Mike PleziaInside service technician, Clark Associates · Commercial Equipment, HVAC, and Refrigeration professional

Gas furnace flame sensor

10 Oct 2024 · 1 min

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1:05

In this informal service‑call video, a HVAC technician explains the operation of a gas furnace flame sensor. He describes how the furnace’s control board sends a small current to the sensor; the flame, being ionized, conducts that current through the ionized gases to the furnace frame (ground). The control board detects this current flow as proof that a flame is present. After a short delay, the board then activates the blower motor. The technician frames the explanation as demystifying “black magic,” emphasizing that the process is simple physics rather than mystery.

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Hey guys! So, just wrapping up another service call here. Um, so, I don't know about you, but when I was a new tech, it took me forever to figure out how the flame sensor worked. Like, it just seemed like black magic. How does this thing that doesn't actually touch anything sticking into the flame tell the furnace that there's a flame present? So, it's actually kind of neat. There's a little bit of black magic to it. No, actually, it's really just science. So, all it is, is a current is sent from the circuit board to the flame sensor. The flame itself contains positively and negatively charged ions. Those ions carry the current across that gap to the frame of the furnace, and to ground. The circuit board is able to detect that current, And that proves that there is flame. Once that flame is present, then after, you know, a few seconds, the blower motor will turn on. So there's really no black magic involved. It's just science. And it's pretty cool.