The Johnson Controls A19ABC-24 is a remote bulb temperature control designed to maintain a consistent temperature in heating or cooling systems. It utilizes a liquid-filled sensor bulb connected by an 8-foot capillary tube to the main control unit. As the surrounding temperature changes, the liquid in the bulb expands or contracts, triggering a switch that turns your HVAC equipment on... https://www.partshnc.com/30100f-8cap-spdt312-adif-for-johnson-controls-part-a19abc-24