In the dog days of summer, the temperature isn't the only thing heating up. Chances are, your energy bill is rising, too. You're not the only one trying to stay cool by turning down the temperature in your house. According to the U.S. Department of Energy, about one-seventh of all the electricity generated nationwide is used to air-condition buildings.

The typical central AC system consumes more than 2,000 kilowatt-hours of electricity per year. And older models, not known for being energy-efficient, become even less so as they start showing their age, so it may be time to retire your long-lived system. How can you tell? Think about a replacement if it's more than 10 years old, or if your rooms are cooling down either too much or too little. Consider it a major red flag if your system is getting noisier, needs frequent repair, or is causing a steady rise in your electricity bills even when it's not steamy outside.

Spending several hundred dollars on repairs for equipment on the verge of failure doesn't make sense. And while getting a new air conditioner can be costly, the good news is that the latest central air-conditioning systems can start saving you money on utility bills from the get-go, and they are far more durable and reliable than 20th-century relics. Today's systems use up to 40 percent less energy than air conditioners made just 10 years ago. What's more, central AC is far better at removing humidity and controlling indoor temperatures than room air conditioners sitting on the windowsill.

If you're thinking of upgrading your old central AC system or installing one in a house that never had it, you shouldn't just buy the biggest, most efficient system you can afford. There's a bit more to consider.

Cool running: how it works
You might think an air conditioner does its job by adding cool air to a space, but it actually removes heat from the air. It then circulates the cooler air through a system of supply ducts (openings in the walls, floors, or ceilings covered by grilles). As air gets warmer again while circulating through the home, it flows back to the central air conditioner through return ducts.

Central AC is typically a split system, with some components inside and others outside. An outdoor metal cabinet contains the condenser and compressor (the heaviest and noisiest parts), while an indoor cabinet holds the evaporator. Refrigerant fluid flows from the outdoor system to the indoors via copper tubing toward the evaporator coil. It absorbs heat and moves it outdoors through the condenser coil. The cycle continues until the thermostat senses that the room has reached a set temperature and shuts off until the room starts warming up again.

A split system requires ducts, but houses without them can use a ductless central AC system. An outdoor compressor serves several small evaporator coils inside, each in its own box and with its own blower fan. These components are usually mounted on exterior walls upstairs, where most heat accumulates, while condensation, refrigeration, and electrical lines are routed outside. Ductless systems have long been popular in Europe and Asia, but they are only starting to catch on in the U.S. Their advantages: They can also provide heat, and they distribute air more evenly. The downside is that they're typically more costly than ducted systems.

Seek the SEER: gauging efficiency
Because of the multiple dioxides they emit, central AC systems are one of the most regulated home appliances, and even the federal government encourages you to buy one with high efficiency. Systems are rated according to their seasonal energy efficiency ratio, or SEER, which indicates the relative amount of energy needed to provide a specific cooling output. The minimum SEER allowed for systems manufactured today is 13. To put that ratio in perspective, an AC system with a 13 SEER is 30 percent more efficient than one with the previous minimum SEER of 10. Some new systems made now have SEER ratings of 20 or higher, while many older systems have SEER ratings of 6 or less.

The new federal standard doesn't mean you have to take out your existing system if it doesn't measure up. Manufacturers will still offer replacement parts and services for a while, since the lifespan of a central AC system is 15 to 20 years. But if one part of your split system breaks, you'll probably have to spring for an entirely new system. The new SEER minimum means that outdoor and indoor units must be perfectly matched to work properly and last longer.

"Replacing only one unit will not achieve a higher efficiency, and it could reduce the entire system's capacity to cool a home by up to 40 percent," says Colleen Hughes, spokesperson for the Air-Conditioning and Refrigeration Institute (ARI), an organization that tests and certifies efficient AC systems. "So you'll end up with higher power bills, because the system will have to work harder and run longer to keep your house cool."

Keep in mind that a unit with a higher SEER rating isn't necessarily the best one for you. Consider the climate and seasons where you live, says Glenn Hourahan, vice-president of research and technology for the Air Conditioning Contractors of America. "It's hard to justify the purchase of a more costly 23 SEER unit in places like New England, because there's not enough cooling hours there. If you live in Phoenix or Miami, you can justify a high-SEER unit, because it's running more throughout the year."

