The article explains the process in a great deal of detail. Imagine that you took an air conditioner and flipped it around so that the hot coils were on the inside and the cold coils were on the outside. Then you would have a heater. It turns out that this heater works extremely well. When the valve is switched one way the heat pump acts like an air conditioner, and when it is switched the other way it reverses the flow of Freon and acts like a heater.
Heat pumps can be extremely efficient in their use of energy.
One problem with most heat pumps is that the coils in the outside air collect ice. The heat pump has to melt this ice periodically, so it switches itself back to air conditioner mode to heat up the coils. To avoid pumping cold air into the house in air conditioner mode, the heat pump also lights up burners or electric strip heaters to heat the cold air that the air conditioner is pumping out. Once the ice is melted the heat pump switches back to heating mode and it turns off the burners. Inc. All rights reserved.