In HVAC rooftop applications, air hoods play a key role that often goes overlooked. While they may look simple from the outside, their purpose, configuration, and impact on comfort and building performance are essential to understand—especially when they are used in place of economizers or integrated into return and supply air duct systems.
This guide breaks down the basics of HVAC hoods from an installer and designer standpoint, with a focus on the MicroMetl offerings available today.
What Is an HVAC Hood?
An HVAC outdoor air hood is designed to:
- Introduce outside air into an HVAC rooftop unit (RTU)
- Minimize the entry of rain, debris, and environmental contaminants
- Control or relieve building pressurization when required
MicroMetl provides two primary categories of hoods:
- Barometric Relief Hoods
- Outside Air Hoods (manual or motorized)
Each type has a specific function, appropriate applications, and performance considerations.
Barometric Relief Hoods — Pressure Control & Airflow Relief
What Barometric Pressure Means
Barometric pressure refers to atmospheric force—typically measured at 29.92″ Hg (760 mm). In HVAC, understanding pressure matters because balanced airflow is crucial to comfort and equipment performance.
How Barometric Relief Hoods Work
A barometric relief hood contains:
- One or more lightweight relief blades
- Hinged metal panels designed to swing open under positive pressure
- Gravity-assisted closure when the RTU is off
These are used when the system does not include an economizer with an integrated barometric relief function.
Common Applications
- Horizontal return air duct configurations
- RTUs lacking built-in relief dampers
- Situations requiring airflow relief to prevent over-pressurization
The hood allows excess indoor air to escape, while preventing outdoor air from flowi