Some burners use a forced-draft system rather than an induced-draft system. The difference here is that the forced draft system
combustion air into the com-bustion chamber, all the way through the furnace, and out the vent. The induced-draft fan
things through the furnace and then pushes them out the vent.
The advantage of the forced-draft fan is that the fan itself is not in the high temperature, acidic environment of the exhaust gases. The forced-draft fan only sees room air. It lives in a much friendlier environment and, therefore, may be less susceptible to failures. Induced-draft fans are one of the most common items to fail on high-efficiency furnaces.