Demand ventilation is not a new idea but technology is impacting building codes and how building ventilation can be implemented, saving you energy dollars. New buildings with a full HVAC BAS (Building Automation System) are taking advantage of these money saving opportunities through some fairly simple control strategies. The goal of demand ventilation is to supply the minimum amount of outdoor air necessary at all times, eliminating energy waste periods in which you are heating or cooling too much outside air.
A first strategy is to implement CO2 control in densely populated spaces. Spaces like gyms, conference rooms, classrooms and auditoriums are now typically designed to include CO2 measurements in the space. Depending on the design, this CO2 measurement might be taken at the space thermostat or in the return air duct of that space. By having a known CO2 value in the space, it’s possible for that unit’s space’s air handler controller to modulate the outside air damper (economizer) to control to an acceptable CO2 design. A maximum of 800 ppm – 1200 ppm (parts per million) while a space is occupied is typical. This CO2 demand ventilation control allows the outdoor air damper to modulate as opposed to open fully while still preventing a stuffy room. The less outside air that requires heating or cooling, the less energy dollars are required to maintain a comfortable space.
A second strategy for demand ventilation is based on your BAS’s knowledge of a space being vacant during occupied hours. If the space is sensed as vacant using occupancy sensors, a significant reduction in outside air to the space can be programmed. It is allowable to decrease the minimum outside air volume below the design conditions in this scenario. If this strategy is selected, make sure your occupancy sensors include an extra set of contacts for BAS integration!
Your existing building might not have a demand ventilation sequence but if it has a BAS control system, it is a relatively easy strategy to implement by upgrading thermostats (to a newer model that measures CO2) and implementing a new demand ventilation program (logic) in your BAS.