I have two Carrier furnaces one in my garage and one in my house, both are condensing. In my garage in 5 years I have gone through 3 heat exchangers, the secondaries were completely rotton (rust). After the third exchanger carrier would not warranty so I opted to replace entire furnace with a keeprite, seems to be working fine but it has only been 1 winter. The furnace in my house went 2 winters and just found out the secondary is rusted and no good. The twist in the story is my condensing hot water tank which is a A.O smith Vortex has been acting up also the condensate line was all plugged up with rusted steel debris kind of flacky material, my guess is the fire tube is rotting on it as well. The house furnace and hot water tank are 2 years old. I have considered getting my natural gas analyized to see if there are impurites that could be causeing this quick failure, We do live in the country so our gas supply is from our county which they purchace from TCPL which is a huge gas transmission pipeline. Has anyone heard of poor natural gas. Thanks in advance for all replies. One other thing to note is all my mid effiecent appliances like the clothes dryer and wall mount on demand water heater havent missed a beat.