Thank you, Jake. Sorry, yes, I meant heat exchanger. The HVAC team I usually work with serviced the furnace in my rental property a month ago, and yesterday the tenant started experiencing pilot issues (the flame would not stay lit for too long). Since my regular team was experiencing a large volume of no-heat calls (it's been extremely cold and windy for the past few days in our area), I called in a different HVAC person. He showed me through a camera inside the heat exchanger what looked to me like cracked dry clay. (Sorry, I have no other way to describe it.) He said that those were cracks in the metal and that the furnace had to be shut down due to a poisoning danger. (He did not test the air for carbon monoxide.) He recommended that a new furnace be installed.
That was the reason why I asked this question. I thought during the annual maintenance service, those "cracks" wouldn't have been overlooked.
My regular team showed up this morning, cleaned the entire furnace, including the heat exchanger and did not find any cracks neither before nor after the cleaning. They said those "cracks" could have been just flaking metal inside the heat exchanger. They also let the furnace run for a while, and the carbon monoxide test in the air came back negative.
