February 2008
send to a friend printable version
Heating with gas
A guide to choosing furnace size, efficiency, and features

Is duct cleaning worth it?
Duct-cleaning services typically promise to rid your heating system of dust, bacteria, or other harmful contaminants. Yet there's little evidence such cleaning is needed, except perhaps as part of an allergy-control program prescribed by a doctor.

If you do get the service, insist on a thorough job; in a pilot project, the Environmental Protection Agency found it took 16 to 30 worker-hours to clean the ducts of a typical home. If the company you hired didn’t spend that much time, it’s likely they were not thorough.

For more information, check the Environmental Protection Agency pamphlet "Should You Have the Air Ducts in Your Home Cleaned?"

Replacing or improving a furnace may not pay off as handsomely as it did during the energy crisis of the late 1970s. But it may deliver steady, modest savings. And efficient heating isn't only about money. Because today's furnaces burn less fuel to generate heat, they are less polluting than their predecessors. Some models with modulating heat output can produce heat more continuously than older furnaces, increasing comfort.

Heat pumps that wring heat from the ground or from outdoor air (and reverse the process in summer, to act as an air conditioner) are the preferred way to heat in the South and Southwest, and oil furnaces have a niche in older homes, mostly in the Northeast. But the majority of new central-heating systems use a gas furnace, the focus of this report.


How do most people go about buying a furnace? First, they contact contractors. To prepare this report, we did, too. More than 500 specialists in residential heating and air conditioning told us about their experiences in installing and maintaining heating equipment.

The two major manufacturers of gas furnaces are United Technology (Carrier, Bryant, Heil, Tempstar, and Comfortmaker brands) and Goodman (Janitrol and Amana). Other brands include Rheem (Ruud), American Standard (Trane), and Lennox (Armstrong). All offer furnaces in a range of capacities and efficiencies, and we think manufacturers generally deliver on those specifications. Each brand offers a generally similar array of key features.

The degree of similarity between manufacturers' offerings is one reason this report does not include Ratings of furnaces by brand. The most important steps in selecting a furnace, we think, are to ensure that the unit's specifications fit your needs, that it is bought from a contractor who installs it well, and that it's adequately maintained. Our survey results help confirm that view: When we asked about the most common reasons for service calls for furnaces, about twice as many contractors we surveyed cited human error—inadequate maintenance, for example, or improper installation—as cited defective equipment.


When it comes to furnaces, size counts—a lot. A furnace that's too small won't keep the house comfortable during extreme cold. Partly to avoid that possibility, most installed furnaces are too large for the home they're installed in. Cost is only one of the problems with such a unit. Compared with a correctly sized furnace, a furnace that's too large will cycle on and off more frequently. That puts more wear on its components, wastes energy, and may cause the temperature to vary uncomfortably. Also, upgrading to a larger furnace may require the installation of bigger ducts to accommodate the increased airflow. Without proper duct sizing, the airflow can be noisy.

To be sure of correct sizing, choose a contractor who agrees to take the time to calculate heating needs using an industry-standard calculation, such as found in the Air Conditioning Contractors of America's Manual J. Such calculations take into account the climate, along with the house's size, design, and construction. The ducts should be designed following a guide, such as Manual D.