How does a heat pump work?
Q: How does a heat pump work? How does it work as a heater in the winter and an air conditioner in the summer, and what does it mean that it pumps heat?
WJ, Boston
A: To understand a heat pump -- first developed by Lord Kelvin (1824-1907) -- it's good to first remember how a refrigerator works.
A refrigerator doesn't actually ''make coldness," and in fact a working refrigerator, overall, acts as a heater, even if the door is open.
In a refrigerator you take some liquid that evaporates easily (usually a gas compressed into liquid form) and let it evaporate next to the thing that you want to have cooled. Evaporating means that molecules of the liquid take heat from the thing to be cooled in order to boil away and escape. So far this is just like the cooling from sweating, or the cool feeling of alcohol evaporating from your skin.
A refrigerator catches the gas and compresses it back into a liquid. This compression produces heat which is carried away by the coils on the back of the refrigerator, and that is why they are warm. The liquid is carried to the place to be cooled and the cycle repeats.
Now imagine a refrigerator with the door off and the cold side facing indoors, and the hot side outdoors. This is a simple heat pump which will cool your house in the summer.
If you set it up to also run in reverse, and cool the outside by evaporating liquid on that side and releasing the heat on the inside, you have a device which will also be a heater. This is the heat pump running the other way.
In winter, because a heat pump basically takes some heat out of the cold air outside (refrigerating the great outdoors), you can get more heating than you would if you just took the same amount of electricity to run a radiant heater, so these things are good to help minimize energy use!
Dr. Knowledge answers your questions about science each week. E-mail questions to drknowledge@globe.com, send a fax to 617-929-9263, or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. Include your initials and hometown. 
