The phrase "you don't appreciate what you have until it's gone" can apply to many things in life, but it's particularly relevant when the heat goes out in the middle of a blizzard or an air conditioner breaks down on the hottest day of the year.

In these situations, most homeowners will do practically anything to restore the indoor comfort level of their homes as quickly as possible - whether it's paying for a quick fix or replacing an entire system. However, in the rush to prevent the family from shivering or sweating all night long, it's easy to make a rash decision that could ultimately be a costly mistake in the long run.

According to Bill Cunningham, a home comfort specialist with Lennox - a leading manufacturer of heating, cooling and indoor air quality equipment - there are three common mistakes people tend to make when the air stops circulating at home:

* Mistake No. 1: Thinking you'll save more money by repairing an old, broken system instead of replacing it. Repairs to an existing furnace or air conditioner may be the least expensive immediate option, but Cunningham says that simply repairing an old system may cost you more in the long run since older systems tend to break down more frequently and consume more energy. Replacement often is a better option, because new heating and cooling systems are much more efficient than those from several years ago and they can save you money, time and headaches in the long run. For example, by replacing an older furnace that is 60 percent efficient with one that is 95 percent efficient, homeowners can save approximately 57 percent on energy bills and up to $5,513 over a five-year period. In addition, new federal tax credits for energy-efficient home improvements make buying a new system more affordable than ever.

* Mistake No. 2: Buying a new system that is too big or too small. "Bigger isn't always better, particularly when it comes to heating and air conditioning equipment," says Cunningham. A correctly "sized" heating and cooling system is crucial to your comfort and the efficiency of the system. According to Cunningham, an oversized system will cost you more to operate and may actually lower your comfort. In fact, an air conditioner that is too large for the home will cycle on and off more frequently than properly sized units, running up your electricity bill, while also leaving rooms cold and clammy. Likewise, if the unit is too small, it will run too often and may be unable to cool or heat your home sufficiently. To help determine the proper size, it's best to enlist the help of a reputable home heating and cooling contractor.

* Mistake No. 3: Failing to take into account your long-term needs. When buying a new system, be sure to consider a system that is priced within your budget, but don't compromise your comfort level, household energy efficiency or long-term savings by purchasing a system that will not satisfy your needs well into the future. Choosing a new heating or air conditioning system that’s right for your home is more than just a matter of comparing the initial purchase price and installation costs. The fuel costs to operate a home comfort system over its lifetime, which can span anywhere from 10 to 20 years, will likely be much more than the initial purchase price. Cunningham says purchasing a new furnace with an efficiency rating of 90 percent or higher, such as the Lennox G71MPP gas furnace , or an air conditioner with a seasonal energy efficiency rating (SEER) of 16 or higher can help offset fuel and operating costs over the long haul.

For more tips and advice on maintaining or purchasing a home comfort system, visit Lennox.com or ItPaysToLiveSmart.com .