First time user... Quick question. I have a carrier gas furnace (roof unit) circa 1979. We have been told we have a cracked heat exchanger. The technician showed my husband. He said that since our unit is so old, they can not buy a replacement part, so our only option is to spend the $15K to replace the whole HVAC unit. THis seems a little extreme to me. Can I get a new Heat exchanger and save my family some $$$ we don't have to spend on a new HVAC unit. Also to add, we have purchased a Carbon Minoxide detector. I just want my unit fixed. Any help is appreciated.