i have a worcester 24i combi boiler and i want to fit new radiator valves?

In this instant I would suggest isolating the boiler ie shut off cold feed v/v,gas & electric. Personnally I would isolate all rads then work systematically thro' those needing v/v replacement remembering that there is system pressure locked in all rads until you vent off as you work thro them and on the first rad v/v,s the pipe work will also be under pressure and contain water dependant on its location,leave water in rads not being worked on saves filling time later.Leave new v/v,s shut off on completion of each rad ensure rad vents are closed.When all v/v,s replaced restore water supply to boiler and using top-up valve put water into CH cct at this stage have a helper fly around to check no leaks present on you new v/v,s if all ok open up new v/v,s allow water to fill rads set pressure to about 2bar and vent off each rad in turn topping up pressure to 1 bar frequently.Open up all rads and vent all until air free ensure pressure is kept up towards 1bar all the time.Restore gas and electric start up boiler and repeat venting when warm

Guytawe
May 2010