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 flowing backwards into the unit.
Additional Benefits
- Useful as a simple, passive exhaust method
- Helps protect building envelopes from positive pressure issues
- Prevents backdrafting when the system is off
Outside Air Hoods — Manual or Motorized Options
Outside air hoods control the volume of fresh air entering an RTU. They come in two primary versions:
- Manual damper hood
- Motorized damper hood
Aluminum Filters — A Valuable Upgrade
Some hoods include or offer optional aluminum filters:
- Lightweight aluminum mesh
- Low resistance to airflow
- Acts as a “pre-filter” to protect the RTU’s internal filter rack
- Helps block rain mists, insects, and small animals
These are strongly recommended in outdoor applications.
Manual Damper Outside Air Hoods
These are a low-cost substitute for economizers. A manual blade is adjusted by the installer to set a fixed opening, introducing outside air through static pressure balancing.
Drawbacks
- Damper stays open 24/7
- Increases heating/cooling loads year-round
- No shutoff during unoccupied hours
- Can worsen fire conditions by feeding outside air
- Negative pressure (bath fans, exhaust fans) can pull unconditioned outdoor air inside when RTU fan is off
For these reasons, manual damper hoods are best suited for very basic ventilation requirements or applications where codes allow minimal fresh-air introduction without modulation.
Motorized Damper Outside Air Hoods
A much more efficient and controlled option, the motorized damper hood uses an actuator to open and close the blade based on RTU fan operation.
How it works
- Damper opens when the RTU blower/fan energizes
- Damper closes when the fan cycles off
- Reduces unwanted outside air during unoccupied periods
Enhanced Control Options
When wired in series with additional devices:
- Timer — limits outside air introduction to scheduled periods
- CO₂ sensor — demand-controlled ventilation
- Smoke detector — safety interlock to shut off fresh air during smoke conditions
This creates a safe, responsive, and energy-efficient ventilation solution.
Where Hoods Fit Compared to Economizers
Hoods provide fresh air but do not modulate based on temperature, humidity, or enthalpy conditions.
Economizers offer:
- Modulating outside air control
- Free cooling when outdoor conditions permit
- Energy savings
- Compliance with codes such as California Title 24
- Integrated fault detection and diagnostics (depending on controller)
Economizers cost more upfront but deliver a far better ROI in long-term energy savings and building comfort.
Key Takeaways
- Barometric hoods relieve building pressure and prevent backflow.
- Outside air hoods provide ventilation but vary greatly between manual vs. motorized operation.
- Aluminum filters provide important protective pre-filtration.
- Economizers remain the superior option when efficiency and code compliance matter.
If you need recommendations for the best hood or economizer solution for your specific RTU application, MicroMetl’s support team can guide you through options, sizing, and compatibility.
