What's the difference between 80 and 90 percent furnaces?

Briefly, an 80-percent unit is a fan-assist draft furnace that does not bring in outside combustion air. It normally vents through the roof using a lined existing chimney or Type B-Vent.

B-Vent is a special double-lined pipe that has one-half inch of airspace between the two pipes to help keep the outside pipe cool. The pipe still has to have a one-inch clearance on all sides from any combustible walls. B-Vent would not be used with a 90-plus unit.

A 90-percent efficient unit is a direct-vent appliance which brings combustion air into the appliance and vents the products of combustion to the outside. Both the intake and exhaust can be vented through the roof or through the side of the building with PVC piping following the manufacturer's installation instructions.

As with all furnace installations, be sure to get permits and follow the building and mechanical codes in force in your area. Updating your furnance and/or sizing the ductwork can make your home more comfortable during cold months and eliminate the need for portable heaters (paid advertisement link).

Copyright © 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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