CPP co-founder and air quality expert Dr. Ron Petersen has spent years helping permitting consultants understand that Good Engineering Practice (GEP) stack height offers more options than a single formula. The GEP stack height is greater than or equal to the height at which plume dispersion from the stack isn’t influenced by downwash. Downwash, in turn, refers to the stagnant, recirculating eddy of air that forms on the leeward side of a building.

While engineers can determine optimal stack height in a number of ways, a persistent myth has it that GEP stack height is limited to 2.5 times the height of a building. This wouldn’t be a bad first guess for a shoebox-shaped building in the middle of the Great Plains, but it’s probably sub-optimal for most buildings of practical consequence. That’s because GEP stack height actually depends upon many variables:

• Building shape
• Building size
• Surrounding terrain
• Nearby buildings and structures
• Local climate characteristics

According to Environmental Protection Agency (EPA) regulations, GEP stack height is defined to be the tallest of the following:

• 65 meters (213 feet), as measured from the ground-level elevation at the base of the stack
• 2.5H (for stacks in existence in January 12, 1979), or H + 1.5L (for all other stacks), where H is the height of the building itself or any significant nearby structure or structures and L is the lesser of the projected height or width of the building in question
• The height demonstrated by a an approved fluid model or a field study that ensures that the emissions from a stack do not result in excessive concentrations of any air pollutant as a result of atmospheric downwash, wakes, or eddy effects created by the source itself, nearby structures or nearby terrain features.

The first criterion simply establishes a minimum stack height, while the second is clearly a very generic method that can’t possibly cover every situation. When stack height is a driving factor in building design, a custom approach can yield more accurate results.

An atmospheric boundary layer (ABL) wind tunnel investigation of a proposed or existing building is an approved method that can be used to justify an optimal stack height that may be quite different than the oft-quoted 2.5-times-the-building-height figure.