Air conditioning is one of those amenities that's easy to take for granted if you have it, and-on a hot, humid summer afternoon-easy to covet if you don't. In fact, in relatively warm climates, central air conditioners have become more the norm than the exception. In addition to cooling, they dehumidify and filter air, making it more comfortable and cleaner.

For those who aren't familiar with the terms, a central air conditioner works from a central location to distribute conditioned air throughout a house. Unlike a small, window- or wall-mounted room air conditioner, designed to cool a small area, a central air conditioner borrows a central air handling unit such as a forced air furnace or heat pump and ductwork for whole-house delivery. The related heat pump is essentially a central air conditioner that can be reversed in winter to heat a house.

Creating the chill

Ever wonder how an air conditioner works? When the household thermostat signals the air conditioning system to lower room air temperature, a whole sequence of events begins. The air handling unit kicks on, drawing room air in from various parts of the house through return-air ducts. This air is pulled through a filter where airborne particln air conditioner works? When the household thermostat signals the air conditioning system to lower room air temperature, a whole sequence of events begins. The air handling unit kicks on, drawing room air in from various parts of the house through return-air ducts. This air is pulled through a filter where airborne particles such as dust and lint are removed-in fact, sophisticated filters may remove microscopic pollutants as well. hen routed to air supply ductwork that carries it back to rooms. Whenever an air conditioner is running, this cycle repeats continually.

But how does that evaporator coil get cold in the first place? That happens through the magic of refrigeration. Every air conditioner has three main parts: a condenser, an evaporator and a compressor. With a typical "split system," the condenser and compressor are located in an outdoor unit; the evaporator is mounted in the air handling unit (which is often a forced air furnace). (With a "package system," all components are located in a single outdoor unit that may be located on the ground or roof.)

A refrigerant circulates through copper tubing that runs between these components. This refrigerant receives and releases heat as it raises and lowers in temperature, changing from liquid to gas and back to liquid.

The refrigerant is especially cold when it begins to circulate through the indoor coil. As the air handler 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 jettisons 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.

The cost of comfort

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, 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). 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 is 10. 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. The York Stellar Ultra features two compressors that work in tandem when high cooling capacity is needed; the rest of the time, only one of the compressors operates. These, combined with a variable-speed motor, achieve SEER ratings of 16.7.

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.

The ozone issue

If you've kept up with the news, you may be aware that the refrigerant used in some air conditioners has been targeted as an ozone-depleting substance. The problem is that aging, sealed systems may develop leaks and allow the refrigerants to escape to the atmosphere. Prior to December 31, 1995, a common refrigerant in older cars, some commercial air conditioners and refrigerators was CFC-12 or R-12. Now the commonly used refrigerant in home air conditioners is R-22, an HCFC (hydrochlorofluorocarbon) that is safer but can still be a problem if it reaches the stratosphere. Production of HCFCs will be phased out eventually, too.

The refrigerant used in new car air conditioning systems is R-134a, an HFC (hydrofluorocarbon) that doesn't contain ozone-damaging chlorine atoms. Now, Carrier is the first manufacturer to produce a residential central air conditioning system that utilizes this refrigerant-the WeatherMaker 134a. Other manufacturers are also introducing systems that use HFC refrigerant blends that have no ozone-depletion potential.

Warranties

Ask about the warranty and be sure you understand the terms. The compressor, the coils, and the miscellaneous parts are generally covered under different terms. The best warranties cover the compressor and heat exchanger for 10 years; many warranties cover the compressor for 5 years. Other parts are generally covered for from 1 to 5 years. By selecting a system that is tailored to your family's needs and backed by a strong warranty, you're sure to enjoy years of comfort.

Reducing Your Air Conditioner's Load

You can reduce the load on your air conditioning system with a few simple measures. First, be sure to clean or replace your furnace filters per the manufacturers recommendations. To reduce heat gain, pull drapes or shades over sun-facing windows. Stir up breezes using a whole-house fan or ceiling fans; or circulate air using the "Fan Only" setting on your heating system. When possible, minimize mid-day activities that add humidity to the air, such as washing and drying clothes, showering, and cooking. When doing these activities, use ventilating fans.

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