If your furnace or boiler is old, worn out, inefficient, or significantly oversized, the simplest solution is to replace it with a modern high-efficiency model. Old coal burners that were switched over to oil or gas are prime candidates for replacement, as well as natural gas furnaces with pilot lights rather than electronic ignitions. Newer systems may be more efficient but are still likely to be oversized, and can often be modified to reduce their operating capacity. -Older, low-efficiency heating systems: Natural draft that creates a flow of combustion gases Continuous pilot light 56% to 70% AFUE. -Mid-efficiency heating systems: Exhaust fan controls the flow of combustion air and combustion gases more precisely. Electronic ignition (no pilot light) Compact size and lighter weight to reduce cycling losses. Small-diameter flue pipe 80% to 83% AFUE. -High-efficiency heating systems: Condensing flue gases in a second heat exchanger for extra efficiency.