The air flow for a 90 percent furnace is the following- a. From the main blower, across the burners, thru the heat exchanger, then the secondary heat exchanger
b. From the main blower, over the secondary heat exchange, thru the primary heat exchanger, over the burners
c. From the main blower over the primary heat exchanger, over the burners
d. From the main blower, over the burners, thru the primary heat exchanger