If chimney flue diameter is oversized for a given gas flow, the exit velocity ($v_s$) drops below $1.5 imes$ prevailing cross-wind velocity. The low-pressure toroidal vortex shed on the downwind face of the stack sucks the raw sulfurous flue gas downward along the outer shell. Instead of ascending into the upper atmosphere via thermal buoyancy, hot flue gases wash down into boiler building fresh air intakes and plant walkways, triggering catastrophic OSHA sulfur dioxide ($SO_2$) ground-level exposure violations.

For fuels with even 1.5% sulfur, trace $SO_3$ and moisture elevate the sulfuric acid dew point to 135°C to 150°C. Operating at low exhaust temperatures or failing to insulate the external steel stack drops the inner wall surface temperature below $T_{adp}$. Concentrated 70% sulfuric acid condenses directly onto carbon steel flues, eating through 8 mm steel plates in under 12 months. When firing sulfur fuels, keep stack temperatures $ge T_{adp} + 15^circ ext{C}$ or install borosilicate foam block linings.

At a critical wind speed, the frequency of alternating aerodynamic vortex shedding matches the fundamental natural frequency of a tall, un-guyed circular steel stack. This induces violent cross-wind (lock-in) resonant vibrations with peak displacements exceeding several feet. The cyclic bending stress shears base anchor bolts and tears circumferential girth welds. ASME STS-1 mandates continuous helical strakes (3-start spoilers projecting $0.10 D$) over the top third of the stack to break coherent vortex shedding.

Natural draft creates negative relative pressure (vacuum) inside the lower chimney and breeching ductwork. If fabric expansion joints or cleanout access doors leak, cold ambient air (20°C) is sucked inward into the flue gas stream. This parasitic air cools the flue gas from 200°C down to 130°C, increasing gas density and collapsing the buoyant draft by 40%. The furnace experiences positive firebox pressure, spewing flame and ash out burner windboxes.

On freezing winter mornings, a tall brick or concrete chimney contains thousands of kilograms of stagnant, freezing air. When boiler burners are initially ignited, the buoyant force of the initial weak burner flame is insufficient to lift the heavy cold air column. The cold plug acts like an aerodynamic damper, forcing combustion smoke backward into the boiler house. Operators must prime the stack draft with warm purge air or pre-heat with gas flare torches.