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The gas furnace inducer fan
When manufacturers were considering making higher efficiency furnaces, one of the problems that had to be dealt with was how to get the exhaust gasses out of the furnace and out of the structure. Earlier furnaces relied on the high temperature, 375F to 450F (190C to 232C) of the gasses to move them out of the structure by natural draft.
Heat exchangers were removing more heat from the burnt gasses, to the point that the temperature of the exhaust gasses was too low to rise by natural draft alone.
At this point, the draft inducer fan was developed. The inducer is a small motor and fan assembly designed to be attached to the outlet of the heat exchanger to pull the exhaust gasses thru the heat exchanger and assist it in rising out of the structure. This puts the heat exchanger under negative pressure. The first furnaces equipped with this part were 80% efficient appliances. These inducers are usually made from galvanized steel. 90+% efficient furnace inducers are usually made from plastic.
Heat exchangers were removing more heat from the burnt gasses, to the point that the temperature of the exhaust gasses was too low to rise by natural draft alone.
At this point, the draft inducer fan was developed. The inducer is a small motor and fan assembly designed to be attached to the outlet of the heat exchanger to pull the exhaust gasses thru the heat exchanger and assist it in rising out of the structure. This puts the heat exchanger under negative pressure. The first furnaces equipped with this part were 80% efficient appliances. These inducers are usually made from galvanized steel. 90+% efficient furnace inducers are usually made from plastic.
The forced draft blower
When furnaces began using fans to assist the removal of exhaust gasses, some used a blower that pressurized the heat exchanger instead of pulling a vacuum. The fan was placed before the burners and forced air thru the heat exchanger, The video below shows how this fan works. For a playlist on this brand and model furnace, copy and paste this link: https://www.youtube.com/playlist?list=PLItXrh64d2JMTpFV00_SSkOOQFZHa3gyX Most furnaces do not use this method anymore, so don't expect to find many of them.
Most furnaces use the negative pressure (induced) draft system. After a few years, virtually all furnaces dropped the ribbon burner and changed over to the inshot style burner. This more or less eliminated the forced draft blower.