Radiator

I am trying to change a thermostatic valve which is in the bathroom...the problem i'm having is that the rad in the bathroom and one upstairs comes on when the hot water is turned on. How do i go about draining just the two of them? Can someone explain how when the hot water is on that just these two heat up but i have to turn the heating on for the rest of them to work?
Many thanks

Barry

Not much info provided so i am guessing that you have a combi if so your diverter valve is passing to rads slightly when in dhw operation so forget about changing rad valves & call in a htg eng

Not necessarilly a combi some conventional systems with hot water cylinders tee off the coil feed to heat up bathroom rads.
If you drain the heating circuit from the lowest point this should enable you to remove the rad valve .
Pete