The DIY part: your pre-installation homework
Central AC is complicated enough that you'll need a contractor to install it, but there are things you can do beforehand to ensure that you get the most energy efficiency from whatever system you choose.

If you're designing your own home, talk to your builder about the right combination of energy-efficiency measures. That means building in proper insulation and ventilation, installing energy-efficient windows and doors, and considering sunlight and shade when situating the house on your lot. Those steps can significantly reduce the need for AC.

If you're installing a new system in an old house, you'll have to consider your electrical setup. It's not uncommon for old houses to have only 110-volt, 60-amp service for the entire home, which is often barely enough juice to handle existing lights and appliances. Central AC needs a dedicated 230-volt circuit and may require up to 50 amps of power, making electrical upgrades a common necessity. Have an electrician evaluate your home's electrical supply before your AC contractor starts work. There may also be a lack of ductwork and little space in which to install it. Lowering ceilings or building out walls to hide ducts is expensive. Sometimes your attic or basement can hold the ductwork, but if your ceilings are plaster, they may be difficult to repair after you break them open. In such situations, a ductless system may be your best bet, because the money you spend could be far less than what you'd pay for carpentry work.

If your home is only 10 or 20 years old, adding central AC is usually fairly simple. You'll just need to focus on properly sealing your home and making sure your ducts don't leak. If you like home-improvement projects, use Energy Star's do-it-yourself guide (PDF) to sealing common air leaks and adding insulation in the attic. If that doesn't appeal, you can hire the contractor to pinpoint and seal hidden air leaks.

Sizing of your system is critical. If it's too small, your system will struggle or even conk out on hot days. If too large, it will cycle on and off too frequently, reducing its efficiency and its ability to control humidity. Efficiency ratings are measured at the factory under conditions that may not closely or even remotely match those in your house. In any case, sizing is a job best left to the pros.

Getting personal: finding the right contractor
Here comes the hardest part: finding a contractor who will pick the right AC system and install it properly. Hughes recommends finding one certified by North American Technician Excellence (NATE), who will have had to pass a rigorous test demonstrating knowledge of how to properly size, install, and repair high-tech AC systems. To find one, go to North American Technician Excellence and type in your ZIP code. Even so, it's a good idea to ask for three to five references.

Beware the contractor who guesstimates by basing the size of the system on a square-footage-per-ton calculation, Hourahan says. "A contractor may recommend four tons for a 2,000 square-foot home, but a house in New England doesn't need the same tonnage as one in Arizona." Same goes for two houses on the same street, he adds: "Two people may have the same model house, but one may be facing north while the other has southern exposure with more sun shining through the glass."

To properly size your AC system, a contractor should first ensure that there is sufficient ductwork capacity to send air back and forth, and/or come up with a plan to upgrade the duct system. Then he should do a Manual J analysis. That's the industry term for a standardized equation factoring in your home's age, the number and quality of its windows, how well it's insulated, the number of stories, the total square footage, and local energy rates. That equation will give a total "heat-load calculation," showing how much heat the house gains in the summer and loses in the winter. To make up for the heat gain, the contractor should pick a system that precisely replaces that amount with cool air. Your contractor should tell you the cooling capacity of the system, either in Btu/h (British thermal units of heat removed per hour) or refrigeration tons (one ton is 12,000 Btu/h, which is the cooling equivalent of a ton of ice.)

An easy way to verify that a recommended contractor is a good match is to ask for the ARI Reference Number or Certificate of Certified Product Performance for both the indoor and outdoor units. Run it by the ARI to see if they'll combine for the highest efficiency. For systems with higher SEER ratings, you can use the certificate for a federal tax credit or utility rebate.

You can also have major input with the contractor when he's installing the system. Make sure the outdoor unit goes in a quiet place that won't keep you or your neighbors up at night, and in a shady, clear site with no objects nearby to block the airflow. Have thermostats placed away from heat sources.

Staying in shape: the importance of regular maintenance
Once it's running, keep your AC cool, breezy, and at peak performance by having your contractor do annual pre-season checkups. They get busy in summer and winter, so schedule appointments for the spring. A typical maintenance call covers thermostat and system settings, tightening of electrical connections, oiling of moving parts to reduce friction, cleaning of dirty coils, and a checking on the refrigerant level and airflow.

You may fret about the upfront costs of installing an energy-efficient central AC system, but once the lower utility bills roll in, you won't even break a sweat.

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