Hi all, as above is it possible to install TRV's without draining the system? I got two radiators downstairs without TRV on them. My water and heating is run by a combi boiler. Each radiators got two lockshield on them so the only way for me to open/close the rads is by manually opening one of each lockshield, and I do this everyday, everytime. I'm hoping by installing TRV's on them would help me regulate the heating. I am thinking to do one of the rads first as the other rad is not being use anyway. Thank you in advance. Any help and tips would be appreciated.