Gas Furnaces

General Recommendations

  • Install a condensing furnace with a 90 or better AFUE
  • Properly size the furnace and ductwork for the required heating load using ASHRAE Manuals J and D

Options and Analysis

Furnace Alternatives

alternatives AFUE cost Estimated annual heating energy cost/sf (habitable) IAQ practice
power combustion furnaces 80 $1000 $0.61 typical Standard
condensing furnaces 92 $1400 $0.52 better Standard
condensing furnaces 96 $2000 $0.50 better Standard

Costs averaged from several online furnace retailers (Alpinehomeair.com and Budgetheating.com). Cost estimates for furnace based on upward flow, 100,000 Btu/h units.
Energy modeling was conducted using REM Rate 12.3 based on an 864 sf Minnesota code-based house, with wood siding, 15% window-to-floor area, unshaded windows with double low-E argon glazing, distributed equally with regard to orientation, AFUE furnace as noted, and 10 EER air conditioning.
Energy costs based on average 2007 U.S. residential rates from EIA: $0.106/kWh for electricity and $1.301/therm of natural gas.

Energy Consumption Comparison

types cost AFUE Estimated heating energy use (therms/yr) Heating cost/sf/yr Est 20-year heating energy cost Heating related CO2 (lbs/yr)
power combustion furnaces $1000 80 407 $0.61 $10,600 4766
condensing furnaces $1400 92 347 $0.52 $9040 4065
condensing furnaces $2000 96 329 $0.50 $8580 3858

Costs averaged from several online furnace retailers (Alpinehomeair.com and Budgetheating.com). Cost estimates for furnace based on 100,000 Btu/h units.
Energy modeling was conducted using REM Rate 12.3 based on an 864sf Minnesota code base house, with wood siding, 15% window-to-floor area, unshaded windows with double low-E argon glazing, distributed equally with regard to orientation, AFUE furnace as noted, and 10 EER air conditioning.
Energy costs based on average 2007 U.S. residential rates from EIA: $0.106/kWh for electricity and $1.301/therm of natural gas.

Regardless of furnace type, ductwork should be correctly sized and run through conditioned space. If it is necessary to run a duct through unconditioned spaces, it must be insulated. Careful sealing of all ductwork—with the appropriate UL 181 A or B-rated mastic, not duct tape—is necessary to ensure that air is delivered to the intended location. Furnace efficiency can be compromised by complex or leaky ductwork.

Cost
Furnace cost is primarily determined by three things: size (measured in Btu/h output), efficiency (measured in AFUE), and unit configuration (up, down, or horizontal flow). Furnaces with higher efficiency result in an increased initial cost, but reduce energy consumption and operating cost, as described above. A 93 AFUE unit costs roughly $500 more than an 80 AFUE unit. This is an increase of $38 per 1% increase in efficiency. Using the same calculation, every 1% increase in efficiency between 93 AFUE and 96 AFUE costs $300. The threshold of affordability and energy conservation diverge at approximately 93 AFUE, after which costs rise rapidly for a smaller return in energy efficiency.

Practice
Installation of gas furnaces is standard practice, as they are the most common choice for residential heating.