heating system (plumbers?)

[evesdad]

You can "usually" change a rad valve on a combi system without draining the whole system. Once you have let the pressure off, as long as you only have one open end (the pipe you are changing the valve on) air cant get into the system so the water cannot fall out.  Note I said usually if the auto air vent on the boiler opens or some worcester boilers with a non return valve lets in air you may get wet!