Here's what experts say to look for when buying a central air conditioner:

  • Your contractor will determine the correct size by estimating your cooling load, which is specified in either British Thermal Units (BTUs) per hour or tons. (12,000 BTU/hr is equivalent to one ton.) Cooling load is computed based on the size of your house, the climate in your area, and other factors such as shade, glass area and ventilation. If your air conditioner is undersized, it won't cool effectively. But if it's oversized, it will cool in short blasts and then turn off. This increases temperature variability and doesn't reduce humidity as effectively. Make sure your contractor does a Manual J cooling-load calculation, however, and not just a rule-of-thumb estimate based on house size.
  • Central air conditioners with high SEER specifications cost more to buy and less to operate. Be sure to find out about local utility and government rebates. They may offset all or most of the cost difference between a 13 SEER air conditioner and a high-efficiency model.
  • Scroll compressors have fewer moving parts than the older, reciprocating compressors, and are more reliable. Scroll compressors are also quieter and more efficient. However, because of their smaller volume, they are less forgiving if liquid refrigerant flooding should occur due to overcharging of the system. Compressors are the most expensive components in air conditioners, but most air conditioners have ten-year limited warranties. Reciprocating compressors are still used in some budget models, but are no longer common.
  • Some cheaper models still use R-22 refrigerant (often called Freon). Freon damages the atmosphere's ozone layer and is being phased out. R-22 units will not be manufactured after 2009 and the refrigerant will eventually no longer be available. Most air conditioners now use R-410A, which in the short run is more expensive.
  • Look for sound-dampening features such as vibration isolation for the compressor and insulation. Also, louvers or heavy-duty wire grids and caps protect the unit from weather and impacts from sports or lawn equipment. If noise concerns you, ask prospective contractors for the decibel ratings of the units they are proposing.
  • A ten-year warranty on the compressor and five years on other parts is typical. Some models come with a better warranty. A variety of extended service plans are optional.
  • Check that the contractor meets your locality's licensing requirements, and is certified to handle refrigerants. The contractor should also be bonded and insured against liability. Hire a technician who is certified by one of the two national programs (Air Conditioning Contractors of America or North American Technical Excellence).
  • Be aware that the cost will vary significantly depending on what systems, if any, already exist in your home. If you have no ductwork, or your ductwork is old and leaky and needs replacement, the project cost will be at least twice as much more than what you'll pay if a new air conditioner can be connected to the existing ductwork.
  • Consider a two-stage air conditioner for maximum comfort and efficiency. These high-end units use two compressors and fans. They cost about 30 percent more than a single-stage air conditioner.
  • You or the contractor will probably need to get a permit. Local building departments often require permits for HVAC installation work. Home improvement columnist Paul Bianchina explains, "In general, you need a permit to expand or structurally alter your home or any of its wiring, plumbing, or mechanical systems."

A good contractor will evaluate your entire HVAC system and make appropriate recommendations, starting with whether to repair or replace an existing air conditioner. House and duct insulation, window upgrades and other energy efficiency improvements could make a bigger difference than a new air conditioner. The ACEEE, GreenGuide.com, Flex Your Power, Sensible Home (Dulley.com) and the Rocky Mountain Institute all have guides and suggestions. (See the ConsumerSearch All Reviews section.)

Alex Walter, an HVAC contractor in Aurora, Colorado, suggests that home buyers bring in an HVAC contractor to inspect heating and cooling systems in addition to hiring a general inspector.

No reviewer rates air conditioners or air conditioner brands. The consensus among contractors and other experts appears to be that all brands are essentially the same. Of course, some tout the brands they sell, but in neutral Internet forums where they aren't going to make a sale, many candidly express no brand preferences and insist that installation quality is more important than hardware quality.

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