Just had someone out for routine furnace check. He discovered what appears to be a cracked heat exchanger. Needless to say my furnace has been shut down. (For obvious reasons.) I have been given 2 options by different contractors: (1) replace heat exchanger(which is still under warranty) in current furnace. (2) replace furnace with new unit. The current unit was built in 1988. (Do not know when it was installed.) Current unit is a Janitrol. The price quoted to replace the heat exchanger is $400. The price for a new furnace is anywhere between $1200 - $2100. My question is - is it worth repairing?