Furnace System GuideChoosing a furnace comes down to three decisions: fuel, efficiency rating, and size. This guide walks each one using dated figures from the Department of Energy, ENERGY STAR, and the Air Conditioning Contractors of America. Claims are attributed to their source, and unknowns are left out rather than filled in.
Start With Fuel
The fuel your furnace burns shapes both what you pay to install it and what you pay to run it. Gas, oil, and electric are the common choices, and the U.S. Energy Information Administration reports that natural gas and propane furnaces account for almost half of residential space heating, which itself is about 40 percent of the energy delivered to homes.
The fuels differ in how efficiently they turn energy into heat. Wikipedia's AFUE reference reports that electric heating is rated at 100 percent AFUE, meaning all of the electricity becomes heat, while standard gas furnaces are rated at 78 to 84 percent AFUE and condensing gas furnaces at 90 to 97 percent AFUE.
A 100 percent rating does not make electric the cheapest to run, because that depends on local electricity and gas prices. In much of the country gas heat costs less per unit of heat than electric resistance heat, which is why gas remains the most common furnace fuel.
Pick an Efficiency Rating
Furnace efficiency is measured by AFUE. A higher AFUE means more of the fuel becomes usable heat. The Department of Energy explains that condensing furnaces reach high efficiency by using a secondary heat exchanger to capture excess heat from the exhaust gases.
ENERGY STAR is the simplest label to look for. ENERGY STAR reports that certified gas furnaces are up to 18 percent more efficient than baseline models in the U.S. North and can save $160 a year, and up to 16 percent more efficient in the U.S. South with average savings of $60 a year. The Federal Energy Management Program lists the ENERGY STAR requirement at 95 percent AFUE in the North and 90 percent AFUE in the South.
Higher efficiency costs more up front, so match the rating to your climate. A cold-climate home that runs the furnace hard recovers the premium faster than a mild-climate home that barely uses it.
Get the Size Right
A furnace that is the wrong size wastes money no matter how efficient it is. Size is measured in BTU per hour of heat output, and Carrier reports that typical residential gas furnaces range from 44,000 BTU per hour up to 120,000 BTU per hour.
The right size comes from a load calculation, not a rule of thumb. The Air Conditioning Contractors of America publishes Manual J as the national residential load calculation standard, and states that a proper load calculation performed to the Manual J 8th Edition procedure is required by national building codes and most state and local jurisdictions.
Carrier notes that an oversized furnace heats the home too rapidly and shuts off before the cycle finishes, a problem called short cycling, while an undersized unit runs continuously without adequately heating the home. If a contractor quotes a size without a Manual J, ask for one before you sign.
Balance Up-Front Cost Against Savings
Once fuel, efficiency, and size are settled, the last decision is budget. Filterbuy reports that average furnace replacement costs $4,000 to $8,000, with a national average around $6,200 installed.
A high-efficiency model raises that price. Filterbuy reports that a 95 percent AFUE or higher furnace costs 30 to 40 percent more up front while saving $200 to $400 a year. The right choice depends on how long you plan to stay and how hard the furnace will work.
Get the load calculation first, then compare quotes at the efficiency level that fits your climate. Buying more efficiency than your climate can pay back is a common and avoidable overspend.