Forced draft fan vs induced draft fan?
In a balanced draft boiler, you actually have both: forced draft
(FD) fans supplying air to the furnace, and induced draft (ID) fans
removing flue gas. Typically, the FD fans control airflow, while ID
Fans control furnace pressure to slightly below atmospheric
pressure.
In general though, the choice between forced draft and induced
draft is based on how "tight" the system is - if you have leaks, it
is better to use ID, as FD will cause product loss.