To prevent damages in the Central Heating System, it is important to vent your radiators frequently. If you want to do this, you will need a radiator key and a towel. To get the key, you can ask Gruno Verhuur. Not sure how to vent a radiator? Please check the following instruction:
If you are still not sure how to do this, check the following video of Energiewacht (in Dutch).
After the procedure, check the pressure of your boiler. The pressure needs to be between the 1 and 2 bar. Is the pressure too low? Then you need to refill your Central Heating System.