Buying Tips

Should I Replace My Existing System?

This can be a difficult question. If your central air conditioner is older than 15 years, it is likely to fail soon anyway. Do your research now to find the best high-efficiency replacement before you are forced to do so by necessity (a typical air conditioner will last about 15 years, and a typical heat pump will last 10 years).

If your air conditioner is 10–15 years old, the decision to replace it depends on its size, condition, and performance. A central air conditioner built between 1992 and 2005 probably has a SEER rating of 10, with somewhat higher values in the later years. If your existing system is SEER 10, even an upgrade to the lowest efficiency new model is likely to save you close to 25% on cooling if the unit is installed correctly. With an ENERGY STAR model of SEER 15, you may cut your bills by 1/3 or more.

If you’ve made upgrades to your house as described in the first section, your air conditioner may well be oversized. If you haven’t already, make these upgrades first before thinking about a new system. A home performance contractor can also help you decide whether it’s time to upgrade. The Energy Auditing page provides more information on hiring home performance contractors and energy auditors.

As of January 2010, U.S. implementation of the Montreal Protocol for atmospheric protection led to a ban on manufacture and sale of new central air conditioners and heat pumps using HCFC-22 (R22). It has generally been replaced by HFC-410A (R410A). Because the two refrigerants are incompatible, and because the inside coil (evaporator) of a SEER 13 system is generally larger than for pre-2006 systems (and it generally uses a different controller), ACEEE strongly encourages replacing the entire system (indoor and outdoor unit) instead of trying to use an existing evaporator with a new outdoor unit.

Selecting a New Cooling System

Choosing an air conditioner (or heat pump) is an important decision. Buying an inefficient model will lock you into high electric bills for years to come. Your decision will depend on your climate, and whether you are replacing an existing unit or installing an entirely new system. If you live in a hot, arid region, such as the Southwest, look into evaporative coolers or other new systems designed for hot, dry climates (see “State of the Art Cooling”). For the rest of the country, compressor-driven air conditioning systems are about the only choice, other than natural cooling, which is covered on the main cooling page. This discussion will focus on conventional compressor-type air conditioners (including heat pumps).

Choosing a Contractor

Room air conditioners can be purchased for as little as a few hundred dollars (or even less), while large central air conditioners and heat pumps can cost more than $5,000. If any modifications need to be made in the ducting system for a central air conditioner, that can add substantially to the cost. It pays to shop around and get bids from a number of different contractors. There is substantially more variation among contractors in the quality of their work than there is among manufacturers, and a significant fraction of “equipment problems” can be tracked to improper sizing or installation.

Try not to use the lowest bid as the main criteria for selecting a contractor. Better contractors might charge more, but they may offer greater value. When evaluating bids, be sure to consider what you are getting for the price. Does the system come with a warranty? If so, how long is it? Air conditioner warranties range from one year for complete parts and labor, to five years for the compressor. Some manufacturers are now offering ten-year warranties on the compressors. If an existing system is being replaced, will the old unit be hauled away? Does the air conditioning contractor offer a service plan, and is it affordable? Make sure the contractor has been in business for a while and is fully bonded and insured. If you are not familiar with the company, ask for some local references and follow them up.

To find a cooling contractor you can trust, consult the list of contractors at the Air Conditioning Contractors Association (ACCA) (certified by NATE).

The Geothermal Heat Pump Consortium lists contractors that specialize in ground-source heat pumps.

A bid should be submitted in writing following a site visit, should include the proposed new equipment, what work is required, and the full cost, including labor. Do not give your business to a company offering to give you an estimate over the phone without ever looking at the job. You should expect a home evaluation, including an inspection of your current system and a cooling-load calculation (called an ACCA Manual J). As explained below, the size of the new system should be based only on this analysis, not on the current equipment. Using the calculation, the contractor should also be able to estimate what your energy bills are likely to be with the proposed system. A good contractor should ask about any problems you have had with your old equipment and offer suggestions for addressing them. Finally, reliable contractors are professional. Their people are prompt and courteous. They should have a published office or shop address. An office or shop is an indication that the company has been in business and intends to remain in business. 

