What Is Barometric Relief?
A barometric relief damper contains lightweight metal blades that open automatically when indoor pressure rises above outdoor pressure. This creates a low-resistance path for excess air to exit the building. When indoor pressure falls, the blades swing closed to help prevent outside air from being pulled back in.

Barometric relief is typically installed on the return-air side of an HVAC system, often integrated directly into an economizer.

Why Building Pressure Matters

Introducing fresh outside air without a way for excess indoor air to escape causes positive building pressure. When pressure becomes too high:

• Doors can be difficult to open or may blow open
• Air can force its way through cracks and openings
• Conditioned air escapes, increasing energy consumption
• Elevator shafts may whistle
• Excessive pressure can even damage roofing systems

A barometric relief damper opens to release excess air and help maintain balance.

Positive Pressure vs. Negative Pressure

Positive Pressure
Occurs when more air enters the building than is removed. Barometric dampers open and allow excess air to exhaust.

Negative Pressure
Occurs when exhaust fans (bath fans, range hoods, etc.) remove more air than is supplied. In this condition, barometric dampers should remain closed—but in reality, they may leak and allow unfiltered outside air to enter the building.

This leakage is regulated by ASHRAE Standard 90.1, which defines allowable leak rates for economizer dampers.

How Much Air Does a Barometric Relief Damper Remove?
There is no single CFM rating. The performance depends on:

• The pressure difference between indoors and outdoors
• The size of the relief opening
• The position of the economizer blades and overall airflow path

Ideally, building pressure should remain slightly positive—but when it exceeds approximately 0.1 inches of water column (w.c.), barometric relief alone may not be adequate.
When Barometric Relief Isn’t Enough
If the building pressure consistently exceeds acceptable limits, a Power Exhaust system is recommended.

A Power Exhaust:

• Actively removes return air
• Can operate at fixed or staged speeds
• Can modulate via a pressure transducer and VFD for precise control
• Maintains consistent building pressure under varying outdoor-air conditions

For detailed information, see: “What Is a HVAC Power Exhaust — Achieving Proper Ventilation.”

Static Pressure vs. Building Pressure

These two concepts are often confused:

• Static Pressure refers to resistance in the duct system—affected by duct length, size, turns, grilles, and dampers.
• Building Pressure refers to the balance between supply air and exhaust air.

A high-static fan may be needed to move air through a restrictive duct system, but that does not mean the building is over-pressurized.

Summary

Barometric relief is a simple and effective method for removing excess indoor air when outdoor air is introduced through an economizer. However, its performance is dependent on building pressure, damper size, and airflow balance. When higher volumes of exhaust air are required—or when precise building pressurization is critical—Power Exhaust is the more dependable solution.

For support selecting or sizing economizers, barometric relief dampers, or power exhaust systems, contact MicroMetl’s Customer Service team.