NEW YORK (This Old House) - Nothing in your house affects your comfort more than your heating and cooling systems. Yet unless the heater conks out during a blizzard or the air-conditioning goes on the fritz in the middle of a heat wave, most of us pretty much ignore our heating, ventilation and air-conditioning (HVAC) equipment.

We shouldn't. When it's not kept in shape, even the best system can cost you. How much? Depending on how you heat and cool your home and the climate of the area you live in, clogged filters, dirty thermostats, sooty flues, leaky ductwork and unlubricated fan motors can reduce heating and cooling efficiency by up to 25 percent.

Some of these maintenance tasks are simple, while others require a trained pro. You'll also need an HVAC contractor if your system is at the end of its useful life. Here are some tips for dealing with HVAC equipment and the pros that service it.

The good news here is that some systems require little attention. A heat pump only needs a yearly service call by a technician who will check belts and filters and replace them as needed. He should also oil moving parts and inspect the wiring.

A gas-fired, forced-air heating system has simple requirements too. Furnace filter should be changed every month or two during heating season, and the circulating fan oiled once a year. Call in a pro to check the heat exchanger, flue and ducts and to adjust the burner every other year.

Other systems, like an oil-fired boiler, require annual maintenance—flue cleaning, a fuel-filter change, cleaning and adjustment of the jets—and often need attention more often than that. These chores should be handled by a professional.

Air-conditioning units are a little less maintenance-intensive. At the beginning and end of each cooling season, you should clean or replace the filters, vacuum out the unit and lubricate the motor. If the unit is not cooling properly, call a technician to check the pressure level of the refrigerant.

Arrange for service calls before the start of heating or cooling season. You'll get better attention and have more flexibility when scheduling the appointment.

When hunting for a company to maintain your system, look for one that designs, installs and services the type of system you have. Full-service companies tend to be up to date on the latest advances in the field.

Besides checking that liability insurance and workers' compensation policies are in force — standard operating procedure with any hire — check with neighbors, friends and family who have used the company over several years. How did the system run under the company's care? Did the technicians always leave the working area clean? How quickly did the contractor respond to emergencies? Were the service people punctual when you called with a problem? A quality provider will have an emergency number that's staffed 24 hours a day, seven days a week and enough technicians to respond when the weather is awful and the calls pile up.

Heating and cooling equipment is designed to last at least 15 to 20 years. If your system is older than this, you might want to have its condition assessed. Although replacing HVAC equipment is a major expense, modern systems operate much more efficiently than the older units they replace.

Most HVAC contractors specialize in designing and installing the systems of a few manufacturers, so no one shop is going to carry every major brand. But before you worry about the equipment, it makes sense to find contractors in your area that are knowledgeable and service-oriented.

Start your search by asking neighbors, friends and family what companies they hired to replace a furnace or air-conditioning system. If they were happy with the installation, ask their contractor to come over and talk to you about heating or cooling your house. You should meet with at least two contractors; hire someone who installs products from at least two manufacturers. Don't forget to consider your service company, if you have one. You're under no obligation to hire the firm for the new system, but its technicians do have a good understanding of the conditions in your home.

When picking a contractor, remember that sizing an HVAC unit by matching it to the home and existing ducting requires skill and experience. A poor design typically results in a system that doesn't deliver a consistent temperature from room to room and costs more to operate. But it can be even more serious than that. In very tight houses served by ductwork, poor design can lead to backdrafting, a dangerous situation where flue gases are sucked back into the house.

Most HVAC shops are small, so the owner should be involved with the system design and either participate actively in the installation or inspect it when it's done. You don't want your system designed by a salesman with no field experience.

Any contractor you're considering also should offer these products and services:

Heat-loss calculation. This process estimates the Btu capacity needed to heat or cool your home. The calculation should include the amount and type of insulation in the walls, attic and floors of your home, as well as the type, number and location of windows and doors. This data is combined with your regional climatic conditions to determine the size unit you need. Software has made these calculations relatively easy. HVAC technicians who don't perform them often specify oversize equipment to be safe. That's dollars out of your pocket now and each time you get an utility bill.

Energy advice. When sizing an HVAC unit, a good contractor will advise you of energy upgrades, such as adding another layer of insulation to the attic. These may allow you to buy a smaller HVAC unit. Efficient equipment. Although it's often not cost effective to buy the most energy-efficient unit on the market, there are minimums to shoot for. Here's what a contractor should offer:

• An air-conditioning unit (if below five tons) with a SEER of 10, preferably 12 (see "Hot & Cold Tech Speak," below).
• A high-efficiency natural-gas heater with an AFUE of around 90 percent.
• A fuel-oil burner with an AFUE of around 85 percent.

A heat pump with an EER of 12.

Automatic controls. A setback thermostat ($40), which contains a timer, should regulate all HVAC systems.

Payback calculations. A quality HVAC contractor will show you payback calculations for the various units he offers, and those calculations should give you estimates of seasonal operating costs.

Variables the contractor will use in his calculations include your regional heating or cooling load, the heating or cooling capacity of the units you are considering and the costs of various types of energy, so you can compare the costs of electric, gas and oil.

Once you receive itemized estimates, you compare costs and do some of your own research on equipment. Start by visiting www.eren.doe.gov/hvac.html or www.consumerreports.com (this site is fee-based), or contact your utility company for comparative lists. Look at operating efficiency and costs as well as consumer-rated reliability. Then compare your knowledge of the contractors involved and make your decision.

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Confused by HVAC lingo? Believe it or not, it's meant to make understanding and buying the equipment easier. These terms allows you to compare apples to apples among units in the same fuel category. Knowing what the terms listed here mean will come in handy.

SEER: The seasonal energy-efficiency ratio (SEER) rates how many Btu an air-conditioning unit will remove for each watt of electricity consumed. The higher the SEER, the less you spend on operating costs. Federal law mandates a minimum SEER of 10 for all new air-conditioning units.

Tonnage: An air-conditioning ton equals 12,000 Btu per hour. That means a three-ton air conditioner can remove about 36,000 Btu of heat per hour from your home.

AFUE: The annual fuel-utilization efficiency estimates how much heat a unit delivers for every dollar spent on fuel. The higher the AFUE, the lower your heating bills.