Not really true (as in anything that burns always produces Carbon Monoxide). But it is true that anything that burns CAN produce Carbon Monoxide.
My bad, I should have specified “gas furnace” because those are the ones that I know about and don’t know about wood, oil, coal or any other fuel mentioned.
Efficience has nothing to do with it, an older 60% efficient gas furnaces can have and do have complete conbustion and complete combustion = CO 2
I do know for a fact from experience a good clean burning gas furnace does not produce Carbon Monoxide unless something is wrong with it.
I have been checking every furnance I’ve serviced for many years now and anytime I find Carbon Monoxide readings within the flue or draft hood or in or around the furnace I find out why and repair it or shut it down.
That is in no way to say I’m making light of a cracked heat exchanger at all. When I find a cracked heat exchanger I always shut the furnace down imediatly, producing Carbon Monoxide or not, many are not but the potential is still there.
I’ve had homeowners relite and turn the furnace back on after I left and probably even called me a crook for fullfilling my obligation by shutting down their furnace but a least I can sleep at night knowing I did my part.
We file our customers by addresses (as well as by name and by phone number) and I’ve gone back a year or two later when the house had a new owner expecting to find a new furnace since I had been there and I see the previously history on our computer before going and found the same old furnace with the same cracked heat exchanger that I had shut down previously, now back in operation.
That’s usually when the new home buyer starts getting on the phone to the realtor about failure of disclosure.
I only tell that story to demonstrate that a cracked heat exchanger doesn’t necessarily always mean Carbon Monoxide.
Maybe I shouldn’t have even mentioned it since it’s relly a moot point anyway and besides I can’t say that the people living in the house haven’t been having too many headachs.
Again though, I think everyone knows a cracked heat exchanger certainly does mean danger and the furnace should be shut down ASAP even though it might be burning perfect at the time.