Return to Topics Types of Main Central Heating Systems Warm-air furnaces are the most frequently used main central heating system, followed by steam and hot water systems and heat pumps. In 1978, heat pumps, used in only 2 percent of all housing units, were relatively rare. By 1997, the use of heat pumps had quintupled to 10 percent of housing units. As the name implies, secondary heating equipment is that which supplements main heating systems. Return to Topics Secondary Heating Fuels Secondary heating fuels are used as supplements to a household's main heating fuel.
Some households use more than one.
In 1978, wood was the most commonly-used secondary heating fuel. By 1997, electricity had caught up to it. By 1997, the use of air-conditioning equipment had increased to 73 percent. As would be expected with its warm climate, the South consistently had the highest percentage of households using central air-conditioners, followed by the Midwest, the West, and the Northeast. The 19 years covered by the RECS have seen continuous and large increases in appliance use in U.S. households.