Venting design has always been a prediction, and the thing we are predicting has stopped holding still. Every section of this hour is one more way the operating point moves away from the single steady-state condition our design methods were built to evaluate — fan-assisted appliances removed the draft hood, modulation moved the operating point continuously, condensing temperatures took the buoyancy away, and the answer the industry offered for Category IV was a certification envelope rather than a calculation. Mechanical draft is the design response to all four.