Answer
The device is known as a heat pump. In the winter, it is moving heat from a colder place (the outdoors) to a warmer place (the indoors). In summer, it does the reverse.
Work Step by Step
This is discussed on pages 328-329.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.