Complete guide to room and central air conditioners

Complete guide to room and central air conditioners

The right air conditioner can keep costs down as the mercury rises

Last updated: July 2015

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With temperatures rising, one thing’s for certain: your utility bill will, too. Now’s a good time, before the thermometer hits triple digits, to assess your cooling needs for the summer. At the very least you should check and clean your equipment, whether you cool your home with central air, room air conditioners, or good, old-fashioned ceiling fans.

If you’re replacing an old room or central air conditioner, the choices on the market today are likely to be more energy efficient than what you have. But don’t buy too little or too much—getting a cooling system that is the wrong size is the most common mistake people make, regardless of the type. Underestimate your cooling needs and you could be hot and sticky and still increase your electric bills. Buy more capacity than you need and you may wind up with a cool, damp space.

To keep your cool, get the best performing and most reliable equipment. Consumer Reports has new Ratings of window air conditioners, and this year we talked to 34,000 readers about the reliability of their central air conditioning systems. We learned what made readers hot under the collar and which systems cooled when called upon. During our research and testing, we discovered which units to buy and which to avoid.

Top-rated window air conditioners

Although more and more homes have central air conditioning, about 6.5 million window units are sold each year. Our latest tests of small, medium, and large window air conditioners found that all were excellent at cooling. What distinguished the best from the rest was quiet operation, convenient controls, and whether they kept working under brownout conditions. All of our top picks exceed federal Energy Star standards and use at least 10 percent less energy than conventional models. Those energy-savers often include other features, such as timers, digital displays, remote controls, and directional vents, which coax the most comfort from the machine.

How to choose a room A/C

Before going to the store, determine the size of the space you need to cool and where you’ll place the unit. An air conditioner that's too small won't cool the room. One that's too big will cool so quickly that it won’t have time to remove enough moisture, leaving your room cold and clammy.
Get the right size. When calculating the size of the air conditioner you'll need, take into account not only the size of the room to be cooled but whether the unit will be placed in a window that gets shade or direct sunlight, the height of the ceilings, and even the part of the country where you live. The Association of Home Appliance Manufacturers has a worksheet on its website that will help you make the right determination. All you need to get started is a tape measure, a scratch pad, and a calculator.

Assess the airflow. Air conditioners generally do a better job blowing air in one direction than the other. To uniformly cool a room, you'll need to ensure that air is distributed throughout. When the window air conditioner is located near a corner, it must be able to direct air to the center, so check whether your air conditioner needs to blow air to the right or to the left.

How quiet? If the unit is going to be placed in a bedroom or another quiet area, check our Ratings for noise. Models that scored excellent or very good in our noise tests are so quiet that the only sound you might hear is the fan running. But air conditioners that scored fair or worse for noise could disturb light sleepers when set on low and are distracting on high.
Our latest tests of almost three dozen room air conditioners include an $580 model that cools superbly and quietly, and even comes in colors that match the drapes. But you don’t have to spend a lot to cool down as the mercury climbs; other top performers start at $160.


If you’re planning to install the air conditioner yourself, consider buying one with a slide-out chassis. That way you can attach the cabinet and adjustable side curtains to the window before sliding in the heavy working parts of the machine. One person can do it, but it’s easier with two.
Check the electricity. Before installing an air conditioner, be sure that the electrical circuit to the room can handle the electrical load of the unit. Read the owner's manual; larger models usually need a dedicated circuit. Never use an extension cord with an air conditioner.
Secure the unit. Always use the manufacturer's safety hardware, such as sash locks and mounting brackets. Unless the manufacturer's directions say otherwise, the window air conditioner should be level from left to right and pitched slightly toward the outdoors so water that condenses on the evaporator drains properly to the rear of the unit and doesn't leak into the home. Seal around the perimeter of the unit with new weatherstripping.


A clean machine will keep you cool and cost less to run. Plan on a thorough cleaning before and after the cooling season and regular filter checks during the season.

