Air Source Heat Pumps

A heat pump is one of the most energy-efficient heating and cooling systems available today. Unlike other types of heating systems that convert fuel or electricity directly to heat, a heat pump is designed to move heat from one place to another. In winter, it can absorb heat from outdoor air and transfer it indoors to heat your home. In summer, it works just like an air conditioner, absorbing heat from inside the home and rejecting it outdoors.

The most common type of heat pump is called "air source" because it uses outdoor air as the source of heat in winter. Even air at 30° F. contains some heat. By using a cycle in which a refrigerant is alternately evaporated and condensed, the heat pump is able to absorb heat from relatively cold outdoor air and use it to heat the home.

How It Works

The heat pump takes advantage of several basic principles - when a liquid evaporates it absorbs heat, and when a vapor condenses it gives off heat. A heat pump system contains two heat exchanger coils, one in the outdoor unit and one indoors. In winter, even though the outdoor air is cold, the refrigerant in the outdoor coil is much colder, allowing it to absorb some heat. As it begins to absorb some heat, the refrigerant evaporates, absorbing an even greater amount of heat. The refrigerant then circulates to the compressor where it is pressurized, increasing its temperature. From the compressor, it goes to the indoor coil. The fan or blower in the indoor unit circulates air from inside the home across the hot coil, where it absorbs some heat. As the heat is absorbed it causes the refrigerant to condense, giving off more heat that is then circulated throughout the home.

In summer, the heat pump "reverses" the cycle to absorb heat from inside the home and reject it outdoors. The indoor and outdoor coils simply trade functions - either coil is capable of either absorbing or giving off heat, depending on its temperature relative to the air passing over it. When running in cooling mode, a heat pump works exactly like an air conditioner. In fact, heat pumps are sometimes referred to as "reverse-cycle air conditioners".

Heat Pump Efficiency and Efficiency Ratings

By using this cycle to move heat from one environment to another, a heat pump is able to deliver much more useable heat for each unit of electricity than other systems that convert electricity directly into heat. In fact, heat pumps are two to three times as efficient as systems that convert electricity into heat. As a result, their operating costs are much lower. And unlike other heating systems whose maximum efficiency is limited to a maximum of 100%, a heat pump's efficiency is limited only by how efficiently it can absorb heat from one environment and give it off to another.

A heat pump's heating efficiency is rated with a Heating Season Performance Factor or HSPF. New heat pumps have HSPF ratings ranging from 7.6 to over 10. For cooling, heat pumps are rated with a Seasonal Energy Efficiency Ratio or SEER, just like central air conditioners. New heat pumps have SEER ratings from 13 to as high as 20. When purchasing a new heat pump, refer to the EnergyGuide label for the HSPF and SEER ratings.

New high-efficiency heat pumps incorporate some or all of the following features to maximize efficiency:

  • Variable speed blowers, compressors, and motors that increase efficiency by matching the speed (and therefore the power consumption) to the operating conditions.
  • More efficient compressors that use less energy to operate (and also run more quietly).
  • Larger coil surface areas to achieve maximum heat transfer efficiency.
  • Time delays that vary the on and off cycles of compressors, motors, and supplemental heat packages to capture the maximum amount of heating or cooling.

Other Energy Saving Options

In addition to the HSPF and SEER ratings, there are other energy saving options to consider when purchasing a heat pump.

  • Dual fuel systems combine a heat pump with a fossil-fuel furnace. The furnace replaces the electric heating elements that supplement the heat pump in very cold weather. With this type of system, the heat pump provides the heating in relatively mild weather, when it is most efficient, and the furnace takes over in very cold weather, when it is most efficient.
  • Programmable thermostats designed for heat pumps allow multiple "setbacks" while ensuring that the heat pump alone is used to bring the temperature back to normal settings. This eliminates reliance on the more expensive electric heating elements.

Operation and Maintenance Recommendations

  • Avoid manual setbacks - when you set the temperature back you obviously save energy, but when you turn it back up the more expensive electric heating elements can kick in, reducing or even eliminating any savings. If you want to save by setting the temperature back, use a programmable thermostat designed for your heat pump (most new heat pumps come with their own programmable thermostat).
  • Other than programmed setbacks, try to keep the temperature settings consistent. Every time someone adjusts the temperature setting, there is a chance the back-up heating elements will turn on, increasing operating costs.
  • Check the filter monthly and replace it when it shows signs of dirt or dust. Dirty filters can restrict airflow, reducing the system's efficiency.
  • Don't block supply registers or returns. Proper airflow is critical to system performance.
  • Have the system checked annually by a qualified service person. Just like with a motor vehicle, regular maintenance can help maintain efficient operation and extend equipment life. Many heating contractors offer annual service contracts so you don't need to remember to call to schedule annual maintenance.

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