HVAC systems often lack the ability to regulate the motors and fans that ventilate a space in response to environmental changes like the weather or building occupancy. Instead, California's building code requires the air damper to be fixed open wide enough to ventilate to the demands of peak occupancy, no matter how many people are actually inside the building. In addition, the supply fan is required to constantly re-circulate air.

Fans in the VAV system

A ducted system imposes additional pressure requirements on the fan. The purpose of a fan is to generate pressure and airflow. Zone dampers in a ducted VAV system impede airflow in response to the zone cooling or heating requirements. Closing one or more dampers creates an increased pressure effect at the same fan speed, but the extent to which they impact the system pressure depends on which dampers are closed, whether they’re those close to the fan or those farther down the duct.
To determine how much a VFD can slow down a ducted fan, we need to consider system effects and the control method. Some VAV systems are operated to maintain a fixed static pressure at some location in the supply duct, the setpoint of the fan capacity control modulation to ensure proper airflow in all system conditions.

With proper control of the fan speed to maintain duct static pressure, significant energy savings can be realized. However, the VAV system modulation curve, the static pressure sensor location, and static pressure setpoint together determine how much fan energy savings can be achieved with variable-speed modulation.

Exhaust fans in enclosed parking garage

When it comes to capturing energy savings in commercial garages, retrofitting a garage ventilation system (remove ‘have’) becomes the first go-to (remove hyphen) action. All enclosed parking garages in the U.S. are subject to ventilation standards established by the International Mechanical Code and ASHRAE. The IMC and ASHRAE stipulate that garage ventilation systems run continuously during building occupancy hours, with an exception made for those that deploy carbon monoxide(CO) sensor based, demand controlled ventilation systems.

Air Handling Unit-VFD Configuration

  • 1. Variable Frequency Drive
  • 2. Static Pressure Sensor
  • 3. Building Management System

Parking Garage Exhaust Fans-VFD Configuration

  • 1. Variable Frequency Drive
  • 2. Carbon Monoxide Sensor
  • 3. Controller