Secondary gas line

[seanie12]

Being a newer of around 5 years, it was definitely taught to me at my initial training class, as well as, CE classes that you disclaim the heat exchanger and trying to actively view it because of its inaccessability. If I come across a 20 year old furnace that has a draft inducer, instead of fan of inducer that the furnace is "near the end of its serviceable life and the buyer should budget for replacement within 5 years" I must admit though because it's never been taught, I don't know if I have the knowledge to adequately inspect a heat exchanger. I don't like disclaiming portions of the inspection unnecessarily, but I really don't know that aspect of the furnace well enough. If someone has pictures or a method, that would be appreciated it.