In the winter are you letting the cold air in or the warm air out when you open the door
I wanted to answer "Yes." But BobPursley explained it to me this way:
"No air can come in/out, without an equal volume coming out/in. You are letting HEAT out.
"Cold is the absence of heat. So heat goes out, and absence of heat comes in."
When you open the door in winter, you are essentially letting the cold air in. This is because the surrounding air outside is colder than the air inside your home. The warm air inside your home seeks equilibrium with the colder air outside, so it naturally flows out of your home through any openings like an open door. At the same time, the cold air from outside rushes in to occupy the now vacant space, causing the overall temperature inside to drop.
To understand this concept, you can experiment with a simpler scenario. Take two cups of water, one filled with cold water and the other with hot water. Place them side by side and leave them undisturbed. Over time, you will observe that the heat transfers from the hot water to the cold water until both reach an equilibrium temperature. Similarly, when you open the door, the warm air inside seeks equilibrium with the colder air outside.
This process can be explained by the principle of thermodynamics, specifically the second law of thermodynamics, which states that heat naturally flows from hotter regions to colder regions until thermal equilibrium is reached.