Clean or replace dirty filters. You’ll need to clean the filter regularly. Depending on how much time the unit is actually operating and how clean the air is, cleaning may be needed every few weeks to monthly during the cooling season. With that in mind, make sure you determine how easy it is to remove the filter when selecting a new unit—some are trickier than others. Remove debris with a vacuum then wash the filter in warm, soapy water; be sure filters are dry before you reinstall them. Replace damaged filters.

Vacuum coils and fins. When the filter is removed for cleaning, it's also a good time to check the surface of the evaporator coil, which will now be visible. If there is dust or debris on the surface, gently remove it. Taking care not to deform the soft fins, use an upholstery-brush attachment to vacuum the coils. If your unit has a slide-out chassis, you will usually have good access to the condenser coil when the chassis is removed from the cabinet. That's a good time to inspect and clean any debris off that coil.

Seal the perimeter. Be sure to seal any air leaks around the unit.

Avoid "short cycling.” Though most models with electronic controls now have built-in timers to prevent the unit from restarting immediately after shut-down, those with the “old-style” mechanical controls may not. Wait 5 minutes after shutting off the unit to restart it. That allows pressure in the refrigeration system to equalize, avoiding stress on the compressor.

The pros and cons of portable air conditioners

If your room has only one window or if window units aren’t allowed in your building, a portable air conditioner might seem like an ideal solution. But our latest tests found that portables aren’t as good at cooling as manufacturers claim. Plus they’re pricey and use more energy than similarly sized window units. And because all the mechanical parts are sitting in the room, they can be noisy.
Even portable models with dual hoses, which vent through a window, didn’t impress in our tests. One hose brings air in from the outside to cool the condenser, and the other hose directs heated and moisture-laden air back outside. Dual-hose units did a slightly better job cooling off our test chamber than the single-hose models we tested, but their performance fell far short of similarly sized window units.
If a portable is your only option, choose a dual-hose model. But in our tests, even those models produced less cooling than they claimed and didn’t cool the room to our required temperature. And rolling 85-pound “portables” around on carpeting isn’t for weaklings.

The most reliable central air conditioning

Consumer Reports asked 34,000 readers about central air conditioning systems purchased between 2007 and mid-2013. Based on their experiences, you may want to give three brands the cold shoulder. All logged the most repairs in our latest reliability surveys. The good news: Choosing one of the more reliable brands can boost the odds that you’ll keep comfortable.

How to choose central air

Adding a central cooling system to your home can be relatively straightforward if you already have ductwork. But not all ductwork is equal, and duct systems that were originally designed for a heating system may not be able to handle the air volume required by a cooling system. Another obstacle can be the placement of supply registers. Systems originally designed only for heating might have registers placed in the floor or located low on the walls—good locations for heating but not the best choice for cooling. The less obvious issue is the amount of air being provided to each room, which really determines the amount of heating or cooling being supplied. For example, some rooms may actually require additional supply outlets in order to deliver the necessary cool air in summer, which would likely make them too warm in the winter.
Your contractor should use a duct-sizing method such as the Air Conditioning Contractors of America (ACCA) Manual D to make sure that the main plenums and all the supply ducts are adequately sized and properly constructed. Further, the system must have the proper number and location of supply registers to deliver sufficient air to the right spots. Leaky or uninsulated ducts can reduce system efficiency considerably. In fact, one of the most beneficial improvements to a ducted system is to have it properly sealed and insulated. If your home doesn’t have ducts, adding them can be expensive and messy, though that is the best option when cooling an entire home.
When replacing or upgrading a central air system, don’t automatically buy the same-sized system. Any changes you’ve made to improve your home’s energy efficiency, such as replacing windows or adding insulation, can reduce your cooling needs. On the other hand, if you’ve added rooms, you might need more cooling.
Have your contractor do a load calculation based on a recognized method, such as Manual J from the ACCA. The contractor’s evaluation should include whether the ducts need to be resized, sealed, and insulated, or replaced. Remember that an indoor evaporator coil and an outdoor condenser must be a matched set, or the performance, efficiency, and capacity claims might not be accurate. After the required cooling capacity has been determined, focus on installation.


