Air conditioners don't live forever, and it's no fun if yours suddenly dies in the middle of a summer heat wave when you need it most.

If your central AC unit is 15-20 years old and showing its age with problems like refrigerant leaks, motor failures or inefficient cooling, then it may be time to shop for a new one.

In addition to keeping you comfortable, a new high-efficiency AC can reduce home cooling costs by 20-50 percent, according to the U.S. Department of Energy.

Here are a few tips to consider when shopping for air conditioners:

Proper sizing. Over-sizing an air conditioner is the most common mistake made by consumers who may think “bigger is better.” In fact, buying a unit that's too large is not only expensive, it can increase discomfort by not removing enough humidity from the air.

Air conditioners are designed to both cool and dehumidify. A system that is too large often achieves the desired temperature before the humidity is adequately removed. If a system is too small, it may dehumidify well but not cool the air sufficiently. 

A properly-sized unit will have an operating cycle that is balanced to remove both heat and humidity. Ask your contractor to do a heat-gain calculation to ensure the right size..

Efficiency ratings. Since 2006, all residential central air conditioners sold in the U.S. must have a seasonal energy efficiency ratio (SEER) rating of at least 13 (on a scale of 20). The higher the SEER rating, the more efficient it is for cooling. 

ENERGY STAR®-qualified units, which must have a SEER rating of at least 14, use eight percent less energy than conventional new models. The Center for Energy and Environment (CEE) provides an HVAC Calculator to compare operational costs, energy consumption and carbon dioxide emissions.

How to pay for it. Many utilities offer rebates for high-efficiency central AC units. Loan programs, such as the Home Energy Loan Program from CEE, offer low-interest loans to homeowners who make energy improvements. See the Database of State Incentives for Renewables and Efficiency at www.dsireusa.org for information on utility rebates, tax credits, loans and other incentives.

Resources. To learn more about air conditioning, check out the Home Cooling section in the Minnesota Commerce Department's Home Energy Guide (pdf) and the Home Cooling fact sheet from the U.S. Department of Energy.

Minnesota Energy Tips is provided twice a month by the Minnesota Department of Commerce, Division of Energy Resources. Contact the division’s Energy Information Center at energy.info@state.mn.us or 800-657-3710 with energy questions.