Boiler Stack Draft
& Damper Sizing
Audit flue gas thermal buoyancy against stack heights to size barometric dampers and ensure combustion stability.
Combustion Draft Physics, Bouyancy, & Barometric Pressure Control Overview
A chimney works because of the temperature-density differential between the hot combustion gases inside the flue and the cooler outdoor air. This “thermal buoyancy” creates a negative pressure gradient, or “draft,” that pulls air into the burner for combustion and pushes exhaust gas out of the building. However, draft intensity is not constant. It shifts with every variation in outdoor temperature, barometric weather fronts, and wind speed. If the stack draft is too high, it pulls excess air through the boiler, wasting energy by heating air that just goes out the chimney (heat loss). If the draft is too low, the burner fails to evacuate exhaust gas, leading to “spillage” of toxic carbon monoxide into the boiler room. Barometric dampers solve this by automatically swinging open to allow room air into the flue, essentially acting as a mechanical regulator that prevents the stack draft from exceeding the combustion design limit.
Frequently Asked Questions
A: A barometric damper acts as a “relief valve” for the chimney. If the damper is too small, it physically lacks the open surface area to dump enough room air into the flue when the chimney draft spikes (e.g., during a cold weather front). The stack will continue to pull excessive vacuum through the burner chamber, potentially lifting the burner flame off the burner ports, destabilizing the ignition, and causing inefficient, sooty combustion.
A: As a modulating boiler changes its firing rate, the velocity of the exhaust gas changes. At high fire, the mass of the exhaust gas creates a high draft. A barometric damper senses this change; the gate swings open more at high-fire and closes slightly at low-fire. By constantly balancing the pressure inside the flue, the damper ensures the “Over-Fire Draft” (the pressure right inside the burner) stays perfectly flat, keeping the combustion Air-to-Fuel ratio constant regardless of how hard the burner is working.