Which type of draft creates pressure that is lower than atmospheric pressure in the furnace?

Induced draft is the type of draft that creates a pressure lower than atmospheric pressure within the furnace. This occurs through the use of a fan or exhaust system that actively pulls exhaust gases out of the combustion chamber. As these gases are removed, a vacuum is created, which leads to lower pressure inside the furnace compared to the outside atmosphere.

This lower pressure helps facilitate the flow of air into the combustion area, enhancing the efficiency of the burning process. It prevents the buildup of harmful gases and ensures a constant supply of fresh combustion air, crucial for optimal combustion and maintaining safe operational conditions.

In contrast, naturally ventilated draft relies on natural convection and does not actively create a pressure differential, while forced draft uses fans to push air into the furnace, increasing pressure. Combustion draft typically refers to the overall drafting associated with the burning process itself but does not specifically denote a lower pressure system like induced draft does.