Examples of how heat flows from hot to cold:

• In a refrigerator, the cool reservoir is the inside of the refrigerator, and the warm reservoir is the room itself. From this, one can see that leaving a refrigerator door open will not cool off the room that it is in.

• In an air conditioner, the cool reservoir is the inside of a house, and the warm reservoir is the outside. This is used to cool a house in the summer.

• In a heat pump, the cool reservoir is the outside of a house, and the warm reservoir is the inside. This can be used to warm a house in the winter. The heat pump is thus just the reverse of an air conditioner, and indeed some heat pumps have a switch which allows them to function as an air conditioner in the summer.