The silver ring below the black plastic secures the thermostat to the valve. If you loosen that screw, it will release pressure on the valve which in turn opens -> heating starts. It is rather tricky to fasten that ring again unless you can turn the thermostat to MAX though. Is the silver ring tight right now?
Not KDE but might be similar: For the MATE desktop it is
mate-screensaver-command --inhibit
. I would expect something similar for KDE.