Replacing central air conditioning equipment, especially components that are more than 15 years old, can result in energy savings. And if you choose an Energy Star qualified model, you can save even more—but savings will only be realized when a new matched system is correctly installed. A matched system is one in which the indoor evaporator or fan coil and the outdoor condensing unit were meant to be installed together—the manufacturer considers the two components a system with its own unique efficiency rating. If you allow a contractor to install an unmatched set, your home has essentially become a test site and your contractor is the system designer. Though the system may function, it's unlikely to deliver the claimed efficiency that was based on a matched set.
Beyond the cooling hardware, there are other important issues that a good contractor will address in the installation. Under- or overcharging the refrigerant on even a matched indoor/outdoor split system can cause a loss of capacity, efficiency, or both, so proper system charging is critical. Likewise, the proper amount of airflow across the indoor coil (evaporator) is critical for proper operation. Finally, if the air is allowed to leak from supply or return ducts, those leaks will have a significant impact on the operating efficiency and costs. So ensuring that system ducts are properly sealed and insulated is one of the most important improvements you can make.
Finding the right contractor is often a challenge. North American Technician Excellence (NATE) offers a certification program for contractors focusing on specific skill areas. Contractors who participate in that voluntary program differentiate themselves from their competitors. You can also find an installer at contractor associations such as the Air Conditioning Contractors of America. Energy Star follows the ACCA guidelines and recommends the following:
Size the unit properly. Installing equipment that is the correct size is essential for getting the best performance. Bigger isn't always better—a system actually operates best when each component is properly sized. Oversized equipment may cycle on and off more frequently, which can make the home less comfortable and shorten the equipment’s life. Larger capacity cooling equipment requires greater airflow. If the duct system was not sized for that flow, it can become noisy or restrict the flow, causing performance or operational problems.
Seal ducts. Ducts circulate air from the central air conditioner or heat pump throughout the house. The duct system is actually many individual pieces, meaning there are lots of seams and joints. Without sealing, air escapes from those cracks, sending your heated or cooled air directly outdoors, which is not a good use of your energy dollars. Sealing ducts can greatly improve the efficiency of your system.
Optimize airflow. Every evaporator or fan coil is designed to have a specific amount of airflow to meet its efficiency and capacity claims. A duct system that is too small can restrict airflow, which not only negatively impacts efficiency and capacity but can cause operational problems as well. Too high of an airflow is not good either, as it can mean a noisy system.
Check the refrigerant. It's important for a central air system to have the correct amount of refrigerant, or correct refrigerant charge. An improperly charged system may consume more energy and provide less cooling capacity.


One of the best ways to keep your air conditioner humming is to keep it clean. That means changing the filters regularly and making sure that no debris accumulates around the outside unit. Here’s some guidance from our experts.

Call a pro. Have a licensed professional clean and flush the coils, drain pan, and drainage system; vacuum the blower compartments; and check the refrigerant charge and mechanical components.

Seal and insulate ductwork. Make sure that ducts are sealed and insulated. Up to 40 percent of cooling energy can be lost due to leaks or when uninsulated ducts pass through uncooled spaces such as attics.

Conduct seasonal checks. Clear debris and keep vegetation at least 2 feet away from the outdoor unit. Clean indoor grills and filters monthly.

Use a programmable thermostat. You can reduce cooling costs by up to 20 percent by programming the thermostat to raise the temperature when you’re at work and lower it when you return home. Consider using a ceiling, table, or floor fan in occupied rooms so that you can set the thermostat to a higher temperature. For every degree you raise the setpoint, you will save about 2 percent on your cooling costs. And remember, don’t operate a fan in an unoccupied room. That just wastes energy because the breeze doesn’t cool the room, it cools people.

