I just had my fall furnace check by my HVAC contractor. The tech told me he had to red tag our furnace because we had a cracked heat exchanger. He showed me how he put a little bit of puffed smoke into the heat exchanger and it swirled around and some came out. He said air backflow means the heat exchanger is cracked. From then it was high pressure sales to replace the furnace and, by the way, while we're at it, why don't we replace your ac as well?
Being a conservative homeowner, I got a second opinion. This contractor laughed at the idea of a smoke test. He used a CO detector and registered no CO, so he said our furnace was fine.
Being curious, I've started doing some reading and I'm finding conflicting ideas on how one should check for a cracked heat exchanger. Would anyone care to elaborate on what sort of tests or checks should be done for a cracked heat exchanger?