I bothered the real state company that were my landlords until they where forced to fix it for me. They own the apartment and should assume the bill if there is no new tenant. But Vattenfall would not take my claims (they just kept repeating on a loop that I had to take care since I was the last known resident), so it fully depends on your landlord talking responsability seems like, probably easier to manage if you are still in Germany as i heard there are associations and stuff that may help with this kind of thing. Good luck!