A variable speed furnace refers primarily to the blower motors performance. Most furnace are single stage with a “multi-speed” blower motor. Which means you can set one speed for heating and another for cooling.
However, on the next step is the “Variable Speed Furnace”. This furnace features a high efficient motor that actually converts your homes AC voltage to DC (like in your car), in turn greatly reducing your electrical consumption for heating and cooling. This blower motor takes the two stage furnace to the next level. This unique motor ramps up and down reducing drafts during the heating season and provides greater de-humidification during the cooling season.
This motor also allows for a low cost, low speed continuous fan to be used. This makes creating and maintaining the proper indoor air quality (humidity, filtration, circulation, comfort) in your home not only practical and comfortable but affordable too.