Furnace Buying Guide

When replacing the old furnace in your central heating system energy efficiency and price are the primary considerations.

Getting started  Undersize matters. The furnace should match or exceed the size determined by engineering load calculations commonly called heat loss / heat gain. A furnace that’s too small won’t keep your house comfortable during extremely-cold weather.

The furnaces in most homes are larger than necessary. Initial cost is usually insignificant for the slightly larger size.

To be sure of correct sizing and a proper installation, choose a reputable contractor who will take the time to calculate your heating needs according to an industry standard such as in “Manual J HVAC Residential Load Calculation” of the Air Conditioning Contractors of America. Such calculations take into account the climate and the size, design, and construction of your house.

Efficiency also matters … Gas is currently the most common heating fuel and most new central-heating systems use gas. How efficiently a furnace convert’s gas into heating energy is reflected in its “annual fuel-utilization-efficiency” (AFUE) rating, which is measured as a percentage. The higher the number, the more the furnace converts each therm of gas to heat. Since efficient furnaces generate fewer emissions, environmental considerations might also influence your decision.

Furnaces have become more energy-efficient over the years. A gas furnace made in the early 1970s typically has an AFUE of about 65 percent. The lowest efficiency allowed by the EPA for new gas furnaces is 78 percent, and some new models achieve 97 percent—near-total efficiency.

The price of a furnace generally rises in step with its fuel efficiency. A furnace with a 90 percent AFUE might cost $1,000 more than a similar size unit with an 80 percent AFUE.

Select models in a range of efficiencies and calculate the life cycle cost of each model you’re considering. The contractor can complete those calculations by plugging information on each unit’s AFUE and electrical consumption, local utility rates, and characteristics of your home into one of several computer programs.  Make sure that the quotes also include the cost of any changes to venting required by any appliances in the home.

Other questions to ask the contractor: Is the model you’re considering fairly new—introduced, say, two years ago or less—and thus relatively untested? If it’s an older model, has the contractor noticed any reliability problems with it?

 Types you can choose from among several energy sources. Prices vary widely according to the type of furnace and the installation.

Gas furnaces … Gas is currently the most common heating fuel. Most new central-heating systems use gas.

 Oil furnaces … These models are still available, mostly in the Northeast.

Electric heat … Heat pumps or reverse acting air conditioners are inexpensive as an add-on when installing a new air conditioner. Heat pumps are the preferred way to heat in the South and Southwest, where winters are typically short and mild. Ground Source Heat Pumps (Geothermal) that draw their heat from the ground are much more expensive to install. GSHP are suitable for cold climates because they can maintain their operating efficiency.

Inexpensive electric-heat options include electric furnace, resistance heat strip heaters, and permanently installed baseboard unit’s electric units. Before you consider any type of electric central heating, keep in mind that electricity rates are much higher than those for natural gas and are likely to remain that way for the foreseeable future.

Features … Features generally found on higher-efficiency furnaces.

  • The indoor fan must be matched to the duct system.  Some units have 3 speed motors.  Others have variable speed motors that save energy.
  • Most current furnaces have an electronic ignition system eliminating the standing pilot and adding increased safety.
  • Heat exchangers that heat and cool constantly can crack allowing carbon monoxide to enter the airstream.  Pay close attention to heat exchanger warranties.

Brands … There are very few differences in HVAC equipment.  Check features and benefits closely for apples to apples comparison.