THE STACK EFFECT: HOW IT WORKS AND ITS IMPACT ON YOUR ENERGY EFFICIENCY
By Patti Dees
Stack effect, also called chimney effect, drives airflow through buildings. Controlling air movement into and through a building can increase energy efficiency and save money.
What Is Stack Effect?
Stack effect is air movement caused by thermal differences. Higher-temperature air is less dense than cooler air. As the warmer air rises, it creates a pressure difference, with lower pressure below and higher pressure above. In buildings during the winter, the lower pressure allows cooler air from outside to move into the bottom floors. The temperature of the cooler air starts to increase, continuing the cycle. During summer or in warmer climates, the stack effect is reversed. The hot air outside enters the upper portion of the cooler building and creates a draft down.
Every building has a neutral pressure level (NPL), where the pressure difference between the building and its environment are the same. Air movement into or out of the building is reduced along this plane and increases further from it. Equipment that actively moves air in or out of the building, such as exhaust systems, will move the NPL location in the building. Knowing the NPL of a building allows designers and building managers to focus on control measures where they are most needed.
When the Stack Is Against You
The stack effect can be beneficial, but only if designs have planned for its effects. Without planning, stack effect can cause some serious issues, especially in tall buildings. Examples include:
- Strain on HVAC equipment as it struggles to deal with air losses or additional load. Inefficient HVAC systems increase energy and maintenance costs, and inadequate temperature regulation decreases occupant comfort.
- Moisture enters the building, setting the stage for mold and bacterial growth. Moisture intrusion is a serious issue that damages buildings and causes occupant health issues.
- Doors and elevators may require more force to open or close, or the opposite may be the case and they may slam easily.
- Air passing through cracks creates noise.
- Air movement can impact fire prevention, spreading flames and smoke. In some cases, building areas exposed to the coldest airflow may have frozen sprinkler system piping.
Designers, builders and facility managers can take steps to mitigate the impact from stack effect. Some are basic energy-efficiency measures, such as properly sealing and insulating pipe openings in the building envelope to stop in-leakage or designing tight exterior walls. Determining where air is most likely to enter the building, either through openings or via leaks, is the first step in controlling stack effect. Modeling and evaluating scenarios with wind pressure and temperature extremes are also important tools that professionals use to set a plan for specific building needs.
Stacking Your Benefits
Natural ventilation can save owners and building managers money, up to 30 percent in some cases. Design features should focus on mixing fresh air with current air and on controlling stack effect within the building.
Indoor transom windows and interior walls redirect flow through occupied areas. An indirect path allows the movement of fresher air to mix with and remove stale air. Interior exhaust openings should be above the supply openings so that the stack effect sweeps stale air up and out. New buildings designed for natural ventilation should be positioned to catch wind during the summer and block exposure during winter.
Before relying on stack effect to provide natural ventilation, consider some of the limits. Natural ventilation doesn’t include humidity control. Building height and width matter. Central areas of very wide buildings will not benefit from natural ventilation efforts alone. Additionally, buildings with low ceilings will not see significant airflow from stack effect. Fan-assisted ventilation may be required. Modern safety and fire codes may also limit some design options historically used in locations such as stairwells.
Since stack effect is unavoidable, building designs can limit unwanted effects while taking advantage of airflow patterns.
Patti Dees is a chemical engineer turned writer. She spins her polymath nature to produce clear and informative pieces on technical topics.