Stack effect or chimney effect is the movement of air into and out of buildings resulting from air buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The stack effect helps drive natural ventilation, air infiltration, and fires.
Since buildings are not totally sealed the stack effect will cause air infiltration. During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or unintentional holes in ceilings, like ceiling fans and recessed lights. The rising warm air reduces the pressure in the base of the building, drawing cold air in through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences.
In a modern high-rise building with a well-sealed envelope, the stack effect can create significant pressure differences that must be given design consideration and may need to be addressed with mechanical ventilation. Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, while interior partitions, floors, and fire separations can mitigate it.
Fig 1. Air Flow
When designing natural or passive ventilation systems, both stack ventilation and Bernoulli’s principle, or Bernoulli effect, can be applied. designing for these effects requires a large difference in height between air inlets and outlets. The bigger the difference, the better.
Passive or natural ventilation supplies and removes air from indoor spaces without the assistance of mechanical systems. Passive ventilation instead relies on pressure differences of natural forces to facilitate airflow. When used in a system, this type of ventilation works to regulate temperature through inlets and outlets (i.e., windows, vents, etc.) in three methods:
- Wind-driven ventilation: Driven by pressure differences between the internal and external air around the building
Fig 2. Passive ventilation being employed using wind through openings & window installations
- Buoyancy-driven ventilation: Driven by temperature differences between the internal and external environments (i.e., stack ventilation). After wind-driven ventilation, which is chiefly facilitated by the opening and closing of windows, stack ventilation is the most regularly used form of passive ventilation. Stack ventilation, along with Bernoulli’s principle, can be remarkably effective and inexpensive to achieve within a building design. It is important to consider buoyancy-driven strategies like stack ventilation in addition to wind-driven strategies when considering passive ventilation tactics, because natural forces such as wind usually slow down at night and can, therefore, make wind-driven strategies less effective and inefficient. Stack ventilation (also known as stack effect or chimney effect) creates airflow using the natural force that emerges from changes in air pressure, temperature, and density levels between corresponding internal and external environments. The relevant variables that create this environmental effect are thermal contrasts paired with the height of the given structure. Airflow through a chimney is one example of stack effect due to its inclusion of the two main variables at play; height and temperature difference.
When designing natural or passive ventilation systems, both stack ventilation and Bernoulli’s principle, or Bernoulli effect, can be applied. designing for these effects requires a large difference in height between air inlets and outlets. The bigger the difference, the better.
Fig 3.Stack Ventilation
- Night-cooling ventilation: Driven by the release of heat absorbed by the building’s thermal mass during the daytime.
Fig 4.Night cooling Ventilation