AC in the winter? Brrrr-rilliant!

Equipment that's too powerful will cool your house too quickly before dehumidifying; a weak system will run incessantly, for high utility bills.

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(Money Magazine) -- Nobody thinks much about air conditioning this time of year. Heck, depending on where you live, your heat may already be on for the season and your flannel pajamas pulled out of storage.

But if your house doesn't have central air, or if the system you have is about 15 years old (and therefore probably nearing the end of its life span), you should get the job done this winter. Not only will it automatically raise the value of your property, but you'll snag an iceberg-size seasonal discount.

Winter is a slow time for contractors. "In December we're trying to find enough work to keep our crews busy, so our pencils are a lot sharper," says Robert Wilkos, general manager of Peaden Air Conditioning in Panama City, Fla.

How sharp? Like many contractors around the country, Wilkos typically knocks 10 percent and sometimes as much as 20 percent off his wintertime bids. That's a savings of up to $3,000 on the $6,000 to $15,000 central AC costs.

Even at full price, adding a well-designed central air system is a no-brainer. Not only does a system cool and dehumidify the air far more effectively than window units, but it filters out allergens and dust and lets you use your whole house, instead of confining you to a couple of rooms.

What's more, central AC instantly increases the value of your house by at least as much as it costs to install, and in warm climates by up to 10 percent more, according to appraiser Alan Hummel, a spokesman for the Appraisal Institute, a national standards-setting organization.

"It's not just in the South, where air conditioning is expected," he says, but anywhere summers get hot. Here are the steps to take to get the job done right:

Here's the techie stuff you need to know to get an efficient system. Central air has three main components: an air handler, which gets installed in the attic or basement; ducts that bring household warm air to the air handler and cold air away from it; and a condenser, that noisy box that sits in the yard. Copper pipe running between the condenser and air handler carries refrigerant, which absorbs heat blown across it in the air handler and then dumps it outside through the condenser.

If your house has forced-air heat and a contractor can use the existing ducts, you'll probably pay about $6,000 to $10,000 for your system. If the ducts are leaky, however, you could lose 25 percent to 40 percent of the cooled (and heated) air flowing into your attic and basement, and you'll probably have to pay another $1,000 to $3,000 to have them sealed.

If you have to install new ducts, count on spending an extra $2,500 to $4,500. A contractor may put the new duct-work in the attic to push air out into rooms through vents in the ceiling. That's generally a more efficient way to deliver cooled air (which falls) than running it up from the basement and through floors.

To ensure your system is efficient and safe, do the following:

Feel the power Air-conditioning capacity is measured in tons - the cooling power of a ton of ice over 24 hours. You need just the right tonnage for the system to operate at optimum efficiency. Equipment that's too powerful will cool your house so quickly that the system shuts down before dehumidifying the air. If the system is too weak, it will run incessantly, producing high utility bills and premature wear and tear.

Contractors who bid on your job should show you their "heat load calculation," a computer printout specifying the tonnage you need based on your ZIP code, cubic feet of living space and other variables (Cost: $150 to $200). If a contractor has no computer analysis, hire someone else.

Pay attention to the ratings Condensers come with federal seasonal energy efficiency ratio (SEER) ratings ranging from the minimum-allowed 13 up to 18 and beyond. Higher SEERs cost more but will lower your electric bill. Check your heat load calculation to make sure you don't buy more than you need.

Avoid eco-unfriendly systems After 2010, manufacturers will no longer be allowed to produce systems with the ozone-depleting refrigerant R22, though there are no restrictions on your buying one or installing it in your house. However, you'll pay extra for repairs and service after manufacturers stop producing such units.

Add a filter One with a MERV (minimum efficiency reporting value) rating of eight will add $500 to $1,000 to installation but should reduce pollen and dust and cut maintenance costs by about $500 every two years.

Hire a good contractor Look for installers with NATE (North American Technician Excellence) certification, a rigorous testing program that indicates a high level of technical expertise. Search for contractors with certified crews at natex.org.

Installation takes only a day or two (even with new ducts). And then you'll be eagerly awaiting a hot spring day, so you can enjoy not feeling it - and not being among the herd of people trying to book an air-conditioning contractor.