You should have asked last night, I would have given you the same answer. 
You wouldnt have had to freeze then!

Thank you very much for all your advice, you’re a life saver! I rather freezed the past days than look into it but yesterday night was too cold haha

Okay, open it to the max and let it circulate. 
If you have to, open some windows. 
You want to clean your piping out if you can. 
If there is some sediment, it could mean that your heater wont work as efficiently. 
Also air will make it less efficient. 
Let it run at max for an hour or so if you can. 
Dont be frightened if there is gurgling sound. 

Satosora

Shouldn't my heater work if I can move the valve?

I am on the fourth floor. I have put it on full power now and water is pouring in. The heater is definitely getting hot, maybe it just won't get as hot as possible with some air in it.