The Worcester Bosch compact remote pressure relief valve kit is compatible with the Greenstar 4000, Greenstar compact combi, and system boilers. It is supplied with a G 12. 7 mm thread and a 15 mm compression fitting adapter elbow and O-ring. The pressure relief valve helps to control pressure in a closed system by proportionally opening to relieve excess pressure, preventing catastrophic malfunction. The Worcester Greenstar 4000 Remote PRV Kit enables the PRV to be fitted outside of the boiler, improving flexibility of boiler siting. Perfect for central locations such as airing cupboard installations. Compatible only with the Worcester Greenstar 4000 and Compact boiler ranges.