Install with an Air of Confidence
Integrating demand control ventilation is heavily influenced by your existing HVAC system, such as whether your ventilation is combined with heating and cooling or is a standalone function.
“For example, adding DCV to a packaged rooftop unit may be as simple as including the CO2 sensor with a controller that has the DCV control logic built into it. Such a system likely serves only one or a few occupied zones, making it simpler to control CO2 levels,” explains Lawrence. “A larger building with central air handling, however, may serve many occupied zones. Determining the proper amount of outdoor air to bring in at the central air handling unit is also complicated by the variable occupancy patterns within the multiple zones.”
Say your VAV system supplies air to a large conference area and a group of private offices. To scale back the ventilation when the conference room is empty means that you risk the possibility of underventilating the offices at the same time. To avoid this scenario, you will need air flow sensors that measure the amount of air going to each space as well as the outside air that’s being drawn through the air handling unit, says Nall.
CO2 sensors are typically installed in the occupied space instead of ductwork because return air is an average of all conditioned spaces rather than an individual area, state ASHRAE members Mike Schell and Dan Inthout in their article Demand Control Ventilation Using CO2. Duct sensors can be used if all ventilated spaces share common occupancy patterns; otherwise, sensors should be wall-mounted.
“Avoid installing in areas near doors, air intakes or exhausts, or open windows,” advise Schell and Inthout. “Because people breathing on the sensor can affect the reading, find a location where it is unlikely that people will be standing in close proximity (2 feet) to the sensor. One sensor should be placed in each zone where occupancy is expected to vary. Sensors can be designed to operate with VAV-based zones or to control larger areas up to 5,000 square feet.”
Switching to DCV will typically require additional building management system points, new setpoints, and new control codes for dampers, Nohra notes. This may include a controller or DDC programming to communicate either directly with the economizer controller or a central control system, specifies the DOE in its 2012 report on demand control ventilation.
You should also make sure outdoor dampers are operable and not stuck in fixed positions, stresses Lawrence. It’s not unusual to find air intakes blocked in a misguided attempt to save energy. There may also be missing equipment, such as economizer controls with modulating air dampers that were specified but never installed.
Once the DCV sequencing has been established, the system requires minimal maintenance. CO2 sensors should be recalibrated periodically as their accuracy will drift over time. Consult your manufacturer guidelines, which may recommend recalibration every five years, annually, or every six months. Lawrence also recommends sensor testing prior to launch in case the product is deficient or was damaged during installation.