What does the hot surface ignitor do in a gas furnace?

In a gas furnace, the hot surface igniter is responsible for starting the combustion process that creates heat. When the furnace thermostat detects that the room temperature has dropped below the desired level, it sends a signal to the furnace control board to turn on the furnace. The control board then sends a signal to open the gas valve and turn on the igniter.

The igniter heats up until it becomes red-hot, at which point the gas valve opens and gas flows into the burner assembly. The gas is ignited by the hot surface igniter and combustion begins, producing heat which warms up the air that is circulated through the home's duct system. Once the desired temperature is reached, the furnace control board shuts off the gas valve, and the hot surface igniter cools down. 

Overall, the hot surface igniter plays a crucial role in the operation of a gas furnace, providing a reliable and efficient way to ignite natural gas and produce heat.