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 in - automatic venting (it will shut off automaticaly when there is no more air in the radiator).
2. Middle knob partially screwed in - vent open.
3. Middle knob unscrewed - vent closed.