Thanks for the input on what to call the “error” term. “Delta” isn’t a bad suggestion but maybe just calling it “remainder” as we have been doing here is actually the most elegant way to describe it. I’ll revisit this when I finally get the chance to implement a “construction storage” term.
FYI, what I was trying to explain above is that you should have at least 1C difference between your indoor cooling setpoint (27 C) and the max_in_temp_ (aka. the indoor temperature at which windows open). Otherwise, you’ll have both the cooling system on and the windows open within the same timestep, which is what’s causing your air heat balance equation to be all wacky.