Installation Checklist

To maximize efficiency, the outside part of a central air conditioner — the condensing unit — should be located in a cool, shaded place. The best place is usually on the north side of the house under a canopy of trees or tall shrubs. However, it shouldn’t be choked by vegetation; the compressor needs unimpeded airflow around it to dump waste heat effectively. Never place the compressor on the roof or on the east or west side unless it is completely shielded from the summer sun, because sunlight shining on it will heat it up and reduce its efficiency at dumping heat.

Also, the compressor may be somewhat noisy. Try to keep it some distance from a patio or bedroom window. If you’re concerned about noise, ask to see (and hear) one in operation before buying it.

With heat pumps, location of the outside unit is more complicated. Because a heat pump is used for both cooling and heating, it usually makes more sense to locate the compressor on the south side — especially in colder climates, and shade it with a sunscreen or tall annuals during the summer. In the winter, when the compressor is trying to extract heat, a southern location will allow it to absorb solar energy, which will boost its heating efficiency.

The location of room air conditioners is constrained by available walls and windows. To perform most efficiently, these units should be out of direct sunlight; if you have a choice of walls, the north is best and the south is second best; avoid east or west walls if at all possible. If you are shopping for a room air conditioner, ask the sales staff if you can listen to different models in operation.

Choosing the Right Type of Air Conditioner

Whether you should choose central or room air conditioning depends in large part on your climate and cooling loads. In small homes and those with modest cooling needs, room air conditioners often make the most sense. In fact, in a small highly insulated house, even the smallest central air conditioner may be too large. If you are considering room air conditioners, you will need to decide between units that mount in the window and those that are built into the wall. Wall-mounted units are often a better choice, both for aesthetic and practical reasons, though they will cost more to install, because an opening has to be cut through the wall. Window air conditioners are harder to seal, they block views and light, and they prevent the use of the window for natural or forced ventilation.

Central air conditioners have a number of advantages. They are out of the way, quiet, and convenient. If you already have a forced-air heating system, you may be able to tie into the existing ductwork. Whether or not your existing ducting will work for air conditioning depends on its size and your relative heating and cooling loads. Ask your air conditioning service technician. Plus, central air conditioners are more efficient (see “Efficiency Recommendations”).

Heat pumps, though more expensive, provide heat in addition to air conditioning all in one unit. If you already have a satisfactory gas or oil heating system and have decided to add air conditioning, it still may combined with your gas or oil heating system (see the discussion on heat pumps in Heating), this would be a “dual fuel” system that uses the heat pump in mild temperatures and the fossil fuel unit when temperatures are very cold. There are other situations where heat pumps can be beneficial. If you currently have electric resistance heat and you live in a relatively warm climate (winter temperatures seldom dropping below 30°F), a heat pump may be a good choice.

If you’re unsure about which type of air conditioner makes the most sense for your house, ask for opinions and bids from several local air conditioning installers.

Sizing the System

No matter what type of system you choose, make sure that it is sized properly. Most air conditioners are rated in Btu/hour, but central air conditioners and heat pumps may also list cooling capacity by the ton.

One ton is equivalent to 12,000 Btu/hour. With air conditioning systems, equipment cost is much more proportional to size than it is with heating equipment. Don’t let a salesperson convince you to buy an oversized system. In addition to the higher cost for an oversized system, it will run only for short periods, cycling on and off, which will increase electricity use and decrease the unit’s overall efficiency. If it just runs for short periods of time, it also won’t do as good a job dehumidifying the air (see “Dehumidification”).

Find a qualified air conditioning technician or energy auditor to determine your cooling load. Do not rely on simple rules of thumb by air conditioner salespeople, but insist on thorough analysis, including local climate information and calculations of heat gain through windows and walls. Insist on an “ACCA Manual J” load calculation to size the equipment, and do not allow oversizing relative to the load calculation.

Wait to size the system until after you’ve taken measures to reduce your cooling loads as described on the first page of this section. Make sure conservation efforts are taken into account when the technician is figuring out how large a system you need.

