Gas furnaces have been around for quite some time. In 1885, the Bunsen burner was invented. With its natural gas flame that was capable of being regulated, this set the stage for the modern gas furnace. But it wasn’t until the 1940s that gas furnaces became common in US homes.
The technology behind furnaces continues to evolve. One way they’ve evolved is how the gas burners are lit. Many older gas furnaces still have a standing pilot light to ignite the gas burners at the start of the heating cycle. You can think of a pilot light as a mini Bunsen burner in the furnace: a small, constant flame. When the furnace is turned on and the gas flows, the pilot light would ignite the gas burners.