Diogo Martinho

When I read the answers I understand but then I am trying to model it the question comes up again... So, basically, by specifying the fluxes where/how do you specify the background values. Like the same as having a room where I would have a flux coming in from the door, a flux going out through the window but how would I define the value of the temperature itself? 
Sorry if you already answered and I didnt get it.