Heat Pump Systems

For climates with moderate heating and cooling needs, heat pumps offer an energy-efficient alternative to furnaces and air conditioners. Like your refrigerator, heat pumps use electricity to move heat from a cool space into a warm, making the cool space cooler and the warm space warmer. During the heating season, heat pumps move heat from the cool outdoors into your warm house; during the cooling season, heat pumps move heat from your cool house into the warm outdoors. Because they move heat rather than generate heat, heat pumps can provide up to 4 times the amount of energy they consume.

The most common type of heat pump is the air-source heat pump, which transfers heat between your house and the outside air. If you heat with electricity, a heat pump can trim the amount of electricity you use for heating by as much as 30%–40%. High-efficiency heat pumps also dehumidify better than standard central air conditioners, resulting in less energy usage and more cooling comfort in summer months. However, the efficiency of most air-source heat pumps as a heat source drops dramatically at low temperatures, generally making them unsuitable for cold climates, although there are systems that can overcome that problem.

For homes without ducts, air-source heat pumps are also available in a ductless version called a mini-split heat pump. In addition, a special type of air-source heat pump called a "reverse cycle chiller" generates hot and cold water rather than air, allowing it to be used with radiant floor heating systems in heating mode.

Higher efficiencies are achieved with geothermal (ground-source or water-source) heat pumps, which transfer heat between your house and the ground or a nearby water source. Although they cost more to install, geothermal heat pumps have low operating costs because they take advantage of relatively constant ground or water temperatures. However, the installation depends on the size of your lot, the subsoil and landscape. Ground-source or water-source heat pumps can be used in more extreme climatic conditions than air-source heat pumps, and customer satisfaction with the systems is very high.

A new type of heat pump for residential systems is the absorption heat pump, also called a gas-fired heat pump. Absorption heat pumps use heat as their energy source, and can be driven with a wide variety of heat sources.

U.S. Department of Energy - Energy Efficiency and Renewable Energy
A Consumer's Guide to Energy Efficiency and Renewable Energy
Content Last Updated: September 12, 2005

Central Air Conditioning Definitions

Air Conditioner: Assembly of equipment for the simultaneous control of air temperature, relative humidity, purity, and motion.

Air Cooled: Uses a fan to discharge heat from the condenser coil to the outdoors.

Air-Source: Air is being used as the heat source or heat sink for a heat pump.

BTU: British Thermal Unit. The amount of energy needed to change the temperature of one pound of water by one degree Fahrenheit. In practical terms, it represents a unit of measure of heat extracted from your home for cooling.

Central Air Conditioner System: System in which air is treated at a central location and carried to and from the rooms by one or more fans and a system of ducts.

Compressor: The pump that moves the refrigerant from the indoor evaporator to the outdoor condenser and back to the evaporator again. The compressor is often called "the heart of the system" because it circulates the refrigerant through the loop.

Condenser Coil: A series or network of tubes filled with refrigerant, normally located outside the home, that removes heat from the hot, gaseous refrigerant so that the refrigerant becomes liquid again.

Cooling Capacity: A measure of the ability of a unit to remove heat from an enclosed space.

COP: Coefficient of Performance of a heat pump means the ratio of the rate of useful heat output delivered by the complete heat pump unit (exclusive of supplementary heating) to the corresponding rate of energy input, in consistent units and under operating conditions.

EER: Energy Efficiency Ratio means the ratio of the cooling capacity of the air conditioner in British Thermal Units per hour, to the total electrical input in watts under ARI-specified test conditions.

Evaporator Coil: A series or network of tubes filled with refrigerant located inside the home that take heat and moisture out of indoor air as liquid refrigerant evaporates.

Free Delivery: There are no ducts and the unit may be installed in the field without ducts if needed.

Ground-Source: The ground or soil below the frost line is being used as the heat source or heat sink for a heat pump.

Ground Water-Source: Water from an underground well is being used as the heat source or heat sink for a heat pump.

Heat Pump: An air conditioner capable of heating by refrigeration. It may or may not include a capability for cooling. Outside air or water is used as a heat source or heat sink, depending upon whether the system is heating or cooling.

Heating Capacity: A measure of the ability of a unit to add heat to an enclosed space.

HSPF: Heating Seasonal Performance Factor means the total heating output of a heat pump in British Thermal Units during its normal usage period for heating divided by the total electrical energy input in watt-hours during the same period.

SEER: Seasonal Energy Efficiency Ratio means the total cooling output of a central air conditioner in British thermal units during its normal usage period for cooling divided by the total electrical energy input in watt-hours during the same period as determined using the ARI-specified test procedure. This rating is only for units with cooling capacity less than 65,000 Btu/hr. The higher the SEER, the more efficient the unit. The more efficient the unit, the lower the operating cost.

Single Package: A central air conditioner which combines both condenser and air handling capabilities in a single packaged unit.

