If you have a 90% AFUE furnace, 90% of the fuel is used to directly heat your home. The remaining 10% is lost. High-efficiency furnaces have AFUE percentages above 90%. Think of AFUE as the gas mileage of your furnace.
Ask your technician for ways to increase your furnace's efficiency.
To be clear, you may have a different heating system, such as a boiler, heat pump, or active solar heating. If your heating system heats water and you have radiators, you have a boiler. A heat pump is basically an air conditioner that works in reverse. Keep in mind that many homes have a hybrid heating system, which combines the energy efficiency of heat pumps for mild weather with the powerful heating capacity of furnaces during more extreme temperatures.
In order to maintain an efficient furnace, annual professional maintenance is not a suggestion—it's a requirement. AFUE will decline over the years due to ductwork leaks and inefficiencies, dirty furnace components, loose and damaged parts, and clogged air filters.