Types of furnace: which one fits your home?

What are the four types of furnaces?

No official source on this page names four furnace types; the four on this page are sorted by fuel and venting. The Building America Solution Center (BASC) says the National Fuel Gas Code “(NFPA 54) categorizes furnaces based on pressures in the exhaust flue” and flue gas temperature, into Categories I to IV, and that “Category II applies to some commercial furnaces but not residential furnaces.”

Of the other three, BASC says “Category 1 induced draft furnaces typically have efficiencies of 80% to 86%”; for Category III, “Most Category III furnaces are oil furnaces with gun type burners” and “These oil furnaces have an efficiency range of 82% to 88%.” For Category IV, the condensing type, BASC says the heat recovered from condensing water vapor ends up “enabling gas-fired Category IV furnaces to achieve efficiencies of >90% AFUE”.

Electric furnaces sit outside those combustion categories. BASC lists them among fully electric heating systems, “such as electric furnaces, traditional split heat pumps , ductless heat pumps , and ground-source heat pumps”, that “can offer an alternative to fuel-fired furnaces”.

What is the most common type of furnace?

A forced air gas furnace. BASC says: “Forced air furnaces are the most common heating source in America ( EIA 2020 ). Forced air furnaces are typically fueled by natural gas”.

For new and replacement systems, EIA (2024) says: “Most new and replacement heating systems in homes use natural gas or electricity.”

My furnace is 25 years old. Should I replace it?

Age alone does not settle it: no official source on this page sets an age at which a furnace must be replaced. For its 2023 furnace rule, DOE estimated an average lifetime from shipments and survey data, which places a furnace of that age past the average: “DOE estimates the average product lifetime to be 21.5 years for NWGFs and MHGFs.” (NWGF means a non-weatherized gas furnace.) DOE also used “a median lifetime of 16 years (low lifetime scenario) and 27 years (high lifetime scenario)”, and says “Electric furnaces are estimated to have the same lifetime as NWGFs (21.5 years)”. These are DOE planning estimates, not a measure of any one furnace.

If you do replace a gas furnace, the federal minimum efficiency is scheduled to change. The federal furnace standards in the eCFR list “(A) Non-weatherized gas furnaces (not including mobile home furnaces) 80.0” percent AFUE today and “(A) Non-weatherized gas furnaces (not including mobile home gas furnaces) 95.0” for units manufactured on and after December 18, 2028, a level DOE says effectively requires condensing technology. Treat that date as scheduled, not settled: a petition DOE published for comment asks it to move the compliance date for “consumer furnaces (currently December 18, 2028) until January 1, 2030, at a minimum.”