From Japan to China and the US to Scandinavia, heating with air conditioning has long been standard practice. Now air conditioning systems and heat pumps for heating are becoming increasingly popular worldwide. This extra step makes them less efficient than air conditioners that warm the air in a room directly. Because air conditioners heat using warm air, a room warms up within minutes, reducing the need for continuous operation. Compact air conditioners do not require an outdoor unit and need only two holes in the outer wall for air exchange.