There are 3 Thon Hotels in Trondheim and I chose this one as it was 200kr cheaper that the other ones. I soon found out why. It was so hot in the room that I could not sleep. I think I managed about 3 hours. Had the window open (it opens about 3 inches) which did not help. The next night I asked if it was possible to transfer to another Thon Hotel that a colleague was staying at but this was not possible. He said he slept great as there was central ventilation system. The following night was the same story, even with the fan. I was SO tired when I left the Hotel that I fell asleep on the plane, which I never ever do.
The girl at reception said that it was an old building so it gets warm. The weather at the time was nice - around 18 degrees during the day and a little colder at night.
I would suggest chosing another Thon Hotel (the other one just around the corner was the one a colleague stayed at). The extra money is worth it if you want a good nights sleep. I am sure it is ok in the winter months, but I would not recommend staying here from May to September.
The breakfast was ok (the bacon was super hard and not eatable) and staff friendly. I stay at Thon hotels frequently, but this was, unfortunately, not a good experience.
Trondheim is a nice city and the people are friendly. I would recommend a trip down to Solsiden where there are restaurants and bars. The old bridge is also cool to see the canal from (good for pics).