Can't stand the heat? Get a new air conditioner

The contractor said that the energy efficiency of air conditioning equipment had improved markedly in the last decade and that new "ultra energy efficient" equipment could cut cooling costs in half. Needless to say, the prospect of cutting an out of control utility bill had great appeal.

First, it may be useful to share a bit of information on how a central air conditioning system works. Most residential central air conditioning units consist of three main elements - a compressor, a condenser and an evaporator. With a conventional "split system," the evaporator is located within the air handler - typically a furnace - and the compressor and condenser are located within a radiator-clad box somewhere outside the home.

Refrigerant circulates through copper tubing that runs between the condenser and the evaporator. This refrigerant receives and releases heat as it raises and lowers in temperature, changing from a liquid to a gas and back to a liquid. The refrigerant is particularly cold when it starts to circulate through the indoor coil.

As the comfort systems fan pushes warm room air across this coil, the cold refrigerant absorbs so much heat from the air that it turns into a vapor. As a vapor, it travels to a compressor that pressurizes it and it moves through the outdoor coil, which gets rid of the heat. A fan helps to dissipate this heat. The refrigerant then passes through an expansion device which converts it to a low-pressure, low-temperature liquid, which returns to the indoor coil. And so the cycle continues.

According to Energy Star, a government-backed program helping businesses and individuals protect the environment through energy efficiency, you should consider replacing your air conditioning system with a new, more energy-efficient model that bears the Energy Star label if it is 10 years or older.

Replacing a central air conditioning system is not a do-it-yourself project. We suggest that you contact a qualified heating and air conditioning contractor who will be able to discuss the various considerations that must be made when purchasing and installing a new system.

Among the most important considerations is size. When it comes to central air conditioning, bigger isn't necessarily better. An oversized unit will experience increased operating costs and result in less comfort. Conversely, an undersized system will run much longer than it should and will likely never do an adequate job of cooling your home.

In the past, when it came to sizing a central air conditioning system, the rule-of-thumb was one ton of air conditioning capacity for every 500 square feet of living area. Today, sizing an air conditioning takes into consideration many factors and is far more complex than the old rule-of-thumb method.

Most professional heating and cooling contractors use a "Manual J" calculation to determine the size of a central air conditioning system. The Manual J Air Conditioning Sizing System was developed by the Air Conditioning Contractors of America and takes into consideration factors such as insulation, window area, roof overhang, shading, duct location - plus other relevant information.

Oversized equipment will operate in short run times or cycles, not allowing the unit to reach efficient operation or deliver even temperatures throughout the home. Another disadvantage to an oversized unit is that it will not run long enough to adequately remove excess humidity.

Studies show that summertime operation at 78 F and 30 percent relative humidity provides the same level of occupant comfort as does 74 F and 70 percent relative humidity. This lower humidity level will provide increased comfort, lower utility bills and less risk of health issues associated with high humidity.

To reduce wasted energy, the U.S. Department of Energy has established minimum efficiency standards for air conditioners. Every unit is given an efficiency rating, called a SEER (seasonal energy efficiency rating). The higher the SEER rating number, the more efficient the unit and the lower the cost to provide a given amount of cooling.

Though the minimum allowable SEER rating for a new central air conditioner has been 10, it will increase to 13 in January 2006. Ultra-efficient models have a SEER rating of 15 to 18.

When it comes to efficiency, don't be confused by terminology. "High efficiency" models meet the minimum SEER standard of 10. "Super-high" efficiency models have a SEER rating of 11 to 12 and "ultra-high" efficiency models are 12 and up.