Aha! I had the same difficulty and had to do some investigation to figure this out.
First, you can change the heat setting by going to the hospital map menu (the green icon in the menu bar). There you can view radiator coverage (click “Toggle radiators”) and also adjust the heating.
Second, I noticed that even empty buildings affect this, so I got in the habit of placing radiators as soon as I’ve bought a new block. Then when creating rooms you can either place new radiators or move the existing ones (but make sure to move them before trying to place the room!).
Finally, I cheated a little and did some reading. Apparently the radiator coverage map matters, but also the temperature of each room is just the mean, i.e. the number of radiators divided by the room’s size: so essentially you want to put more radiators in large rooms. I’m not sure how they knew this, or exactly what ratio to use, but I started putting a second radiator in every room which was larger than about 25 cells, and this seemed to work.