Further experiences with the room temperature experiment

I've been trying to live with a room temperature of 20 to 21 degrees for a few weeks now, with mixed results. I almost always feel *slightly* cold now (despite on average three layers of warm clothing), with cold feet and cold hands and a cold nose, which is uncomfortable but not unbearable. On some days it crosses into "so cold I can't concentrate on anything" territory, though, which is obviously bad. Still, the fact that I don't feel like that non-stop, and that most of the time it's only uncomfortable is encouraging. I'm thinking about cutting up an old camping mat to put it under my desk for some extra insulation for my feet against the cold floor...

The bigger problem is that my radiator seems unable to actually keep my room's temperature constant. It seems like every day I have to turn up the thermostat a bit higher to still get to 20 degrees - and only on days when there's a lot of sun shining through my windows will the temperature go to 21 without having to turn the heating up to almost the highest possible setting. What confuses me about this is that for a while - a week and a half or so - the setting #3 seemed enough to achieve 20 degrees - yet when it got colder outside, I had to put it on setting #4, or the temperature in my room would go down to 17 degrees. The way I understand thermostats, this shouldn't be happening: a thermostat is supposed to always keep a room's temperature constant, so if #3 meant 20 degrees two weeks ago, it should still mean 20 degrees now. But here, it doesn't.

So, I think, my thermostat is broken. I will have to talk to my landlord about that.