Newer models of furnaces use gas, electricity and propane, being very efficient, with ratings up to 98% annual utilization efficiency.
A modern furnace is a complex system that works with other systems to provide the indoor thermal comfort you need and to allow you to set the level of comfort at the temperature you wish, by using a thermostat. If you are not familiar with this HVAC system, know that it is not at all complicated in its process. The temperature is permanently measured by the thermometer, and when it dips below the setting you have specified, fuel is sent to the burners, the furnace turns on automatically and starts heating the house. It sucks in air, heats it up and redistributes it through the ducts.
There are also complex systems that are not limited to a furnace and its thermostat, but completed with air conditioning, air filtration and humidifier that provide you full control of the climate in your house. Now that summer is over there is not as much a need for air conditioning repair as there is furnace repairs.
The air in your home will remain safe to breathe due to the flue and vent pipe that exhaust harmful carbon monoxide and other noxious fumes resulting from the combustion process.
When the temperature reaches its set level, the system stops until it receives the signal to start again.