What are the main types of draft used for combustion air in boilers?

• A. Forced draft, induced draft, and balanced draft.
• B. Forced draft, induced draft, and balanced draft (and natural draft in some designs).
• C. Natural draft only.
• D. Only forced draft.

Answer: (B)

Draft is the pressure-driven flow of combustion air and flue gases in a boiler, and there are several ways to create that flow. The main types of draft are forced draft, induced draft, and balanced draft. In addition, some designs rely on natural draft, which uses buoyancy of hot gases in a stack rather than mechanical fans.

Forced draft pushes air into the furnace with a blower or fan, creating positive pressure that ensures a steady, controllable supply of combustion air. Induced draft, on the other hand, pulls the flue gases out of the furnace with an exhaust fan, creating negative pressure inside to move gases through the system and prevent leaks of combustion products back into the boiler room. Balanced draft uses both a forced-air supply and an induced-draft exhaust, maintaining about zero net pressure in the furnace for tight control of air and gas flow and often improved efficiency and emissions performance. In some boiler designs, natural draft occurs without fans, relying on the stack effect to move air and gases, though it offers less controllability.

This is why the best answer includes forced draft, induced draft, and balanced draft, with natural draft noted as present in some designs. The other options are incomplete because they omit one or more of these important draft mechanisms.