i am having a house built in Kentucky next year and my contractor says he always uses these..Well i live out here in california right now and do not know how these operate..On my central heat&air i have a furnace in the garage and a A/C unit out side on the dide of my house..How do these heat pumps differ?? thanks

That AC unit on the outside of your house is most likely a heat pump. Typically round in shape with fins like a radiator with a big fan inside.
 

There are two styles.  The first looks like a big AC unit.  In the summer it blows cold air inside and hot air outside.  In the winter is reverses which side blows hot and cold.  It will loose efficiency when it gets below freezing and won't be able to heat the house.  So it has a backup plan.  Either it will have a gas furnace or an electric backup.  The electric backup is more or less a really big hair dryer element.  The heat pump costs less to run than gas when its above 40* or so.  I have one with propane backup on my house.


 

From a physics point of view it's like this.... Heat and temperature are not the same thing. Heat is the kinetic energy of atoms as they vibrate and bounce-off each other. Temperature is the frequency at which the atoms hit each other. Heat energy goes from areas of high temperature to ares of lower temperature. If you compress a gas you will raise the temperature but you will not add heat energy (the atoms are still moving at the same speed but impact more often due to the closer proximity). Thus, a heat pump works by compressing a gas (i.e. the "refigerant") in the hotter area. The temperature of the gas increases dramatically and heat energy dissipated to the hot (but not as hot as the gas) area. Once the gas cools it is moved to the colder are and allowed to expand. This results in the gas getting colder (colder than the cool area) which causes it to soak-up heat from the cold area. The refrigerant is pumped to the hot area and the loop continues.

Imagine a spong soaking-up the heat then the heat being squeezed-out someplace else.

Heat pumps vary in efficiency. Newer models are much better than the ones made a few years back. I would get recommendations from someone who has a newer model. My buddy has one he got two years ago and it seems to work quite well compared to the one I had some years back. I work with the "ground water" style one's but its on our ship and the river water stays above freezing. The four units I work with seem to do a good job at keeping us warm. My buddies new heat pump serves him well. He has propane back-up heat. The ones with electric heating coils can get expensive to run if you are in a very cold area. The ground water units cost less to run but more to install as you have to bury all that piping.

I would do some searching and try to find a good site that could give you more technical data on actual operating costs comparing different types of heating systems. The heat pump concept is sound, if the out door temps don't get too low. It can get pretty cold here in Kentucky so you need to give careful consideration to the cost and efficiency of your B/U heating. .

I will admit I have no experience with newer units but heat pumps are/were dry air. Static electricity and really low winter humidity are a problem.

If I was building a new house, I would get a new high efficiency gas furnace if at all possible.

I would be wary of propane due to the cost.

I forgot to mention efficiency.... thank you for reminding me. Back when I was taking thermodynamics at Speed most air conditioners required 1 BTU (equivalent) of electricity to move 3 BTU of heat from inside your house to the outdoors. I assume it's like 4 or 5 BTU now. Thus, since you get 3 BTUs of heat or cold for every 1 BTU you burn a heat pump theoretically should save you on utility bills.

You can run a heat pump down to 25 degrees if you want. I have a hybrid system heat pump and gas furnace. Basically depending on how much you want to save and what not. I have it set for 30 degrees and then my gas furnace will kick on after. I installed one at my mom's house and saved her 100 bucks a month in heating cost. Hybrid systems are the way to go.

Geothermal is good stuff but it also depends on the area you live in, in order to have one. Though I have been out of the geothermal game for about 7 years now.