The air handlers of forced air gas furnaces provide the energy to move air through the furnace (and central air conditioner), and distribute it to the rooms of a house. The amount of electricity these require varies enormously, even within size categories, but averages about 3.4% of the site energy use of the furnace, or 10% of the source energy. Improving the motor and fan systems of residential furnaces and heat pumps promises substantial, cost-effective efficiency gains.