The efficiency of a room A/C is measured by its energy efficiency ratio (EER). This is the ratio of the energy the unit removes from the air to the electrical energy the unit uses. A system with a high EER may be more expensive to purchase, but the higher the EER, the lower your bills. Look for the ENERGY STAR label to evaluate a given unit’s efficiency.

The efficiency of a central A/C is measured by its seasonal energy efficiency ratio (SEER). SEER indicates the cooling output in Btu during a typical cooling season, divided by the total electric energy input in watt-hours during that same period. A/Cs sold in the 1970s and early 1980s had SEERs of about 6. Today you can buy an A/C with a SEER of 18, or even higher. As with EER, the higher the unit’s SEER rating, the more energy efficient it is—and the higher purchase price will be offset by lower electric bills. Again, look for the ENERGY STAR label.

Air-conditioning capacity is expressed in tons. A ton equals just a little more than 12,000 Btu per hour of A/C capacity and is approximately the amount of cooling you’d get from melting a ton of ice.

Contractors often size A/Cs 0.5–1.5 tons larger than necessary, to make sure you have plenty of cooling power. But correctly sizing an A/C can save you money, because running a smaller unit steadily for a longer time is more energy efficient than running a more powerful unit that turns on and off frequently. Having a correctly sized unit (rather than an oversized one) can also reduce demand on the electrical grid during the hottest afternoons, when utilities are stressed to meet the load.

To avoid buying an oversized A/C, have your contractor measure your house, take into account the window area and the direction the house faces, and measure insulation levels. These data are entered into formulas that are used to calculate the amount of cooling your house needs. Ask your contractor to show you his or her ACCA Manual J calculations, and make sure the a/c recommended by the calculations is the same size as the one the contractor wants to sell you. If the contractor is unable to show you these calculations, or persists in trying to sell you a larger unit than you need, hire another contractor.

Another important consideration is the A/C’s ability to remove moisture from the air. Your contractor should understand how to select an A/C that is suitable for your house and climate. If you have hot, humid summers, ask for an A/C with an evaporator coil that has a sensible heat ratio (SHR) of 0.75 or lower. This means that 25% of the coil’s capacity is used for humidity control. The lower the SHR, the more of the coil’s capacity is being used to control humidity.

It’s also important that the condenser and the evaporator coil be matched in capacity. Some firms install mismatched equipment—for example, a 3-ton condenser (outdoor unit) with a 3.5- or 4-ton evaporator coil (indoor unit). This is not advised; the air delivered is likely to be warmer and more moist than you want it, since it will not be sufficiently dehumidified.

Troubleshooting and Maintenance—Especially for Older Central A/Cs

If your A/C isn’t old enough to replace, but it isn’t working well, there are several ways to increase its efficiency. If some rooms cool fairly rapidly, and others never cool, your problem is probably in the ductwork; you need to fix leaks and insulate. Replacing your compressor and evaporator won’t fix this problem.

Another straightforward fix is to increase the flow of return air to the A/C by adding a second return air grille and duct. Many A/Cs don’t get all the return air they require, decreasing efficiency.

If cooling is inadequate throughout your home, look at maintenance. Do you clean the filters on the manufacturer’s recommended schedule? A leaky or inefficient compressor is also a possible culprit. Another problem might be leaking refrigerant—bad for both your wallet and the environment. Contractors who sell A/Cs and heat pumps usually also service them; they can check to make sure your refrigerant level is where it should be.

Extending an Existing Air Conditioning System

If you have central air conditioning, and you’re adding space to your house, perhaps you can simply add another duct run to the addition. Because many A/Cs are oversized, your existing unit may be able to handle a whole new wing, especially if you find opportunities to tune it up and seal leaky ducts. Duct additions—even those as simple as a single new run—require an understanding of duct geometry and airflows, proper installation, sealing, and balancing, so have an HVAC contractor figure out what will work for you.

Using a Room A/C in an Addition

Some of the cooling energy in central systems is wasted via air friction in ducts, making the fan work harder. Room A/Cs don’t have ductwork, but the units themselves are generally less efficient than central A/Cs. The strength of room A/Cs is that they let you cool just the area you’re using. Look at the EnergyGuide label to compare the energy efficiency of different models.

If you get a window A/C, take it out of the window and store it in winter to prevent air from leaking in and out around it (and to get your view back). If you get a wall A/C, insulate it and cover it inside and out for the winter, sealing the cover to the wall.

Adding an A/C to Your Furnace

If you have a furnace and want to add an A/C, check to see if you have sufficient space next to the furnace. The indoor portion of the A/C needs to sit between the furnace and the supply ductwork. This will probably require a clearance of about 3 feet. You will also need a new heating-and-cooling thermostat. The outdoor unit needs room to discharge hot air and should be located where the noise it creates will cause the least annoyance. If you also plan to replace your furnace and space is tight, you could consider buying a shorter low-boy furnace—or a heat pump.

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