QWe have a forced-air heating system that was designed to handle the 15-year-old, 2,500-square-foot house we bought a little more than a year ago. Last winter, we found that several rooms remained relatively cold and drafty, while the main living area and kitchen were amply heated. When we turned up the heat, the kitchen and living area got too warm and the back bedrooms remained cool.

We called a heating contractor, who recommended we replace the system. I can't believe this is necessary for a well-constructed house with a heating unit that's only 15 years old. I know I can get a second opinion, but I want your advice on what might be common problems with this type of heating system. I would like to be better informed before dealing with another sales-oriented

professional.

ABecause I don't know what diagnosis the heating contractor gave to indicate a new system is in order, I can only provide general troubleshooting information.

Comfort problems caused by minor and major heating-system ailments are probably more common than you suspect. Yet 90 percent or more of these problems can be cured.

Some solutions are of the low-cost or no-cost type that a handy homeowner can perform. Many require a qualified heating contractor to correct the problem. However, replacing the system would be rather drastic unless your current system was never adequately sized and installed.

Before calling a pro, there are a number of things you can do. No matter the type of heating system, the first basic is to make sure your home is adequately protected from drafts and cold entering from the outside. Insulating and weather-stripping your home are just as important as fine-tuning your mechanical system.

Old weather stripping can lose its elasticity and become worthless, leading to costly heat loss. You may need to add insulation, caulk baseboards, windows, door trims or outside vents, or add storm sashes. Strong winds can drive into small cracks left by failed caulk and make windows drafty and uncomfortable. Remove dried and crumbling caulk by scraping it away with a putty knife and replacing it with a new bead of caulk.

Replacing furnace filters is vital to keeping your heating system at top performance. A clogged filter that hasn't been changed in a year may reduce the total air delivery of your furnace by as much as 50 percent. Check furnace filters every two months during the heating season; clean or replace them as needed.

It's also important to clean fan blades annually. Keep the area around the furnace housing free of dust, lint and litter.

Inspect heating ducts annually for leaks and repair them with quality duct tape. Seal all duct joints with duct tape and look for disconnected ducts in locations such as basements or crawl spaces. Heating ducts and water or steam pipes that pass through unheated areas, such as attics, crawl spaces and basements, should be covered with duct insulation or un-faced R-11 insulating batts, or blankets of insulation.

If the ducts are used for air conditioning as well as heat, use faced insulation and place the vapor barrier on the outside to prevent condensation from forming on ducts. Look for objects stuck in ducts or for furniture blocking the wall or floor registers inside the room.

The problem may be register dampers that do not open completely. The two most common problems with forced-air systems are disconnected ductwork and blocked registers.

Older systems often have dampers, which are used to balance airflow to rooms, within the ducts in the basement or crawl space. Check to be sure these dampers aren't closed. Then, too, you might want to damper down the ducts to warmer rooms slightly to direct more heat where you want it.

Sometimes the duct supplying a room splits off and heats the basement as well. Shutting the basement register will supply more heat to the cold room.

Poorly designed ductwork can result in cold rooms. Undersized ductwork should be replaced, if possible, with larger ducts. Long duct runs with sharp bends can restrict airflow. Have a contractor install a "scoop" or baffle inside the main supply plenum to divert more air to one particular duct. Another solution, which also works on undersized ducting, may be to install an auxiliary in-line booster fan that comes on whenever the furnace is running.

If heat distribution is generally uneven, the problem is often worse at slow furnace-blower speeds. If possible, speed up the blower to increase the airflow.

Although some homeowners complain about the draftiness of forced-air systems, they rarely notice the difference between lower and higher fan speeds as long as the air temperature is warm enough. The only problem might be that a higher blower speed will create more noise.

The blower on your furnace should run almost continuously on a cold day. If not, the furnace may be too large. This is frequently the case if a home was insulated after the furnace was installed. An oversized furnace or burner wastes fuel. Have your utility company or heating contractor test the system and advise you if the size of your furnace burner can be reduced.

If your furnace runs constantly on a cold day, yet your home does not warm up to the thermostat setting, the furnace is either too small or not operating properly. Get professional advice.

Ceiling-mounted registers can also create problems, because heat rises and the warm air never makes it to the cold floor. Increased blower speeds can help, as can installing in-line auxiliary booster fans.

Adding a return duct might increase the heat delivery to a room. A ceiling fan is an inexpensive way to force hot air down from the ceiling to the level where it's needed.

Check your thermostat. If the furnace heats up, then shuts down almost as soon as the blower kicks in and hot air starts to circulate, it can be because of a faulty setting of the anticipator. This is a part of the thermostat that controls burner shutoff.

The anticipator can be easily adjusted. A bad thermostat location also causes problems. If the thermostat is near the fireplace, or in the kitchen where warm air is created by cooking, you are going to have a problem with the heat source shutting down too soon. Consider moving the thermostat to achieve a better balance throughout the house.

Dry indoor air is another heat problem that can be corrected with the installation of a good, in-line furnace humidifier, which will increase comfort level. Be sure the humidifier is attached far enough away from the furnace that drips won't rust out the heat exchanger.

It's best to have your heating equipment serviced by a reputable heating specialist before the start of each heating season. This can reduce your fuel bill by as much as 10 percent and save you the discomfort and expense of equipment failure during the winter.

If your furnace is fired by oil or natural gas, annual maintenance should include cleaning of the furnace and flue outlets, an inspection of the fan belt for tension and wear, oiling of motor and fan bearings, replacement or cleaning of fuel filters, a check of the combustion and safety devices, and any other adjustments or tests recommended by the owner's manual.

Good furnaces last for many years but not forever. When you need to replace yours, make sure the system you choose serves your needs and the size of your home.

Deal with a reputable heating firm. It's best to have more than one estimate and recommendation if you are considering furnace replacement. Be sure to thoroughly read the contract and warranty coverage.

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