Answer by
elden (116)
A flame sensor uses something called a thermocouple. When heated a thermocouple (two different metals soldered or welded together) produces a small amount of electricity. That voltage change is used to operate a transistor switch (either on or off). If the flame goes out the transistor operates some control or larger switch.
The answer by elden is true for some older furnaces but newer furnaces use a metal rod in the flame that actually flows some very small electrical flow through the gas flame itself, without the flame electricity cannot flow and the control box shuts of the gas to prevent calamity. add a comment