I was told today by a heating company that was out to replace my humidifier that the Lennox G20 series has heat exchanges that crack after as little as 4 years of use. Mine is just 4 years old. I was told that Lennox is keeping a lid on this problem, but stopped producing this line 3 years ago. Do you know if this is true? Also, my unit is a G20Q5. It was installed without a chimney liner. I was told that Lennox marketed it that way, although it actually should have a chimney liner. I'm getting rust on my unit and was told that it was due to condensation from the duct leading to the chimney. The installer was out to clean the furnace 3 weeks ago and said nothing. Whom should I believe? Thanks for any help you can give.