OK, so if an air conditioner moves heat outside, then in reverse, a heat pump moves heat inside the conditioned space. The coils (condenser and evaporator) reverse roles and hot gas is redirected indoors (via the reversing valve) during the winter and our cold house air blows across the coil and voila!! it is now warm.
Heat pumps are great but they have their limitations. If temperatures drop below the mid 30 degree range they start to loose their power. They need to find heat outside to transfer to the refrigerant and there just is not enough at the lower temperatures. That is why heat pumps work great in Phoenix but not so hot in Omaha. Sometime there are heat strips or small heating coils that can provide supplemental heat but by and large gas is the best option in real cold weather climates.
Heat pumps must also have an additional part known as a defrost board. Over time the outdoor coil will get cold and any moisture will adhere to the coil and freeze. The defrost board checks the coil temperature from time-to-time to see if it must redirect hot gas to the outdoor coil to melt that ice. Many problems with heat pumps during the winter revolve around the defrost board and its operation (or lack thereof). For more tips and help troubleshooting heat pumps and air conditioners or if you want to DIY your next project visit me at http://www.thermal-medics.com/diy-ers/ for more.
