gas furnace flame safety controls

The gas appliance, as originally designed, simply had a manually operated hand valve that was opened as a lit match was held over the burner. I had one of these in a rental house I lived in many years ago. I would learn very quickly that if the lit match was not placed above the burner before the valve was turned on, there would be a rather violent rapid expansion (explosion).
After many fires and explosions, the industry developed the flame safety control.
This control had to be designed to keep the gas from flowing if there was no automatic method of safely igniting the fuel.
This page will be exploring the ways this is done.

The pilot safety

This is generally considered the first effective flame safety control.
This system uses a standing pilot light to accomplish 2 things. First, it provides a flame placed near the burner to light the gas as it comes out the burner.
Second, it heats a thermocouple or thermopile which produces a small voltage that holds open a valve that will interrupt the gas flow to the burner.
The flame safety works this way: If the pilot heats the thermocouple sufficiently, there is sufficient power to hold the pilot valve open. If the pilot light goes out, the power from the thermocouple reduces to the point that the valve cannot stay open. When the valve closes, no gas can flow to either the pilot or the main gas. Then the pilot must be relit while the pilot is manually held open. When the power is sufficient to hold the valve open, the manual control can be released.