Air conditioners have come a long way from the days of fans and window units. Early air conditioners were noisy, inefficient and costly to operate. Today's central air units have made great strides in all three areas.

The cost to install a central air conditioner will vary greatly depending on the size of your home and the amount of work that needs to be done. If the air conditioner can be attached to duct work, the cost will be much less than if new duct work has to be added, particularly if the ducts have to be run through walls (thus requiring drywall repair and replacement). There are ductless systems available, but they're not widely used in the United States, where most homes have duct work, or at least the space for it. The duct-free systems tend to be more expensive, but it may be worth comparing price if you have to have ducts added anyway.

Do you need help deciding which brand air conditioner to purchase? Our contractors can help you choose a top brand that is right for you: Trane, Carrier, Goodman, Bryant, or Amana. When considering a new air conditioning system, there are two factors, or gauges of measurement, to consider:

Energy efficiency. The U.S. government has created a set of standards, known as the Seasonal Energy Efficiency Rating (SEER), which manufacturers use to test their central air conditioning systems. The systems are then assigned a SEER rating. The higher the rating, the lower the operating cost, because the higher rating indicates a system which uses less electricity. Government standards require new systems to have a SEER rating of at least 10, but many newer models boast a rating of 18 or more. Be aware that the higher the SEER rating, the higher the price. However, many HVAC experts and homeowners feel that the additional cost will be paid back in long-term energy savings, particularly in climates where an air conditioner is used year-round.

Cooling capacity. This is not as easy to define as the SEER rating. Cooling capacity refers to the number of "tons" of cooling, with one ton equal to 12,000 Btu per hour. This is harder to gauge, because the "tons" will vary hour to hour, morning to night, depending on the weather and the home's cooling needs. A general rule of thumb is that the larger the capacity, the higher the cost to pay for the equipment, and the more electricity needed to run it.

Determining what cooling capacity your home requires is tricky, and it's wise to go over your home's environment with your contractor so they can help you make an informed choice. Factors involved include the size of the home, level of insulation, amount of direct sunlight (particularly from windows which face east or west, which let in the most light and heat), and whether the house is in the open or shaded by trees. If anything, an undersized air conditioning system is better than one that's too big. An overly large system will run more often but won't dehumidify the home as efficiently as a smaller system in Charlestown, New Hampshire

Note: many power companies can put a device (often called a "saver switch") on your central air conditioner at no charge. This switch operates during peak energy consumption hours (usually during the work week), and it turns the air conditioner off for a few minutes every hour. This may raise the overall temperature in the home by a degree or two, but it provides considerable cost and energy savings. It's well worth the slightly warmer temperature.