Barometric Relief
What Is It And How Much Air Does It Remove?

A reader recently asked; What is Barometric Relief, and how much air does it remove?

It’s important to provide fresh, healthy air to a building’s occupants. Therefore, when introducing fresh air into a building, how do you exhaust the present air? One method is with a Barometric Relief Damper, sometimes referred to as Gravity Dampers.

Positive Building Pressure

A Barometric Relief Damper is comprised of thin metal blades, or flaps. The damper provides a low resistance path for excess air to exit the building when indoor pressure exceeds outdoor pressure. A relief blade reacts to positive building pressure by forcing gravity operated relief dampers open; allowing excess air to exit the building. Inversely, the relief blades will seal closed under negative pressure conditions. The Barometric Damper is located on the return air side of the blower, and prevents the indoor fan from over-pressurizing the space.

Barometric Relief is often incorporated into an economizer (See What Is An Economizer – AKA Mixing Box? Economizers Class 101 for additional information). The economizer is a set of dampers which mixes the building’s return air with fresh outside air. The amount of outside air brought into a space is regulated by Enthalpy, or Dry Bulb Controls. As an economizer opens to allow outside air to mix with building’s returning air, the space pressurizes requiring a higher equilibrium, so a barometric damper is forced open to compensate. When a Barometric Damper is incorporated into an economizer, the outside & inside pressures often balance themselves out.

The Result of Positive Building Pressure

As economizer dampers open and close, the amount of outside air entering a building’s space is increased or reduced. However, when forcing more air into the building than can be removed, problems occur. The higher pressure of the building is pushing to get out. Doors standing open, and whistling elevators can result from excessive positive building pressure due to air rushing out the building. A substantial positive pressure differential could result in the excessive loss of conditioned air, leading to increased energy costs. An extreme pressure differential could even damage roof systems.

Negative Building Pressure

Exhaust-only ventilation, common with bath fans and range hoods, can cause a space to be negatively pressurized. Barometric Damper Blades are designed to seal off outside air under negative pressure conditions. However, in reality, the negative pressure causes the Barometric Dampers to leak outside air into the building.

ASHRAE regulates the amount of leakage allowable under ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings. It includes requirements for economizer dampers, including those used in barometric relief economizers. Sorry… MicroMetl cannot provide the ASHRAE documentation; it must be purchased from ASHRAE.

When Barometric Relief Is Not Enough

How much air will a Barometric Relief remove is part of the initial question. The answer depends upon the building’s measurement of positive pressure, the size of the Barometric Relief opening, and the position of the economizer blades with proximity to the relief damper. Ideally, air pressure inside a building should be equal to, or slightly more positive than the air pressure outside the building. When building pressure measurements exceed more than 0.1 w.c. of static pressure (refer to the building engineer, or local code for specific pressure tolerances and ranges), and a Barometric Relief damper is present, a Power Exhaust should be considered.

A Power Exhaust is an air movement system designed to forcibly remove exhaust air from the space. It consists of fans or blowers set at a fixed speed, sequentially staged, or coupled with a pressure sensor to modulate pressure relief. The modulating Power Exhaust operates from a pressure set point, and is the best method available for building pressure regulation (See — What is a HVAC Power Exhaust — Achieving Proper Ventilation — For Additional Information).

Don’t Confuse High Static Pressure With High Building Pressure

Static pressure is the resistance air encounters when being forced throughout a building’s HVAC system (duct size, duct obstacles, duct distances, and dampers to name a few). There are times when a high static HVAC fan is required to over-come high static building designs. More powerful fans are required to adequately push air throughout the building, back to the HVAC unit. The additional fan power is designed to properly distribute the air, and in no way results in over building pressurization.

Feel free to contact the MicroMetl Customer Service Department with any questions you have concerning Barometric Relief, Economizers, or Power Exhaust for your specific application and need.