smoboBonafideI put my window open at a crack, and turn on the heating around 22c, if I leave the heating on without opening a window once in a while the air will get sooo dry
what you're doing is both wasting electricity and making the air drier.
outdoor winter air at a temp of -10C can hold one tenth the moisture that room temperature air can hold. Doing some rough comparison, if you left your window open and heated the outside air up to room temperature, the expected humidity% would be around 5-8% in the winter. that is much lower than any comfortable level and certainly wouldn't be an improvement from where you're at.
stop doing that
This is not how it works. You are assuming that opening the window to a crack will somehow replace the entire air in the room, then heat that replaced air without replacing it by any new air. Furthermore, precisely because hot air can hold much more humidity, hot radiator air will feel dryer than fresh air from outside, even if it contains more water.
Lastly,
if you left your window open and heated the outside air up to room temperature, the expected humidity% would be around 5-8% in the winter. that is much lower than any comfortable level and certainly wouldn't be an improvement from where you're at.
this implies that the air in the room at any given point in time would come from anywhere other than outside and magically have a much higher water content. But if it didn't come from outside, it would be all CO2 and he would die. The only special extra way water could be added to interior air is through hot showers, which probably don't take place directly next to his room. This also happens outside in the form of rain.