Most new 80% gas furnaces are vented in a properly sized B-Vent or masonry chimney that terminates through the roof. Make sure you consult with a licensed heating professional to make sure your existing chimney is in good condition and appropriately sized for the load. Condensing furnaces, or furnaces with rated AFUE efficiencies of 90%+, can NOT be vented into a conventional chimney because flue-gas temperatures are too low to properly vent. Condensing furnaces are typically vented with plastic venting like PVC either out the sidewall or through the roof. Condensation is produced as a by-product of combustion in condensing furnaces, and will need to be properly drained.