A single stage furnace comes on at 100% capacity when there is a call for heat, therefore, you may notice swings in temperature in the home because of it. Two stage furnaces have been on the market for over fifteen years now and are built more for comfort. The two stage gas valve allows the furnace to come on at a lower capacity - when there is a call for heat - thereby allowing the furnace to run a little longer so as to maintain more of an even temperature in the home. The furnace only kicks in to the second stage - or full capacity - on the really cold days. Two stage furnaces operate a great majority of the time on first stage - which as an added benefit -allows them to be quieter.