The best modern high-efficiency boiler heaters can achieve average fuel use efficiencies of 95 percent or more under optimal conditions, but efficiency for specific boiler heaters is heavily dependent on design, fuel type and installation. Older models often only achieve efficiencies of 65 percent or less.
The fuel efficiency percentage refers to the amount of heat produced by the heater relative to the amount of energy it consumes.
Two major factors in determining the effective efficiency of a boiler heater are the fuel type and the method the boiler uses to vent exhaust gases. Systems using propane or natural gas tend to be more efficient than those using fuel oil. Designs that use a direct-venting exhaust system burn more fuel than older designs and are more efficient overall in comparison.