I don't really get how heat pumps work in a cold climate. In New England where I live, most energy is spent heating the house in the winter.
During the winter the outside is ALWAYS colder than the house. So, normally the outside temperature will vary between -15F and 50F. The house on the other hand (at least my house) is kept at 68F to 72F constantly. So, the house is ALWAYS much warmer than outside during the winter.
So, if this is the case, how is a heat pump of any benefit?
A heat pump transfers energy. I expect you've noted that the outside part of your A/C unit is really really hot in the summer. That's the energy from inside your house being transferred to the outside.
In the winter, the direction is reversed. The energy in cold air is substantially less than that of warm air, but there is energy there. The heat pump removes the energy from the cold air, making the outside discharge even colder (just like inside, during the summer) and transfers that energy inside to warm things up.
Heat pumps are usually equipped with a resistance heater for those days when the cold air is too cold to have any energy transferred effectively. The colder parts of the world will have less effective heat pumps.
High Performance HVAC suggests that in colder areas, the energy source for a heat pump during the winter would not be the outside air, but rather a warmer source for heat exchange. A geothermal source (ground water loop) will contain far more energy during the winter as well as provide a good heat sink for summer operation.
Such systems will be more expensive to install, but will be less expensive (more effective) during the winter months, keeping the abode warm.
The above link also references electricity costs as a factor to determine suitability for heat pump selection, as well as other considerations.
I live in the Southeast and the heat pump aux heater never engages during the winter. It is a very efficient air conditioner in the summer, saving electricity cost over the equivalent non-heat pump design. Of course, the heat pump is a newer technology than the A/C unit it replaced, which may have provided for lower cost operation as well.
A heat pumps works by moving heat from one space to another space. When you want more heat in your house, it will take heat from outside and move it inside. When you want it cooler in your house, it will move heat from inside to outside.
Even when it is seems cold, there is still heat energy present. There is heat energy even in frozen water. So long as the temperature is above absolute zero (−459.67 °F) there is still thermal motion and thus heat.
Now heat pumps are not able to extract the heat in extreme cases and in fact they work best in moderate climates. Often in cold climates, you will need a supplemental heating source as the heat pump on it's own is not able to maintain the desired temperature.
More information, as well as the below excerpt can be found here: Heat Pumps – How Well Do They Work?
Heat pumps work very efficiently when the outdoor temperature is in the 50 F range. As the outdoor temperature drops, the heat loss of a home is greater and the heat pump needs to operate for longer periods of time to maintain a constant indoor temperature.
Around 37 F many heat pumps reach what is called the balance point. At or near this temperature the heat pump needs to run constantly to produce enough heat to maintain a comfortable indoor temperature.
As the outdoor temperature continues to drop, the heat pump needs help from traditional electric resistance heat coils. These coils resemble the glowing wires inside your toaster and consume vast amounts of electricity as they burn to keep you warm. Your thermostat will most probably have a light that comes on when this happens. It is usually labeled as emergency or auxiliary heat. If this light is on whenever your heat pump is working when the outdoor temperature is above 40 F, you should have a professional service your system.
