I have a 23 year old Carrier nat gas furnace, model 58GP125. Last spring and now in the early fall I noticed a strong 'hot metal' smell coming from the furnace when it is on. After a few heat cycles the CO detector in the house goes off. So I think that the flue is blocked and it's not venting properly, but it couldn't be any cleaner. Nothing but some light rust dust in the vent pipes. The furnace otherwise runs fine. No weird noises, burners have a nice blue flame, fan runs fine, new air filter. Good air flow at the registers.
I'm stumped. I haven't taken the furnace apart to fully inspect the heat exchanger.
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