a coil install question

i have an old furnace i'm removing and installing a 90% unit when i took the cover of the a coil i found 1/2 of an a coil and the other half was block off with sheet metal now that seems to me to be counter productive the a coil itself blocks some airflow and the other half being completely blocked can't give the new furnace much airflow. and my guess will be that i will see some kind of fault with that diminished air flow, so can someone tell me how i can install half of an a coil and still get cooling from it? maybe some type of block that gets opened in the winter to let the heat thru the heat exchange better? or some instructions on how to install half of an a coil unit. thanks