is it a standard valve or thermostatic? ifs a standard white valve, remove the white plastic head then there may be a brass nut tighten it and see if helps. if it doesnt u will need to drain the water from the system. also turn of the boiler. un screw it and fit the new one. then refil the system. u will need the right tools spanner, allen key if needed, ptfe tape. here is a youtube guide.
http://www.youtube.com/watch?v=0cqDY9H_XO4
kash, December 2012
