Air Handling Unit Control: Night Setback & Free Cooling
The building automation system (BAS) has the capability to perform whole building, complex sequence of operations. Two simple sequences that lead to big energy savings are night setback and free cooling. A complex air delivery system or a basic packaged rooftop unit with a stand-alone thermostat can perform these two functions. Mechanical cooling and heating components (and systems) also benefit from these strategies because run times can be significantly reduced, extending service life.
Night Setback: Proper scheduling and temperature setpoints are the most commonly overlooked items we see when returning to a job during a one-year warranty review. Both occupied and unoccupied building periods require thoughtful setpoint scheduling to reduce energy consumption.
Step 1: Set the building occupied schedule for the hours of occupancy. This is an easy way to ensure that your units are performing only when necessary. Building automated systems allow for holiday, event and specific zone scheduling requirements to be accounted for.
Step 2: Set the occupied cooling and heating setpoints to maximize energy savings while maintaining comfort. A 74F cooling setpoint and 68F to 70F heating setpoints are most common. Adjustable setpoints allow for you to control to occupant comfort, climate and energy saving goals.
Step 3: Set the night setback, also known as unoccupied cooling and heating setpoints. These setpoints are the most critical and can provide a large reduction in your energy consumption. An 80F cooling setpoint and 60F heating setpoint are common night set points. Make sure that in unoccupied modes, the outside air damper remains closed and the air handling unit remains in 100% recirculation mode. Because the building is vacant, fresh air requirements do not have to be satisfied.
Free Cooling: Proper air handling unit damper operation in cooling mode is the next most common HVAC issue we see. During the spring and fall seasons especially, the opportunity to use outdoor air to provide free cooling saves energy costs while also boosting indoor air quality. To accomplish this sequence your air handling unit needs to be equipped with modulating return and outdoor air dampers. Temperature (or enthalpy) sensors are also needed for your BAS to determine which flavor of air is most appropriate for cooling (outside air for free cooling or return air for mechanical cooling).
It’s simple – when conditions are favorable (outside air temperature is below return air temperature), the outdoor air damper shall be the first stage of cooling. Even when additional mechanical cooling is required, if the outdoor air temperature is favorable when compared to the return air, the outdoor air damper should be open and continue working its magic!
If you would like help determining your building’s scheduling or energy saving sequence opportunities, contact Elevate Building Commissioning today! We strive to provide helpful, long-lasting solutions for your building system operation and energy use strategies.