This week I would like to follow up on the 80% versus 90% debate. The primary argument for installing an 80% furnace is the lower cost of the equipment. However, a colleague of mine suggested that there is really not much difference in the installed cost because of the difference in the cost of the vent pipe. Standard B-vent is far more expensive than the PVC that is used in 90% furnaces. If the furnace is in a crawl space or basement, the 80% furnace must be vented through a chase or closet while the 90% PVC vent can usually be run sideways out through the wall. This also saves on labor. His argument is that the extra cost associated with installing a Category I furnace vent eats up the difference in cost between an 80% furnace and a 90% furnace.
For a changeout, the math will favor an 80% furnace because there is already an existing vent. However, check to see if the vent is adequate. You may need to make improvements to the existing vent for the furnace to operate safely. Often, a fan assisted replacement furnace will use a smaller vent than the older natural draft furnace it is replacing. It is possible for the old vent system to be too big, causing condensation inside the vent which leads to rapid vent corrosion. Many older natural draft furnaces are vented into masonry chimneys, a practice that is usually discouraged when applying fan assisted furnaces. Several manufacturers require a metal flue liner for the masonry chimney, eating up most of the cost savings. Another common practice in older furnaces is the use of single wall vent connector. You really don’t want to connect a new fan assisted furnace in a cold location to a single wall vent connector, especially not an old one. I have seen single wall vent connectors rust through and fall on the ground in less than a year after installation of a new fan assisted Category I furnace. Again, replacing the old vent connector eats up some of your savings.
It is true that there is still quite a cost difference between an 80% single stage furnace with a PSC blower and standard controls versus a 90% two stage furnace with an ECM motor and communicating controls, but the cost difference has more to do with the improved blower and controls. For a true cost comparison, both furnaces should have similar blowers, staging, and control systems.