With heat pumps, proper sizing can be especially difficult, because the same unit is used for both cooling and heating. A heat pump sized for heating loads in a cold climate will be considerably oversized when it comes to cooling, and a heat pump that is sized for cooling loads in a warm climate will tend to be oversized when it comes to heating. If the heating load is larger than the cooling load, some heat pump salespeople will recommend sizing the heat pump for cooling and then adding enough electric resistance heat to make up the difference in the winter. In such a situation, it generally makes more sense to size the heat pump to provide all of the heating requirements in average winter conditions, even though it will mean a larger and somewhat more expensive model. In this situation, a 2-speed or modulating compressor may be a worthwhile investment. A good heat pump technician should be able to help you choose the best compromise between cooling and heating capacity.

Visit the Consortium for Energy Efficiency (CEE) website to learn more about their recommendations or to access a directory of certified central air conditioners and heat pumps. You can limit your search by indicating size, “CEE Tier,” etc.

To identify the most efficient room air conditioners and ENERGY STAR-qualified central cooling systems, go to the ENERGY STAR website and look under products for “Heating and cooling.”

Efficiency Recommendations

For consumers who choose to invest in efficiency beyond legal minimums, there are several alternative information sources that offer sound recommendations. ENERGY STAR establishes reasonable higher efficiency criteria, generally with excellent market availability. The Consortium for Energy Efficiency, the trade association of efficiency programs, publishes guides used to set utility program incentives for high- and higher-performance equipment. All of these should be consulted to find the products best suited to your situation and climate.

For central air conditioners and air-source heat pumps, look for a SEER of at least 15. Besides SEER, there are two other factors to check. The first is high-temperature performance. Although it is not federally mandated, EER at 95°F is available. Look for at least EER 11.6. If this is a replacement for an existing system, it is critical to replace both the condensing (outdoor) unit and the evaporator (indoor “coil”). In general, the old “coil” will have used a different refrigerant, be too small, and have the wrong control device. When shopping for room air conditioners, look for EER ratings of 11 or higher, if available.

High-efficiency units generally cost more, but in hot climates more efficient units pay for themselves over a few years through reduced electricity bills. Central air conditioners are usually more efficient than room air conditioners.

Other energy-saving features to look for include a fan-only switch, which will enable you to use the unit for nighttime ventilation (see discussion on ventilation below). A filter check light to remind you to check the filter after a pre-determined number of operating hours is helpful. With central air conditioners and heat pumps, a variable-speed fan will allow efficient air circulation for ventilation and filtering.


Air conditioners remove moisture from the home as room air is forced past cold coils. Water vapor from the air condenses out on the coils the same way moisture from the air condenses on a glass of ice water on a hot, humid day. This water exits through a condensate drain.

Lowering the humidity in this way is both good and bad. You feel more comfortable at lower humidity levels, so the dehumidification contributes to cooling. But when water vapor condenses into liquid, it releases stored heat, reducing the apparent efficiency of the air conditioner. One of the ways manufacturers can boost the rated efficiencies of air conditioners is by keeping the condenser coils somewhat warmer, thus reducing condensation. Some new air conditioners may not dehumidify air as effectively. This could be a problem, especially in humid climates.

High-efficiency air conditioning systems can get around the dehumidification problem by including variable-speed or multi-speed blowers. High-speed operation leads to high efficiencies but low dehumidification. Lower speeds reduce efficiency but increase dehumidification. Lower speeds are used during very humid weather; the rest of the time a more efficient higher speed is used.

If you live in a humid climate, look for air conditioner models that are effective at removing moisture. Although there is no industry standard for rating the effectiveness at removing moisture, most literature does list water removal in pints per hour, which will help you compare one model to another. If you’re buying a very high-efficiency air conditioner, choose a model with a variable-speed fan to aid in dehumidification, even though you might have to pay more for it. Several manufacturers have models with a variable-speed blower controlled by a humidistat, automatically reducing fan speed at high humidity. Also try to keep moisture out of the house (see “Operation and Maintenance” below).

Did You Know? Proper air conditioner refrigerant charge can improve system efficiency by 20% in pre-2006 models. Correction of airflow rates can improve efficiency a further 5–10%. Have your system tuned up every 2–3 years to ensure the most efficient operation.