This was an unpleasant experience for us as well.
This is not the first time we rent the apartment out in the winter and if you scroll the comments you can check it. I have never had complaints about the heating, never. Nevertheless, I did all I could to make the guests comfortable and sent the handyman to see what was wrong and asked him to modify the settings in order for the heating to be on 24h per day which is by the unlawful. Then they complained again and when I suggested to send the handyman again, they refused him in. They were telling me every day that they would leave and I told them that they were free to do so and I also wished them the next apartment would fit their expectations. When they left (normal departure day), the cleaning lady told me that the temperature in the apartment was up to the sky, higher than the 20 degrees that were set and the balcony door was open. I checked the gas consumption and they spent 7 m3 in six days which never happened with any other winter guest before.
As for the upper room, it corresponds truthfully to the pictures: you can clearly see that it's small and you can clearly see the "hole" in 2 pictures, which is a circle to make the air flow.
To read this kind of comment after all the effort I put in making them feel welcome and considering that the rent is more or less 35 euros per person per night is really disappointing.