Think of a hot summer day. A window air conditioner must remove heat from the room and reject it into the already hot summer air. While the temperature of the refrigerant far exceeds the ambient outdoor temperature it has reduced capacity to release heat as the temperature increases. If the air conditioner could release the heat it has removed from the room into the cool earth below the home it could get rid of the heat much faster than trying to reject it into the hot summer air.

Now think of a cold winter day. The colder it gets outside a home, the less efficient it is to remove heat from the cold ambient air and deliver it to the warm home.