I should mention also we have (3) CO2 detectors running. One in the crawl space (not far from the furnace), 1st and 2nd floors. None have ever gone off.
Here's some old scary pics of the old furnace setup when we bought the place (17) years ago.
http://katmanduonline.net/pics/ACFURNACEPics/index.html
Note the copper pipe running from the AC condensor to the back corner. No drain pipes available. Talk about a damp, moldy mess! I fabricated a piece of PVC out to a 5 gallon bucket that had a sump pump in it. We would have to empty it every few hours when the AC was running. Crude yes, but it WORKED!
Part of the new install will be to fabricate a new automatic sump pump to the furnace/coil. That will be done when the come back in the springtime to recharge the AC.
BTW, the old duct work that was done on this new install was the 2-12" flexible hose connections and the new plenum before the filter. Nothing else was done to the existing duct work.