Venting step by step
If you are a homeowner, check if there is enough pressure in the system and then turn off the circulation pump or the heating. If you are a tenant of an apartment, you probably do not have access to these switches, which is not a big deal. The process changes only minimally. Simply start with the radiator that is farthest from the system and check it again after the process is complete.
To allow all the air to collect in the radiator, wait at least 30 minutes, sometimes an hour is recommended.
Now turn the thermostat of the radiator to the highest level.
Put the square wrench on the radiator and place the container so that the water is collected. If necessary, wrap the towel around the valve.
To allow the air to escape, carefully open the valve. The air can be very hot, so be sure to open the valve in a controlled manner and with sufficient distance.
If liquid comes out of the radiator and the hissing becomes quieter, the air has completely escaped. Close the valve as soon as possible to avoid losing too much water.
The process is as good as finished. Turn on the heater, or the circulation pump, and check the water pressure. The escaping water may cause the pressure in the system to drop and you may need to add more water. The pressure can be read on the pressure gauge, and the ideal range is usually marked in color.
Now their radiators should warm up silently again. If this is not the case and your system remains cold, contact a professional to analyze the problem.