What type of boiler has a furnace pressure that is below atmospheric pressure?

A boiler with only an induced draft fan operates with a furnace pressure that is below atmospheric pressure. This is because the induced draft fan is responsible for drawing combustion gases out of the boiler and creating a vacuum or low-pressure area inside the furnace. As the fan pulls the flue gases out, it lowers the pressure, which results in the furnace operating under negative pressure conditions.

This is important because it helps to ensure proper combustion and efficiency by preventing combustion gases from leaking out into the surrounding environment, thereby enhancing safety and performance. Induced draft systems effectively manage both the air supply and the gaseous emissions, contributing to overall operational efficiency.

In contrast, a boiler with forced draft typically operates at or slightly above atmospheric pressure, as the forced draft fan pushes air into the combustion chamber, increasing the pressure above atmospheric levels. Natural draft systems rely on the buoyancy of hot gases to create draft, which does not necessarily lead to a negative pressure within the furnace. A boiler operating at atmospheric pressure is, by definition, at the same pressure as the surrounding atmosphere, which is distinctly different from the negative pressure created by an induced draft system.