Split System: A central air conditioner consisting of two or more major components. The system usually consists of a compressor-containing unit and condenser, installed outside the building and a non-compressor -containing air handling unit installed within the building. This is the most common type of system installed in a home. Ton: The unit of measurement for air conditioning system capacity. One ton of air conditioning removes 12,000 Btu's of heat energy per hour from a home. Central air conditioners are sized in tons. Residential units usually range from 1 to 5 tons.

Water-Source: Water is being used as the heat source or heat sink for a heat pump. Sources of underground water are wells and sources of surface water are lakes, large ponds, and rivers.

Year-Round: Air Conditioner which uses gas or oil for heating.

Central Gas Furnace Terms & Definitions

Annual Fuel Utilization Efficiency (AFUE): The Annual Fuel Utilization Efficiency (AFUE) rating, indicates how well a furnace converts energy into usable heat. The rating is expressed as a percentage of the annual output of heat (output rating in Btus -- British thermal units, a measure of energy) to the annual energy input to the furnace (input rating in Btus).

Ratings can be categorized in the following way:

• Low Efficiency: AFUEs below 71 percent.
• Mid-Efficiency: AFUEs between 71 and 83 percent.
• High Efficiency: AFUEs of 90 percent and above.

Ratings between 84 and 89 percent are not common. Acidic condensate, harmful to the furnace, forms at these percentages. Public Law No. 100-12, passed in1987, requires that all gas furnaces manufactured after January 1, 1992, have aminimum AFUE of 78 percent.

Atmospheric Vent Combustion: If a chimney is available, furnaces with this system are the least expensive to install. Atmospheric vent furnaces have AFUEs of 60-65 percent when equipped with standing pilots, and AFUEs of 63-70 percent when equipped with electronic ignition systems. With special vent dampers, atmospheric vent units can achieve AFUEs of 78-80 percent.

Condensing (or Recuperative Units): These units are super efficient with some designs reaching AFUEs of up to 97 percent. Unlike conventional forced air furnaces, condensing units capture most of the water vapor and heat contained in hot flue gases that would normally escape up the chimney. The escaping gases then pass through a second heat exchanger and condensate is expelled. The heat exchangers are made of corrosion resistant stainless steel, and many have lifetime warranties. Exhaust is cooler than that of conventional furnaces and can be vented with PVC piping.

Downflow or Counterflow Furnace: These units have a blower at the top to draw air into the furnace. Heated air is blown out at the bottom. This type of furnace is used to supply floor duct systems. Electronic Ignition: An electronic ignition eliminates the need for an energy-wasting standing pilot. Fuel is used only when needed. The pilot is ignited with an electric spark.

Heat Exchanger: The heat exchanger is a metal chamber in the furnace that houses a gas burner. The flame produced by the burner heats the chamber. When the outside of the chamber becomes hot, the air surrounding it is warmed and used to heat the house. Improved designs enhance efficiency and provide quieter operation.

Horizontal Flow Furnace: Air travels horizontally from one side of the heater, across the heat exchanger, and hot air is blown out the other side. This type of furnace is installed in areas with limited head room, such as attics or crawl spaces. They can also be installed below floors or suspended below ceilings.

Nonweatherproof: These units are designed to be installed indoors.

Power Combustion: In this system, combustion is produced by a blower. The blower pushes the combustion gases through the vent, and regulates the amount of combustion air. Power combustion furnaces do not require a draft hood. This reduces off-cycle losses and improves efficiency. Many power combustion furnaces operate at 78-80 percent AFUE. When equipped with an additional heat exchanger, they can operate at AFUEs of 90-96 percent.

Pulse Combustion: (These models are listed under condensing furnaces.) Pulse combustion is produced by self-perpetuated "pulses." This unique system mixes air and fuel in a sealed combustion chamber. A spark ignites the mixture, and the resulting increase in pressure closes the gas/air inlet valve. The combustion products are forced through an exhaust pipe and the pressure in the combustion chamber drops, re-opening the inlet valve. The next combustion cycle is ignited by the heat remaining from the previous cycle. This process repeats itself about 60 times per second. Furnaces with this combustion system have AFUEs from 91-97 percent. Exhaust gases, at 100-200 degrees Fahrenheit, are cool enough to vent through PVC piping.

Sealed Combustion: Sealed combustion systems draw in all the air used for combustion from the outside, and exhaust gases are direct vented to the outside. Since cold outside air is not mixed with the warm indoor air during combustion, efficiency is enhanced. Furnaces with this type of combustion system have an AFUE range of 70-80 percent.

Upflow Furnace: These units have blowers at bottom that draw air into the furnace. Heated air is blown out at the top. These heaters can be installed in utility rooms, closets or basements.

Vent Damper: The vent damper is a "flapper" device installed in the flue. When the heat demand has been met, the damper closes, trapping residual heat for circulation in the home. When heat is needed, the damper opens before the burners are ignited to allow combustion fumes to escape. The damper remains open only as long as the burners are on. Burners cannot ignite if the damper is closed.

Weatherproof: These units are designed to be installed outdoors.

Definitions courtesy of The National Propane Gas Association.