A return fan in HVAC systems is a fan that pulls air from the conditioned space or building and directs it back towards the air handling unit (AHU).
Role and Function
In a typical HVAC system, the supply fan pushes conditioned air into the space. The return fan works in tandem with the supply fan, pulling air out of the space through return ducts. This returned air can then be:
- Recirculated (often mixed with fresh outside air) back through the AHU, conditioned, and supplied again.
- Exhausted from the building (as exhaust air).
Air Balancing and Pressure Control
One critical function of the return fan is to help balance the amount of air supplied to and exhausted from the space. By controlling the volume of air pulled out, the return fan helps maintain desired building pressure.
- Positive Pressure: More supply air than return/exhaust air. Helps prevent uncontrolled infiltration of outside air.
- Negative Pressure: More return/exhaust air than supply air. Can be desired in areas like laboratories or restrooms to contain odors or contaminants.
- Neutral Pressure: Supply and return/exhaust air volumes are roughly equal.
Key Design and Operation Aspects
Based on system requirements, the return fan is designed to handle specific conditions. As noted:
The return fan is then designed to meet the static pressure requirements of 100% exhaust air operation; this includes the return duct, exhaust duct and exhaust damper.
This means the return fan is built to overcome the resistance (static pressure) in the ductwork and dampers even when all the air being returned from the space is being exhausted outside rather than recirculated.
Furthermore, the return fan typically operates continuously:
The return fan will operate continuously in conjunction with the supply fan to balance the amount of air supplied to and exhausted from the space.
This continuous operation alongside the supply fan is essential for maintaining consistent airflow and pressure relationships within the building.
Importance in Different Applications
Return fans are vital in various HVAC system types, particularly in commercial buildings, hospitals, laboratories, and schools, where precise control of airflow and pressure is necessary for comfort, air quality, and safety.
Understanding the return fan's role in overcoming static pressure and its continuous operation with the supply fan highlights its importance in the overall system performance and air management strategy.
