I replaced both limits and the control board.This unit does not have an IDM.Could the heatexchanger be corroded and not allowing the proper amount of heat to escape keeping the blower on in default?

MMMM. Please, start by the beginning... why did you decide to change the limits, try to explain better your problem... what is an IDM for you? Thanks

Upon further inspection found that heat exchanger was corroded and beginning to separate.I also would not doubt the coil is plugged also.Without an Induce Draft Motor the vapors and excess heat caused the heat exchanger to over heat tripping the lim

Be careful! Do not use the furnace anymore... a cracked exchanger allows combustion products pass into your room CO included. So I suggest you to replace your furnace TODAY. There is no way to repair a corroded-broken exchanger. Take care. Miguel