and technology alike to reduce energy consumption and take pressure off the grid.
When it comes to energy efficiency in buildings, the first thing that often comes to mind is lighting and controlling its use through occupancy sensors. However, HVAC systems account for approximately 25% of the energy consumption in commercial buildings. When reviewing the engineering costs of building design, HVAC improvements can affect a quarter of the overall energy use and as a result, occupancy sensors are becoming energy-efficiency staples in commercial buildings across the country.
One of the biggest factors related to HVAC energy consumption correlates to the amount of outdoor air ventilation provided to the building. The introduction of outdoor air in a space changes the temperature, requiring the HVAC system to provide heating or cooling. This wastes valuable energy.
As a result, one of the most cost-effective ways businesses can reduce energy waste is with demand control ventilation (DCV) systems within HVAC systems. DCV systems read the number of occupants in a room through space occupancy sensors. These sensors provide data on actual real time ventilation requirements, reducing the amount of outdoor air and energy consumed by cycling HVAC systems.
Why Demand Controlled Ventilation?
The term may seem esoteric, but demand control ventilation is simply a way to control the amount of outside air that is let into a space at a given time. Up until the last few decades, ventilation of commercial buildings was provided at a constant non-variable rate. In turn, HVAC systems worked constantly to maintain space temperature to counter the temperature of the air coming in from outside. So while air quality was being addressed in this process, energy efficiency was not.
DCV was implemented to alleviate the pressure on HVAC systems. This helped regulate the temperature by providing flexibility and variability in ventilation from the outdoors. This allows the HVAC system to regulate the indoor temperature and reach periods of stasis so that the system can shut off compressors, heaters, and fans for a time period.
Using a controlled ventilation system in a commercial building can provide a savings of 5% to 80% on energy costs depending on building, size, design, and equipment controls. This can create massive operational savings for a building owner or developer.
The Role of the Occupancy Sensor
Scheduled ventilation periods is a start for reducing energy consumption from cooling and heating systems, but a more finite approach is needed to provide maximum energy efficiency and ventilation demands for the entire building.
This is where the occupancy sensor comes into play not only to adjust ventilation demand, but for temperature and humidity as well. Occupancy sensors provide information back to an automation system that lets the HVAC system know whether or not there are occupants in a room or space.
Occupancy sensors come in two varieties:
- Space Motion Sensors: This type of sensor acts in tandem with a light motion sensor and will automatically call for ventilation if motion is detected in a space. The issue with this type of occupancy sensor is that it will call for ventilation whether or not there is one person in a room or twenty. Therefore, it may provide ventilation in a space where little is needed. Think about it like an entire 20-story building with lights on when only 1 person in 1 office is working. Not terribly efficient right?
- CO2 sensors: These are sensors that measure the amount of CO2 in a space. Occupants breathe out CO2. Therefore, a measured amount of CO2 (defined by the design parameters) can inform the automation system. If the amount of occupants in a space exceeds the amount of CO2 allowed, the sensor triggers the HVAC system to turn on. This type of sensor is more precise than the typical motion sensor and is often required to meet the demand control ventilation requirement for LEED certification.
Optimizing Controls for Maximum Efficiency
Occupancy sensors are a cutting-edge tool in providing accurate information to the building automation system, or the direct digital controls (DDC) system for individual HVAC systems. Implementing them in building spaces requires looking at several factors to achieve maximum energy efficiency.
If you are considering occupancy sensors for your HVAC system in your building, consider the following two factors. These are:
- Location: The single most important factor to consider is where to install occupancy sensors. If a building is trying to achieve LEED certification, the parameters are outlined clearly in the credit description. Otherwise, the design engineer should place occupancy sensors in the areas of highest occupancy as a first step. For an office building, this could be in large conference rooms and lobbies. Schools often prioritize sensors to be located in cafeterias, gyms, and auditoriums first, then individual classrooms.
- Time of Day: Building occupancy usually is at its highest during daytime hours, meaning ventilation can be reduced or eliminated all together at night. Manufacturing facilities will rarely have occupants at night outside of graveyard shifts, so the control systems can decrease or eliminate ventilation. However, a hospital may have the same quantity of occupants around the clock and may require more control to meet ventilation requirements. The timing depends on the type of space and type of occupants.
Occupancy sensors are becoming as necessary to controlling building HVAC systems as thermostats once were. Since these sensors regulate and provide demand control ventilation, they not only create more comfortable spaces for occupants, but also sustainable spaces. When space occupancy and daily use are taken into account, occupancy sensors become a key factor to reducing energy consumption in commercial buildings.
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