The Resideo L4064B2210 is a tiny (11.5") fan and limit controller for forced warm air heating systems. This critical component protects your HVAC system by monitoring and controlling both fan operation and high-temperature thresholds. It uses a bimetal detecting device to intelligently turn on and off the fan in response to plenum temperature changes. https://www.partshnc.com/l4064b2210-resideo-fanlimit-11-12-insertion