If you have had a 90%+ furnace installed in the last few years, please read...
The addition of a secondary heat exchanger to extract heat from the exhaust is what makes the efficiency difference
between the conventional 80% furnace and the 90+ units.
Since this exhaust is much cooler than the conventional furnace, condensation occurs quickly and a drainage system of plastic plumbing is used, this system gets clogged or leaks and the water backs up into the inside of the furnace and rusts it from the inside out.The homeowner will not even know this is happening.
At issue here is that the secondary heat exchanger is made out of polypropylene laminated steel. With heat and age , this laminate cracks and allows water intrusion that rusts out the heat exchanger, which will eventually allow deadly carbon monoxide to enter the home. If you notice any water under your furnace (and it is not from a neglected humidifier) you
need to call an authorized repair person to look inside your furnace to determine if the drain lines are clogged and the heat exchanger is rusting.
There was a class action lawsuit and settlement in November, 2007, that included Carrier Corporation which manufactures Carrier, Bryant, Payne and Day and Night brands. Carrier Corporation will cover, for 20 years after initial installation, all parts and labor required to replace the condensing heat exchanger and associated parts in the event of a mechanical failure. (water on the floor and a heat exchanger with rust holes.)
This water condensation problem is an issue with nearly all 90+ furnaces it would be wise to have a safety check performed on the condition of the heat exchanger so you do not become a victim of carbon monoxide poisoning.