A question to HVAC experts

[Yulia Garvanovic]

After two HVAC companies serviced the two furnaces in the duplex I own in the course of the last 12 months, the third one discovered that both furnaces have multiple cracks in the exhaust systems. 

Here is my question:  is checking the exhaust system for cracks part of the annual maintenance service?

[Brian Conway]

Best way to verify a heat exchanger crack is by performing a combustion analysis, you can only visibly see a portion of a heat exchanger. If the company that came out suspected a heat exchanger crack they should use a combustion analyzer to verify. I know for my company we do this during routine maintenance.