Turn down the thermostat. Unscrew the air vent, soak it in vinegar to clean it, then turn the thermostat all the way up. After a few minutes, you’ll hear a hissing sound followed by a little bit of water spurting out. Finally, steam will start exiting that hole. Turn off the radiator valve and replace the vent. It should be straight up and hand tight. You should not need or use a wrench.