A furnace heats air and blows it through ducts. A boiler heats water, or makes steam, and circulates it to radiators, baseboard or in-floor tubing. When one wears out, replacing it with the same type is usually the smaller project, because the ducts or piping already exist. Switching types means building a new distribution system, and that can cost far more than the appliance itself.
The practical differences
| Item | Furnace | Boiler |
|---|---|---|
| Delivers heat by | Warm air through supply and return ducts | Hot water or steam through pipes to emitters |
| Central AC | Can share the same ducts and blower | Needs separate cooling equipment, such as ductless heat pumps |
| ENERGY STAR gas threshold | 97% AFUE or higher in the U.S. North, 95% in the South | 90% AFUE or higher |
| ENERGY STAR oil threshold | 87% AFUE or higher | 87% AFUE or higher |
| Homeowner upkeep | Filter changes per the manual | Per the manual, which varies between hot-water and steam systems |
The thresholds come from ENERGY STAR’s furnace criteria and boiler page . AFUE measures how much of the fuel becomes heat in the appliance. It doesn’t count heat lost from leaky ducts or uninsulated pipes, so a high rating doesn’t fix a distribution problem.
If you’re considering a conversion
- Boiler to furnace: ask where the supply and return ducts would run, which walls and ceilings would be opened, and how each room gets air.
- Furnace to boiler: ask about pipe routes, emitter locations, floor work, and what happens to the existing AC.
- Either direction: price a heat pump too. DOE’s heat pump overview covers ducted and ductless setups. A boiler home doesn’t need a furnace just to get cooling.
If one room heats unevenly, the problem may be distribution or controls rather than the appliance. Replacing the boiler or furnace won’t fix that.
What a replacement quote should include
- a heat-loss calculation for the house, not a copy of the old nameplate size;
- venting, combustion air, and condensate disposal for condensing models;
- pumps or blower, controls and zoning, and electrical work;
- repairs to ducts or piping;
- the exact model, warranty terms and local service availability.
Treat combustion odors, soot or a CO alarm as an emergency: leave the house and get it assessed. Don’t bypass safety controls. Limit your own maintenance to the tasks the manual lists for homeowners.