Heaters and Coolers
Both heating and air conditioning work on the principle that heat always moves from a warm object to a cooler one, just as water flows from a higher to a lower level. Furnaces and heaters put heat into the air; air conditioners remove heat.
All heating and cooling units burn fuel. Air conditioners use electricity. Most heating systems use gas or fuel oil; other systems use electricity. The heat pump - an electrically powered climate control unit - both heats and cools air. In summer it extracts heat from the air inside. In winter it pulls heat from the air outside and uses this heat to warm the air inside.