Split ductless a solution for some

If you live in an older home or one in which it would be difficult to install the ductwork for a central air system, there is another alternative to getting window units. Split ductless systems are similar to central air but need no ductwork. They have an outside condenser and one to four indoor units with blowers mounted high on the wall. Tubing connects the parts and circulates refrigerant. The tubing, along with an electric and drain line, is usually run through a 3-inch hole hidden behind the indoor unit. Each indoor unit cools the room in which it’s installed and has its own remote control.
The systems we tested in the past had a single indoor unit, did an excellent cooling job, and were much quieter than window air conditioners. When they were set on low, they were barely audible. The systems were about 12,000 Btu/hr., enough to cool roughly 650 square feet, and handled brownouts with ease. And they all used an eco-friendly refrigerant.
Split ductless systems are more expensive than window air conditioners, and professional installation is recommended, but it’s a way to add cooling without tearing up walls to install ducts. A drawback is the large indoor unit (evaporator and fan) that must be mounted on the wall in the room being cooled. The systems can be a good choice when you're only cooling a few rooms. But if you plan to cool many rooms, the cost can increase significantly, often making a ducted system the better choice.

Run the A/C without running up your bills

The average home spends almost 20 percent of its utility bill on cooling, according to Energy Star. But there are ways to save even on hot summer days. A good strategy may be to use air conditioning and ceiling fans in concert. Instead of setting the air conditioner at 74° F to 76° F, raise the temperature to 78° F and let the fans do the rest. Each degree you lower the thermostat increases cooling costs by 2 percent. Here are some simple moves you can make that are recommended by our experts and the Environmental Protection Agency.

Switch to energy-saving lightbulbs.
Less than 10 percent of the energy used by an incandescent bulb produces light; the rest escapes as heat. That’s one reason energy-wasting bulbs are being phased out. Energy Star qualified lighting not only uses less energy but also produces less heat, reducing your cooling costs.
Set the thermostat. Use a programmable thermostat or the timer on a window unit to program cooling around your schedule. Avoid cooling an empty house by setting the thermostat a few degrees higher when no one is home and timing your window unit to go on an hour or so before you arrive.
Use ceiling fans. Run the ceiling fan to create a cool breeze. If you raise the thermostat five degrees and use a ceiling fan, you can lower cooling costs by around 10 percent. Remember that a ceiling fan cools you, not the room, so turn it off when you go into another room.
Pull the shades. Close the curtains and shades before you leave home to keep the sun’s rays from overheating the interior. If you don’t have natural shade, move container trees and plants in front of sun-exposed windows.
Reduce oven time. Use a microwave instead of an oven to cook when you can. Ovens take longer to cook food and add heat to your home, working at odds with your air conditioning system. If you have a gas grill outside, consider using that.
Check air conditioner filters. Check your cooling system’s air filter every month. If the filter looks dirty, change it. A dirty filter will slow airflow and make the system work harder.
Plug leaky ducts. As much as 40 percent of your heating and cooling energy can be lost due to leaks and lack of insulation. Seal ductwork using mastic sealant or metal tape and insulate all the ducts that you can access (such as those in attics, crawl spaces, unfinished basements, and garages). Also make sure that connections at vents and registers are well sealed where they meet floors, walls, and ceilings. Those are common places to find leaks and disconnected ductwork.
Work with your utility company. Many utility companies offer rebates to homeowners who upgrade their cooling systems with energy-efficient equipment. Some also offer homeowners free programmable thermostats or discounts and rebates to use an outdoor digital cycling unit (DCU) that “talks to” the utility via radio signals. When the electrical grid gets stressed during heat waves, the utility cycles your central air conditioner’s compressor on and off to decrease demand. Your home may get a little warmer, but it’s better than a blackout.

—Mary H.J. Farrell

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