We just spent two weeks in Alessandro's apartment, in a heatwave (37-40•), late July. The apartment is small but perfectly formed and there is air conditioning which gave us relief from the heat. It is in the most wonderful neighbourhood in Florence, central yet away from the crowds, near to wonderful restaurants which catered for locals as well as tourists. Do follow Allessandro's recommendations as all the ones we tried were great. Alessandro made us very comfortable and it was a pleasure to meet her.
The only drawback of the flat is that although the street is quiet mostly, when the rubbish trucks or street cleaners go past in the early morning, as they do every day, or even a motorbike goes past fast, it is very loud, and they did wake us up. This is probably less frequent in low season but if you are a light sleeper it is not ideal.