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If you're thinking about buying a new central air conditioner, there are a few factors you should consider. Because buying and installing an air conditioner is not a do-it-yourself project, these are things you should discuss with a qualified air conditioning contractor.
First find out whether your house can readily accept an air conditioning system. Installing one usually only makes sense if a house is being newly constructed or has a forced-air heating system that the air conditioner can piggyback on--retrofitting ductwork is simply too expensive in most cases. Even in a home with usable ductwork, a standard central air conditioning system will typically cost from $3000 to $6000 installed. Because the cost depends upon so many variables, the only way to pinpoint it is to get bids. Be sure to get at least three from contractors.
Central air conditioners run on electricity for the compressor, blower, and fan. In some hot climates, where an air conditioner must run frequently, this can push energy costs sky high. The amount of electricity needed to run a central air conditioner depends on two things: cooling capacity and efficiency.
Cooling capacity
Manufacturers produce air conditioners in a variety of sizes, typically measured in "tons" or "BTUs per hour." A BTU (British Thermal Unit) is a common unit used for measuring heat output, equal to the amount of energy required to raise one pound of water one degree Fahrenheit. One ton equals 12,000 BTUs/hour-a term derived from the amount of energy required to melt 1 ton of ice in a day.
With air conditioners, bigger is not necessarily better. For one thing, the larger the capacity, the more a unit will cost; output is directly related to cost. Also, it's critically important to pick a size that is appropriate for the house. An air conditioner that is too small can't keep up with load requirements on a particularly hot day. One that is too large will cycle off and on too frequently, doing a poor job of dehumidifying the air, which degrades comfort. In fact, it's better to slightly undersize an air conditioner than to oversize it. Also, the air flow into and out of rooms must be carefully balanced to insure efficient operation of the system. These factors, as well how well a house is insulated, how it's used by your family, the climate and more must be taken into account when selecting and designing your system. That's why you should consult a qualified air conditioning contractor.
Efficiency
To reduce wasted energy, there are minimum efficiency standards for air conditioners . Every unit is given an efficiency rating, called a SEER (Seasonal Energy Efficiency Rating). This is listed on the yellow Energy Guide label posted on each unit. It's very important to look at these ratings-the higher the number, the more efficient the unit and the lower the cost to provide a given amount of cooling.
The minimum allowable SEER rating for new central air conditioners has been 10, but is to be changed to 13 in January, 2006. The most efficient models have SEERs of 15 to 17. When buying, watch the terminology. "High efficiency" models are actually those that meet the minimum SEER standard of 10. "Super-high" efficiency models have a SEER of from 11 to 12. "Ultra-high" efficiency models are 12 and over.
To achieve particularly high SEERs, air conditioners may have any number of energy-saving features: typically large coils for more efficient heat transfer and variable speed blower and fan motors to reduce electricity consumption.
Programmable thermostats also help by offering precise control. Another energy-saving feature is a fan that stays on long enough to utilize residual cool air that remains after the compressor turns off. And some units have an indicator light that lets you know when the filter needs changing, which also boosts efficiency by insuring proper maintenance.
Do you need an ultra-high efficiency air conditioner? Not necessarily. If you live in a climate that requires little cooling during the year, it may not be worth the premium you'll pay for it. Or if your home isn't tightly insulated, all the gains of a premium model may not be worthwhile. On the other hand, where an air conditioner must run frequently during the year, a highly-efficient model will pay for itself in energy savings over a period of years.
One note about replacing an aging system: be sure to replace both the outdoor condensing unit and the indoor evaporator. Otherwise, if you just replace the condensing unit, potential gains of a higher-efficiency unit may be lost.
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