Boiler Draft

In a boiler, draft is the difference between atmospheric pressure and the pressure inside the furnace or the flue gas passage, and it is what moves hot combustion gases and combustion air through the boiler, which is essential for burning fuel efficiently and transferring its heat to the boiler tubes. Natural draft arises on its own, from the density difference between hot flue gas and cooler outside air, which pulls gas upward through the stack with no mechanical help. Mechanical draft instead uses fans: a forced draft system pushes air through the fuel bed at above atmospheric pressure, an induced draft system pulls flue gas through at below atmospheric pressure using a fan or a tall stack, and a balanced draft system combines the two so that pressure stays roughly at the atmospheric level throughout. The strength of the draft affects both how completely fuel burns and how much heat the flue gas gives up to the boiler tubes, so it is a central factor in a boiler overall efficiency. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/