Draft - continued

Balanced Draft Furnace

The figure provided here illustrates a balanced draft furnace. In this type of design, an FD Fan and an ID Fan are used to aid the draft created by the chimney. These fans operate together to provide the following functions:

a. Control the furnace pressure at the desired level. Usually the furnace pressure is maintained at slight vacuum. (About 0.25 inches of water-column below the atmospheric pressure. Remember, 1 psig is equal to 27 inches of water column).

b. Provide sufficient combustion air to maintain the excess air at the desired value.

c. In a way a constant push (forced dreaft fan) and pull (induced draft fan) operation exists between the two fans to ensure the furnace pressure is what the setpoint set by operator dictates.