In a multi-story building, why does smoke typically rise through vertical shafts like stairwells during the early stages of a fire, even without mechanical ventilation?

In a multi-story building, smoke typically rises through vertical shafts like stairwells during the early stages of a fire, even without mechanical ventilation, due to a natural physical process driven by convection, buoyancy, and the stack effect.

A fire generates intense heat, producing hot combustion gases, which are visible as smoke. These hot smoke particles and gases are significantly less dense than the surrounding cooler, ambient air. Density refers to how much mass is packed into a given volume; hot gases expand, making their particles more spread out and therefore lighter per unit of volume.

Because the hot smoke is less dense, it becomes buoyant, meaning it experiences an upward force, much like a hot air balloon rising or a cork floating on water. This buoyant force naturally causes the hot smoke to ascend.

This upward movement of heated fluid, in this case smoke, is a form of convection. Convection is the transfer of heat through the movement of a fluid (liquid or gas). The hot smoke rises, carrying heat upwards, while cooler, denser air tends to sink or is drawn in to replace the rising hot air.

Vertical shafts, such as stairwells, elevator shafts, or utility risers, act as natural flues or chimneys. They provide a clear, unobstructed pathway that channels the buoyant hot smoke efficiently upwards. This channeling of smoke through a vertical opening due to temperature and density differences is known as the stack effect, also referred to as the chimney effect.

As the hot, buoyant smoke rises within the stairwell, it creates a differential in pressure. A zone of lower pressure is formed near the fire's origin, which effectively draws more smoke into the vertical shaft. Simultaneously, higher pressure develops at the top of the shaft as smoke accumulates. This pressure difference further enhances and sustains the upward movement of smoke.

This phenomenon is most pronounced in the early stages of a fire because the fire is rapidly generating a large amount of heat, creating a strong temperature gradient that significantly drives this powerful natural stack effect.