The definition of a heat pump

Heat pumps are machines that can take heat from one place and then deliver that heat to another place at a higher temperature. Your home fridge is similar to a heat pump. The difference is that the refrigerator doesn't compress the refrigerant gas to raise the temperature, it just draws the heat from the contents of your refrigerator and dispatches that heat through the heat exchanger (the grill) at the rear of the fridge.

When Choosing An Air Source Heat Pump, consider the following points: 

Choose a heat pump that has defrost control. Basically this reverses the heat flow to defrost the external coil which will reduce the supplementary electrical energy use for heating by the heat pump.Make sure that the outdoor unit is protected from high winds as this will affect the efficiency of the heat absorption process. The right fencing and planting can accomplish this. There are in development 'Cold Climate Heat Pumps' that have a two speed two-cylinder compressor, and a backup booster compressor.

If you are likely to use the heat pump more in the heating mode instead of the cooling mode then installing the indoor unit in a low wall posistion is better as this will increase its heating efficiency. This is because warm air rises. Nevertheless, if you are using the heat pump more for cooling, then install the unit high on the wall as this position will increase its cooling efficiency.There are slim-lined indoor heat pump units available. They can be recessed into the wall to make them less intrusive in the room.