What are some telltale signs a furnace may need to be replaced?

Stewart Unsdorfer: Inefficient operation, high utility bills, frequent breakdown, a cracked heat exchanger and rusting out. With newer furnaces, there are a lot more components in them to fail. With older furnaces, the problems are usually because of a lack of maintenance. It's also very common for us to see oversized furnaces, which do more starting and stopping, and that wears out the furnace. It has to go through that ignition sequence every time. It's only going to heat up and cool down so many times before it [the heat exchanger] cracks.

Tim Flynn: Some people have furnaces that are 25 to 30 years old, while the average lifespan is 12 to 15 years. A quick way to find out [if it needs to be replaced] is to run an efficiency test on your heating system. A lot of people replace their furnaces for oil-to-gas conversions or because their fuel bills are too expensive. A lot of things you can repair. You can replace a gas valve for $1,000, but would you want to put that much into a 30-year-old furnace?

What are some factors to consider when purchasing a new furnace?

Unsdorfer: The biggest factor is the contractor. Make sure they know what they're doing, have a good reputation and have been in the business for a while. Then of course there are economy brands and premium brands. You really get what you pay for. New furnaces aren't very forgiving. You've got to move the correct amount of airflow or else they'll overheat.

Joey Dulin: The most important factors are the efficiency rating, size, number of stages and warranties on the parts. The efficiency for all furnaces is measured in an Annual Fuel Utilization Efficiency rating ranging from 78 to 95 percent. The American Society of Heating, Refrigerating and Air Conditioning Engineers does this testing. Most furnaces come in sizes from 40,000 to 120,000 BTUs. Bigger isn't better - the size of the furnace should be based on the home's heat gain and loss calculation. Twenty-year heat exchanger warranties are common, but most 90-percent and up AFUE gas furnaces come with lifetime warranties.

Flynn: Efficiency is No. 1. The warranty is also really important. Our warranties are good for three years. We try to ask customers what's most important to them - efficiency, cost or look. A lot of systems are hot-air based with a heat pump. They don't work well in the Northeast. I recommend a heating system that's natural gas or oil fired. We always take into account the size of the home and its insulation.

What's the most common system you install?

Dulin: Hybrid heating systems are the most efficient way to heat your home and are the most common system we install. They have a heat pump unit outside instead of an air conditioner, as well as a gas furnace. The system is controlled by a combination of indoor and outdoor thermostats. It'll heat the home with the heat pump until it's 35 or 40 degrees outside, then it automatically switches over to gas heat. Hybrid systems are the best of both worlds and range from $7,000 to $16,000, depending on the size and efficiency of the system. The purchase of a hybrid heating system includes the cost of a new cooling system as well.

Flynn: We install a lot of water-based systems with steam. The average cost can range from $6,000 to $11,000, and installation takes about two days.

What can a homeowner do to help ensure a new furnace runs properly?

Unsdorfer: The representative from the company installing the furnace should be properly trained to make sure the equipment is the correct size for the space it's heating or cooling. The equipment also needs to be adjusted and calibrated to operate within the manufacturer's specifications. Make sure the air filter or cleaner is maintained regularly, keep the area around the furnace clean and dry, and have the furnace serviced regularly.

Dulin: The best thing is to have it serviced by a professional once a year. Small problems can become bigger as your system continues to run. A well-maintained system will help minimize costly breakdowns, extend your equipment life, and keep the unit operating at maximum efficiency.

Flynn: You need to buy a preventative-maintenance plan from your provider. We offer a program in which we visually inspect everything annually. For the first two years, you shouldn't require any maintenance at all. After that, you need to be talking about what's going to happen with your furnace and who's going to fix it.