A furnace is primarily controlled by your thermostat. There are many different kinds of thermostats, programmable, non-programable, and smart wifi-enabled thermostats, but they all control your furnace in the same way. When the thermostat or control system requests heat because the internal temperature has dropped below the desired temperature, the furnace temperature control activates the ignition switch and begins the heating process. When the desired temperature is reached, the control shuts off the furnace.