If you are considering replacing your air conditioning unit (or system), you should be aware that energy is an important consideration. In fact, like the mpg rating for cars, most air conditioning units are rated for their relative efficiency, and since air conditioning units often last longer and use more energy, this energy efficiency rating is even more important.

This fact sheet outlines the options available for packaged air conditioning units, such as rooftop units, and split systems. These units use electricity for cooling and ventilating and have a variety of heat sources - gas, heat pump, and electric resistance. The energy efficiency of these types of units is designated in various ways. Generally, the higher the rating, the more efficiently the unit will operate, saving you money for the life of the equipment.

Rating Systems

Cooling energy efficiency is usually designated by the term "EER" (energy efficiency ratio) for units having a cooling capacity of greater than 5 tons and "SEER" (seasonal energy efficiency ratio) for smaller units (5 tons or less). Window air conditioners are rated using "EER", as their use is typically intermittent. Both of these terms are a measure of the unit's output or cooling capacity divided by the power input at standard conditions. New units have EER's/SEER's ranging from 10.0 to 16.0. For split systems the efficiency also varies with the match between the indoor cooling coil and outdoor condensing unit. The manufacturer must be consulted to determine the combined efficiency.

The SEER includes part load operation, and more clearly defines how the unit will operate during the course of a cooling season in a given climatic region. When comparing two different air conditioning units, be sure that the same energy efficiency ratios (EER or SEER) are used for both units. When comparing split-systems with packaged units, make sure that all components such as indoor fans, outdoor fans, and compressions have been taken into account.

In either rating system, a higher rating indicates a more efficient unit.

Purchasing Guidelines

Besides a unit's EER or SEER, there are additional energy saving features to look for when purchasing a new unit or upgrading an existing unit. Some of these features are standard equipment on certain models. Options on others may involve changes to your system over and above the actual equipment purchase. Some of these may also lend themselves to a retrofit application on existing units that will remain in service.

• Adding to or upgrading the unit controls by installing: a time clock (to shut the unit off automatically); a night setback thermostat (to maintain the space at a minimum or maximum temperature when unoccupied); a warm up cycle which holds the outside damper closed until the building warms up in the heating season; a tamperproof thermostat; or interlocking associated equipment (such as exhaust fans) to operate only when the air conditioning unit operates.

• Adding an outside air economizer and "free cooling" controls to allow outside air to be used for cooling when feasible.

• Installing high efficiency motors on supply and return fans.

• For larger size units, installing a variable volume system or two speed supply fan motor for low speed operation during the majority of non peak load hours.

• Installing a waste heat recovery system (or desuperheater) to provide domestic hot water if your facility has a large demand for hot water coincident with the space cooling requirements.

• When replacing an air conditioner, consider replacing electric resistance heat with either heat pump, gas, hot water, or steam heat.

• For units using gas heat, the most efficient units have an intermittent pilot and draft inducer fans. For gas furnaces in split systems, condensing heat exchangers are available that can extract almost all the available heat.

• Correct unit capacity is critical to optimum efficiency. Both over and under sizing lead to poor operating efficiency. If you have made or plan to make significant changes to your building or system and you plan to buy a new air conditioning unit, have the unit sized based on actual conditions. If your current unit cycles off and on frequently, at maximum load, you should consider downsizing the unit, which could save you on both first cost and operating cost. Ask a qualified mechanical contractor for assistance.

• Any deficiencies in the existing system should be corrected prior to or in conjunction with the purchase of new equipment. These can range from poor air distribution, uneven space temperatures, poor location of controls or filters, or just a bad system design.

Installation Guidelines

In addition to purchasing high efficiency equipment, some consideration must be given to both the installation and maintenance of the equipment if it is to deliver energy savings year after year. The following are some general guidelines that should be followed for optimum performance.

• Keep the manufacturer's instructions, control drawings, design documents, and any other relevant information for future reference.

• Maintain adequate space around the condensing unit to avoid short circuiting or restriction of condenser airflow.

• Locate filters, controls and units where they can be replaced or maintained with minimum aggravation and cost.

• Try to arrange ductwork for minimum air resistance and least exposure to unconditioned space. If ductwork must be run outside, add additional insulation and adequate protection.

• Adjust the supply air quantity for minimum acceptable air flow by adjusting the fan speed rather than the dampers.

• Adjust the quality of outside air to the minimum necessary to provide adequate ventilation and a very slight positive pressure in the space.

• Have a qualified Test and Balance contractor adjust your air dampers and control systems after installation. Subsequent tampering with system components such as thermostats and dampers should be discouraged or prevented.