I just learned I had a bad install on a Carrier 2-stage found by a factory authorized service tech. The installer wired it wrong so it could
never fire up on the high setting ever. Would this have caused combustion fumes to enter the house? Service tech said fumes could
enter the house this way, but it doesn't sound quite right to me. He said the furnace needed to fire up in the high setting to create a good seal.
Could the heat exchanger be faulty? I smelled odors coming from the vents and had trouble breathing. The installer blew me off.

Yes, I did my homework before having this unit installed and still got a bad install. I plan to call the installer's boss to complain.