For our reliability survey, we focused on conventional air conditioning systems, which are more common in areas with wide temperature swings. In areas with more moderate cooling and heating needs, heat pumps are a good option. (You can check their reliability in our Heat Pump Buying Guide.)

To provide cooling throughout the home, air conditioners transfer heat from a home's interior to the outside.

If you are installing an A/C system from scratch, your contractor should calculate the size of the cooling equipment you need by using such recognized methods as the Air Conditioning Contractors of America (ACCA) Manual J. If you already have ductwork for your heating, adding a central system can cost less.

Keep in mind that ducts used for heating might not be the right size or in the right location for optimal cooling. Your contractor should ensure duct sections are properly sized and that there are enough supply registers to deliver sufficient air to the right spots. Undersized ductwork can make for inefficient and noisy operation. Here's how to keep yours humming:

Keep it clean. Be sure hedges and plants are at least 2 feet away from the outside unit. Clean grills and filters monthly. Clear debris and dirt from condenser coils and check for blockages in the drainpipe.

Seal and insulate ducts. Cool or warm air can escape through leaks or when ducts aren't insulated properly, wasting up to 40 percent of the energy used to run your system. Sealing your ducts will keep you cooler in the summer and warmer in the winter. That’s why it’s called duct tape, not duck tape.

Seasonal checks. Once a year have a licensed professional change all filters, clean and flush the coils, drain the pan and drainage system, and vacuum the blower compartments. The contractor should also check that the system is properly charged with refrigerant, that there are no leaks, and that all mechanical components are working properly.

Size
A synonym for the air conditioner's cooling capacity, size is measured in British thermal units per hour (Btu/hr.) or in "tons." One ton of cooling equals 12,000 Btu/hr. For sizing guidance, use the calculator on the Energy Star website.

Efficiency
This describes how much cooling the unit delivers for each watt of electricity. Efficiency is expressed as the Seasonal Energy Efficiency Rating, or SEER. The minimum SEER for a split system central air conditioner allowed today is 13, so look for units with SEER ratings of 13 or greater. The higher the SEER, the more you can lower your energy costs.

Maintenance
A service plan that combines regular inspections with discounts on repairs and a labor warranty is worth negotiating into the overall price. Prices for such a service vary widely.

Programmable thermostats
Proper use of a programmable thermostat can reduce your cooling costs by up to 20 percent. And using a box or ceiling fan, which cost little to run, can make you feel 3 to 4 degrees F cooler.

Upgrading an existing system
If you're upgrading your central air, don't assume you should buy the same-sized system. Any changes you've made to improve your home's energy efficiency, such as upgrading your 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 your ducts need to be resized, sealed and insulated, or replaced. Remember that an indoor evaporator coil and outdoor condenser must be a matched set from the same brand, or else the performance, efficiency, and capacity may not meet expectations.

New systems are 20 to 40 percent more efficient than minimum-efficiency models made even 10 years ago. Costs will vary and can depend on whether you need ductwork installed and the particular size and configuration of your home.

Whether you're replacing an older air conditioner or installing one for the first time, finding a trustworthy contractor to install and service an air-conditioning system matters the most. Here's what to do.

Ask around. Seek referrals from neighbors, family, or business associates. It's wise to get price quotes from at least three contractors.

Check their background. Contractors who bid on your installation should show you verification of bonding and insurance, plus any required contractor's licenses. Check with your local Better Business Bureau and consumer affairs office for complaint records. It's a plus if technicians are certified by a trade organization, such as North American Technician Excellence or HVAC Excellence, to service residential heating and cooling equipment. Those and other similar programs assess the technician's knowledge of specific types of equipment and their proper service methods.

Get specifics. Contractors who bid on your job should calculate required cooling capacity by using a recognized method such as the ACCA's Residential Load Calculation Manual, also called Manual J. An additional reference for assessing ductwork needs is Manual D. The calculations produce a detailed, room-by-room analysis of cooling needs. Ask for a printout of all calculations and assumptions, including ductwork design. Be leery of a contractor who bases estimates merely on house size or vague rules of thumb.