Choosing a Central Air Conditioner

During a recent heat wave, the central air conditioning system in one of our homes failed. That sent the indoor air temperature soaring upwards of ninety degrees. Yikes!

An investigation by a local heating and air conditioning contractor revealed that the compressor/condensing unit had met its demise and that the unit would need to be replaced. The news wasn’t all bad. The good news was that our unit was over ten years old – manufactured in 1993 – and even when operating at its peak was at best an energy hog. The contractor went on to explain 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.

Before we go much further, it may be useful to share a bit of information on how a central air conditioning system works. First, 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.

Here’s how it works. 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 system’s 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 superior 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 ten years or older. Earning the ENERGY STAR means products meet strict energy efficiency guidelines set by the US Environmental Production Agency and the Department of Energy. What’s more, ENERGY STAR qualified central air conditioners have a higher seasonal efficiency rating (SEER) than standard models, which makes them about 25% more efficient. More on SEER later.

If your central air conditioning system is over ten years old, if your utility bills are out of control or your equipment is in need of frequent repair, you – like one of us – may be in need of new equipment.

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 over-sized unit will experience increased operating costs and result in less comfort. Conversely, an under-sized system will run much longer than it should otherwise need to and will likely never do an adequate job of cooling your home.

In the old days, when it came to sizing a central air conditioning system, the rule-of-thumb was one ton of air conditioning capacity for every five hundred 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 a host of other relevant information.

Over-sized 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 over-sized unit is that it will not run long enough to adequately remove excess humidity. High moisture levels result in discomfort and annoyances and possibly serious health issues as they relate to bacteria, viruses, fungi, mites (dust mites and mold), allergic rhinitis and asthma. Studies show that summertime operation at 78 ° F / 30% Relative Humidity (RH) provides the same level of occupant comfort as does 74 ° F / 70% RH. This lower humidity level will provide increased comfort, lower utility bills and less risk of the above 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.To achieve a high SEER rating, an air conditioner may employ many energy-saving features: typically large coils for more efficient heat transfer and variable speed blower and fan motors to reduce electricity consumption.

Don’t forget about your home’s duct system. All too often, old heating and air conditioning systems are replaced without giving any thought to the old duct work. Properly installed and maintained duct work can last twenty years or more. But time, heat and humidity can degrade your duct’s insulation. Over the years, your ducts may have collected contaminates that should be removed.

Have some of the rooms in your home been less comfortable than others? Have your air conditioning service contractor evaluate the amount of air each room should get and verify that your duct system is clean and configured to deliver the right air to the right rooms. It may even be time to consider replacing your duct system.

For more home improvement tips and information, search our web site or call our listener hot line 24/7 at 1-800-737-2474.

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