So last December a plumber came over and noticed that the tubes going to the radiators of the central heating were connected the wrong way. I was very pissed, mostly because we didn't notice it earlier. Today, finally, it got fixed. Luckily, the fix was not as expensive as I expected, but still not very cheap. I had to borrow the money from my dad, which I really do not like.
However, it's fixed now and I expect the result noticable on my monthly statement for gas.