In the air handler/heater section of the furnace several limit switches help protect against accidental operation of the system. Depending on the type of system, a limit switch is installed to guarantee forced purge occurs before allowing the gas valve to open. A switch is present to insure the air is hot enough for the fan to turn on. Another switch is for the high temperature limit. Some systems will have a low and working temperature limit switch for operating the furnace. A switch on the filter door will be found on newer units. Many heat related switches are eye shaped with a raised circle in the center of the eye. The fan switch can also be a heat controlled switch. The box or mounting area will usually have a temerature guage that is settable. Momentary connection switches are used to keep panels in place.