Disclaimer: This is not homework, just some project I'm working on.

Say I have a room full of air. I have control over a valve which lets air into the room and another valve which lets air out.
The air which comes in is quite cold, so I want to make use of this temperature to cool down the atmosphere in the room. In other words, when it's too hot inside the room, I want to let out some of the hot air and take in some cold air, without changing the pressure inside.
How should I calculate the amount I have to let in/out? Can use sensors for just about anything, inside temp/pressure, intake air temp...