Heat pumps and air conditioners are two different devices for cooling a home. The two machines work in a similar fashion, although a heat pump can work in reverse. This means that a heat pump can heat a house as well as cool it. The heat pump accomplishes this by acting like an air conditioner switched into reverse.
If you ask the average person how an air conditioning unit cools their home, they are likely to tell you that it works by pushing cold air into the house. This is not the complete story. What is actually happening is that your AC unit is pumping hot air out of the house. This means that another common belief is also wrong, the belief that a heat pump and an air conditioner are completely different machines. They are actually quite similar.
How an Air Conditioner Works
An air conditioner works by transferring heat from the house to the unit, where it is dissipated into the air. To do this, Freon is pumped into a compressor. Inside the compressor, the molecules of the Freon are pushed closer together. This tightening of the molecules causes the temperature of the gas to rise along with its pressure. The Freon then gets pushed into a condenser. Here, the gas is converted into a high pressure liquid and it's cooled considerably. This cool liquid enters an evaporator inside the house, where it needs heat to convert itself back into a gas to begin the cycle anew. It gets this heat from the house. When it does, the house cools.
Reversing the Process
So an air conditioner works by taking heat from the inside of a home and transferring it to Freon, which then moves the heat out of the house. So, what's to stop us from reversing the process? Why not take heat from the outside and move it into the house? That is exactly what a heat pump does, or rather can do. See, a heat pump can function just like an air conditioner in the warm weather. When the weather gets cold, the heat pump has a valve to reverse the process. When this happens, the Freon liquid is converted back into a gas outside the house. As it completes the conversion, it pulls heat from the outside and brings it into the house.
Heat Pump vs. AC
It may be that your climate is warm enough that you never need a heater at all. In this case, obviously all you need is an air conditioner. Most climates, however, get cold enough that you want at least a little bump in the heat. These are the climates that heat pumps really shine in. In climates that are less than moderate, you may need to rely on a furnace to provide enough heat for the home.
Heat Pump vs. Furnace
Heat pumps cannot heat a house sufficiently in extremely cold climates. In these cases, a furnace is the clear way to go. In climates that are moderate enough for a heat pump to handle, the pump provides more even distribution of heat at a much greater energy efficiency than does a furnace. How you choose to heat your house has a big impact on your utility bill and comfort level. A qualified heating and cooling company will be able to advice you on the pros and cons of each option in your climate.