The heating season has definitely started, and to keep your European radiators working properly, you need to vent them regularly. This is the procedure for most radiators in Belgium, Germany, and the UK, please check with your landlord if you live in a different country.
This is how it’s done:
Have your vent key (see picture), an old mug or similar and an old towel ready
Turn on all of your radiators and wait about 30 mins.
Start at the top floor of your house
The venting valve should be on the opposite end of your heating valve. Place the old towel underneath the venting valve on the floor in order to protect it, and hold the mug right underneath the venting valve.
Use your vent key to open the venting valve for a quarter to half a turn to the left (counter-clockwise). DO NOT TURN THE KEY TOO FAR!
You will hear a hissing sound as the air comes out, then eventually some water will leak out (into your mug at the ready).
Leave the valve open until the blubbering noises have subsided, then close the valve with a clockwise turn of your vent key.
Repeat this procedure for all other radiators in your home, going from the top of the house to the bottom.
If you have let out a lot of air, it might be necessary to refill the water in your heating system.
You should do this at the beginning of the heating season, and then about once a month until spring.
You can also check the internet on lots of how-to videos.
Here is some information on how to check and top up the pressure on a boiler.