As the heating season swiftly approaches, the focus often revolves around the absence of draft. However, it's crucial to recognize that inadequately designed venting systems, leading to excessive draft, can detrimentally affect boiler performance just as much.

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.