danryan.us

Let's think (and write) before we draw. What are the stocks here? Really just the warmth in the house. What are the flows? When the heat is on, warmth gets added to this stock, and all the time warmth is removed by the leakage due to poor insulation and the conduction loss due to cold outside. What is the information? The thermostat can measure the current temperature and it knows the set temperature. And how is time involved? The measured temperature of the house probably does not change immediately as soon as more warm air is added.

There are different ways of incorporating time. Sometimes it is just shown by sketching two short lines across an arrow in a loop. Or, in a stock and flow we can add an extra stock (air on one side of the room, air on the other side of the room with time implied in the step between when a volume is one side and when it gets to the next — the school and organizing army models below incorporate time as "stages" in this manner). In equations we sometimes sneak delay in by having, say, $F_{t+2}$ depend on both $F_{t+1}$ and $F_t$.