3 bar 1/2 inch BSP female x female pressure relief valve.
Colour of plastic top may vary from picture (makes no difference to operation of valve).
Most domestic central heating installations and equipment including combi systems are designed to operate at a pressure below 3 bar. To protect the equipment, including boilers and radiators, a safety valve should be fitted to prevent pressure in the system exceeding 3 bar.
The outlet from a pressure relief safety valve should be positioned in a way so as to not scald people or animals with escaping hot water or steam.
** BSP thread sizes are measured a special way - have a look at our information page for more details **