More Information About HVAC

Energy Efficiency Ratios - EER & SEER

EER (energy efficiency ratio)

EER is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (usually 95° F). A higher EER means the system is more efficient. The term EER is most commonly used when referring to window and unitary air conditioners and heat pumps, as well as water-source and geothermal heat pumps. The formula for calculating EER is:

          Btu/hr of cooling at 95°

EER = ________________

          watts used at 95°

For instance, if you have a window air conditioner that draws 1500 watts of electricity to produce 12,000 Btu per hour of cooling when the outdoor temperature is 95°, it would have an EER of 8.0 (12,000 divided by 1500). A unit drawing 1200 watts to produce the same amount of cooling would have an EER of 10 and would be more energy efficient.

Using this same example, you can see how efficiency can affect a system's operating economy. First, you'll need to determine the total amount of electricity—measured in kilowatt-hours—the unit will consume over a period of time. (A kilowatt-hour is defined as 1,000 watts used for one hour. This is the measure by which your monthly utility bills are calculated.) To do this, let's assume you operate your 8 EER window air conditioner—drawing 1500 watts at any given moment—for an average of 12 hours every day during the summer. At this rate, it will use 18,000 watt-hours, or 18 kilowatt-hours (kWh) each day, leading to a total consumption of 540 kWh over the course of a 30-day month. At a summer electric rate of 6.34¢ per kWh, it would cost you $34.24 to operate that window air conditioner each month. At the same time, a 1200-watt, 10 EER system, consuming 14.4 kilowatt-hours per day and 432 kWh per month, would cost you $27.39, a 20% savings over the less efficient model.

SEER (seasonal energy efficiency ratio)

SEER measures how efficiently a residential central cooling system (air conditioner or heat pump) will operate over an entire cooling season, as opposed to a single outdoor temperature. As with EER, a higher SEER reflects a more efficient cooling system. SEER is calculated based on the total amount of cooling (in Btu) the system will provide over the entire season divided by the total number of watt-hours it will consume:

            seasonal Btu of cooling

SEER = ___________________

            seasonal watt-hours used

By federal law, every central split cooling system manufactured or sold in the U.S. today must have a seasonal energy efficiency ratio of at least 10.0.

Efficiency Information for HVAC Systems

The following list provides details about what to look for in different types of HVAC equipment for energy efficiency. Efficiency Vermont does not provide rebates for all of the equipment types listed.

Boilers can be integrated into a range of systems, from radiant slab to hot water baseboard to ventilation fans blowing across hot water coils. Like furnaces, Annual Fuel Utilization Efficiencies (AFUEs) range from 80 to 87 percent for oil systems and from 80 to 98 percent for gas (natural and propane) systems. Larger boiler systems are often rated in thermal or combustion efficiency as well and have similar efficiency ranges to AFUE. To attain efficiencies greater than 90%, you must use condensing gas boilers. Additional efficiency options include "low-mass" (low water content) boilers, increased boiler insulation, control options to reset boiler water temperature based upon outdoor air temperatures, and variable speed control of the circulating pump operation and/or the combustion air or burner blower fan operation. In all cases, efficient motors improve overall system efficiencies.

Central Station Air Handling Units
Heating and cooling efficiencies are based upon either boiler or furnace and chilled water or direct expansion (DX) equipment efficiencies. Other efficiency measures include efficient motors and variable frequency drives for fans, demand-controlled ventilation using carbon dioxide sensors, dual-enthalpy integrated economizers, deck temperature reset controls, heat recovery units, such as enthalpy wheels or air-to-air heat exchangers, and optimum start-stop strategies.

Chilled Water
The primary energy user in a chilled water system is the chiller, and more specifically the compressor. Baseline systems are often reciprocating compressors—while scroll, screw, or centrifugal compressors offer higher-efficiency options, depending upon the size and application. You can also improve the efficiency of chilled water production by reducing condenser water temperature, raising chilled water temperature, varying chilled water flow and condenser fan speed, and installing water-side economizers on the cooling tower systems. Increased efficiencies can be obtained by using variable speed control on chilled water pumps and ventilation fans, and the use of fan motors.

Fan Coil
Heating and cooling efficiencies are based upon boiler, chilled water, and direct expansion equipment efficiencies. The efficiency of the unit itself can be improved by installing a permanent-split capacitor fan motor (for single-phase power applications) or efficient motors for higher horsepower and three-phase power applications. Occupancy sensors and/or programmable thermostats can be used to control fan operation.

Furnaces are incorporated into forced-air systems that can supply a mix of outdoor air and "return" air. These systems can also use condensing units and evaporator coils to provide cooling. Efficiencies are measured by the Annual Fuel Utilization Efficiency (AFUE), with baseline furnace efficiencies of oil and gas (natural and propane) units typically around 80%. With high-efficiency systems, this rises to 87% (oil) and 98% (gas). For packaged rooftops and split air-handling systems efficiencies generally don’t exceed 88%. Energy efficiency measures in furnace systems include variable speed control of ventilation fans (tied to temperature, pressure, or carbon dioxide sensor output) and fan motors.

Heat Pumps
Heat pumps move heat from one location (the 'source') at a lower temperature to another location (the 'sink' or 'heat sink') at a higher temperature using mechanical work or a high-temperature heat source. Heat pumps differ from air conditioners in that they provide both heating and cooling through the use of a reversing valve. Air source heat pumps have historically been the most widely used heat pump type because they are relatively easy and inexpensive to install. Recent advancements in air source heat pump technology has also made this an effective heating and cooling source in cold climates, although both capacity and performance decline as indoor and outdoor temperature are farther apart (colder ambient temps in winter and warmer in summer).

Ground source or geothermal heat pumps extract heat from the ground or groundwater, which in moderate climates is at a relatively constant temperature year-round below the frost line. This constant temperature source improves seasonal performance and provides consistent output; however, these pumps are more expensive to install because wells or trenches must be dug for the pipes that carry the heat exchange fluid.

Packaged Terminal AC
Packaged Terminal AC (PTAC) cooling efficiencies are based on direct expansion air conditioning, but are typically limited to a Seasonal Energy Efficiency Ratio (SEER) of 12. Heating efficiencies rely on electrical resistance, heat pump, boiler, or furnace efficiencies. Heat pump PTACs are more efficient than electric resistance PTACs, but boiler or furnace PTACs will be less expensive to operate. Occupancy control can be used to reduce demand, either centrally or within each room.

Packaged Rooftop Unit
Rooftop cooling and heating unit efficiencies are based on direct expansion air conditioning and furnace efficiencies. For added efficiency, install dual enthalpy-based integrated economizers,and heat recovery units, such as enthalpy wheels, or air-to-air heat exchangers. Occupancy control, dual-speed compressors, and ventilation fan motors will also increase efficiency. As another option, "demand-controlled ventilation" uses a carbon dioxide sensor to determine the occupancy of the conditioned space to vary the amount of outside air thereby reducing air conditioning and/or heating requirements.   

Variable Air Volume Systems
See central station air handling units for basic efficiency components and strategies. Additional measures include replacing inlet guide vane control with variable frequency drives, and static pressure reset control with a direct digital control (DDC) system.

Window AC (Room Air Conditioner)
Window AC unit efficiency is also measured in Energy Efficiency Ratio (EER), however these units are not tested for a SEER rating. Typical units have an EER of 9.8 or less to meet federal minimum requirements. Most high-efficiency window air conditioners are ENERGY STAR® qualified. Energy Star room air conditioners are required to be a minimum of 10% more energy efficient than the minimum federal guideline. Additionally, to reduce building heat loss during the winter months, cover or remove the window unit.