How It Works

A heat pump is designed to move heat from one place to another. The outdoor coil of the heat pump absorbs heat from the outside air, even at relatively low temperatures, and transfers it to your home. In hot weather the heat pump operates in reverse to provide cooling. It transfers heat from within your home to the outdoors.

The outdoor unit of a heat pump contains coils filled with refrigerant which absorb heat from the outside air. Even though the outdoor air temperature may be as low as 0° F, the refrigerant is much colder and is able to absorb heat. As it begins to absorb heat, the refrigerant evaporates, absorbing even greater quantities of heat. The heated refrigerant vapor passes through coils and then on to the compressor where it is pressurized. This process increases the temperature of the refrigerant so that it is now hotter than the inside air. The heated refrigerant travels to the indoor coil. As a fan blows air across the coils, the refrigerant condenses and releases heat which is then circulated throughout the house.

The heat pump also cools your house using the same process, but in reverse. Simply switching the heat pump from a heating mode to a cooling mode reverses the valves that control the refrigerant cycle. So instead of extracting heat from the outdoors, the heat pump will remove heat and moisture from the indoors.

When purchasing a new heat pump, be sure to check the efficiency rating of the proposed unit. A higher efficiency rating will result in lower operating costs. Heat pump efficiency is designated by the terms "HSPF" (Heating Season Performance Factor) for heating and "SEER" (Seasonal Energy Efficiency Ratio) for cooling. New units have HSPFs ranging from 6.8 to 9.0 and SEERs ranging from 13.0 to over 17.0.

For split systems with an outdoor unit and an indoor coil, the efficiency varies with the match between the indoor cooling coil and outdoor condensing unit. The manufacturer should be consulted to determine the combined efficiency. The American Refrigeration Institute (ARI) publishes an annual directory listing various combinations of outdoor units and indoor coils with their SEER rating. Most major manufacturers' product lines are included in this directory.

New high-efficiency heat pumps incorporate some or all of the following improvements:

• Variable speed blowers, compressors, and motors - these features increase efficiency by matching the speed (and therefore the power consumption) to the operating conditions. Heat pumps utilizing multi-speed components will typically start in the first stage or low speed. If comfort levels or control settings cannot be satisfied with the first stage, the second stage or higher speed will activate. Some heat pump systems have more than two stages or speeds of operation.
• Scroll compressors – this improved compressor design increases efficiency while also reducing operating noise.
• Larger coil surface areas - increased surface area provides maximum heat transfer efficiency.
• Time delays - these controls vary the on and off cycles of compressors, motors, and supplemental heat packages to capture the maximum amount of heating or cooling.

Besides a unit's HSPF and SEER, there are additional energy saving features to look for when selecting a heat pump for your home.

• Dual fuel back-up - these heat pump systems are supplemented by a fossil fuel furnace instead of the traditional electric resistance coils. When outdoor temperatures are moderate, the home’s heating requirements can be satisfied by the heat pump alone. When outdoor temperatures are below the economic balance point, the heat pump is switched off and the furnace supplies the heat at close to its peak efficiency.
• Programmable setback thermostats - specially designed heat pump thermostats with an adaptive-recovery or "ramping" feature allow the thermostat to be programmed for one or more "setback" periods per day without relying on the more expensive supplemental heating to bring the temperature back up. Their microprocessor unit senses the temperature difference to be overcome when bringing the space temperature back up, and brings the temperature up gradually over a longer period of time. This allows the heat pump alone to provide the temperature increase and minimizes the use of the more costly electric resistance auxiliary heat.

The following are some guidelines that should be followed to ensure efficient, comfortable operation of heat pumps.

• In the heating mode, don’t set the temperature back at night or when you are at work unless a programmable heat pump thermostat is used. Since heat pumps operate differently than fossil fuel heating systems, setbacks using a standard thermostat can actually increase energy consumption. This is due to the use of supplemental heaters to bring the house temperature back to the desired setpoint. Use of supplemental heaters will reduce the efficiency of the heat pump system and result in higher energy costs.
• Keep the temperature setpoint consistent. A standard heat pump thermostat has two controls, one for the heat pump and one for the supplemental heat. If the temperature difference between the room and thermostat setpoint is more than 2° or 3° F, the supplemental heat will be activated. Manually adjusting the thermostat will result in greater reliance on the supplemental heaters and will reduce the efficiency of the heat pump system and increase operating costs.
• Replace filters regularly. Vacuum dirt and dust from the indoor coil once a year to prevent restricted airflow. Adequate air flow through a heat pump system is critical to ensure efficient and comfortable operation.
• Keep supply vents open and free from obstruction. Closing off supply vents will restrict air flow and reduce system efficiency, and potentially shorten the life of the compressor.