Vent-free ODS pilot assemblies are required on all vent-free heaters and heat sources. The vent-free Oxygen Depletion System or ODS works off of the amount of oxygen in a room, as the oxygen lowers the flame moves farther away from the thermocouple reducing the heat to it and reducing the amount of millivoltage it produces. If the room oxygen level drops to the point the thermocouple does not have enough millivoltage to keep the valve open the fireplace will automatically shut down.