High efficiency gas furnaces, sometimes referred to as condensing furnaces, condense the water vapor from the flue gases (Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a furnace)  and return the latent heat of vaporization (heat required to convert liquid into a vapor, without change of temperature)  to the circulating air in the form of actual heat that goes around heating your house. There is a range of options while selecting a high efficiency gas that are usually measured for cost effectiveness and performance. The principal option is the selection of the furnace AFUE in a range between 90% and 98%.

There are three common product categories, differentiated by efficiency and circulating air blower control (AFUE ranges approximate):

1. Single-stage furnace with blower motor operating at a single speed, generally 90% to 92% AFUE

2. Two-stage furnace with blower motor operating at multiple speeds, can range from 92% to 96% AFUE

3. Step modulating furnace with variable-speed (electronically commutated permanent magnet (ECM) or multitap permanent split capacitance) blower motor, can range from 96% to 98% AFUE.

The higher efficiency products command a higher market price as they offer improved comfort features as well as energy cost savings. High efficiency furnaces with step modulating and 5 multispeed blowers modulate the furnace capacity and circulating air volume to match the heating load of the home under varying weather conditions.