It is used to automatically remove trapped air from a radiator, eliminating the need for this to be manually done. It has a shut off cap that can be used to stop auto venting if so required. Size: 1/2″ BSP. Product has got 3 operating modes: 1. Middle knob fully screwed – automatic venting (it will shut off automaticaly when there is no more air in the radiator). 2. Middle knob partially screwed – vent open. 3. Middle knob unscrewed – vent closed