HSI

Furnace Troubleshooting: Hot Surface Ignitor
Messing about with our furnace I got to wondering about the ignitor (Blogger wants to spell it with an e, not an o, but o is correct). I kind of sort of thought that it was a spark ignition device, but when we pulled ours out, I could see that it was not. It's just a resistance heating element. Okay then. So it must get pretty hot, but we use resistance heating in toasters and small room heaters, and you need to careful with them because they can set stuff like clothes and curtains on fire, so sure, you could use a resistance heater to ignite the gas in the furnace.

Hot Surface Ignitor

So how hot does it need to be in order to ignite the gas-air mixture?

Natural gas has a high ignition temperature, approximately 1163 degrees Fahrenheit. - ERPUD

That's a little warm. Paper, as we all know, thanks to Ray Bradbury, ignites at Fahrenheit 451. So our Hot Surface Ignitor (HSI) must likewise get pretty hot.

The HSI heats up to around 1,800°F to 2,500°F and glows red-hot. - The Spruce

That's almost hot enough to melt steel (2500 degree F). So what are these things made of?

In 1993, the furnace industry moved away from the standing pilot light ignition system in all furnaces. One of the most popular styles of ignition used today is a Hot Surface Igniter (HSI). The first HSI’s looked like a “fork” and were a silicon carbide material. While this type of igniter is very dependable, they are fragile. The newer style HSI is made out of Silicon Nitride, which is a more durable material. This is the type of igniter that our technicians stock on their trucks. While they are more expensive to purchase up front, you can expect a longer life out of them. - Santa Fe Air