If hot air rises and cool air falls, how does an open window downstairs end up cooling the upstairs in winter?


If hot air rises and cool air falls, how does an open window downstairs end up cooling the upstairs in winter?

In: 7

Hot air rises and cool air falls, but hot air and cool air next to one another also mix together and warm/cool one another to some extent. The rising/falling effect is not strong enough to resist mixing from typical air currents.

As people move around the building, and as random air currents flow, the hot air upstairs gets mixed with cold air flowing in from downstairs. It’s also directly cooled by contact with the downstairs air.

Even though the hot air will tend to be displaced upwards, the downstairs air is going to be in direct contact with the outside world through the window. This will cause the cool air to chill further (Bearing in mind that the outside air is likely colder than the “cold” air inside. And the warm air is resting on top of that cooling air, which will tend to swap warmth from that volume of warmer air.

Essentially, there is a thermal conduit from the warm upstairs air and the cold air outside.

Houses aren’t completely airtight. Chimneys, bathroom vent fans, leaky can light fixtures, even electrical sockets can leak air.

Think of your house full of warm air like a plastic soda bottle completely filled with water and the cap put on it. If you poke a few small holes in the bottom, they will dribble a bit but you won’t lose much water in the bottle. Now take the lid off the bottle.

Similar to opening a downstairs window, you’ve now made a path for “makeup” air. Water will stream from the holes at the bottom, and the total volume of water will start to go down. In your house, warm air will stream out through upstairs vents and fixtures, and cold air will rush in through the downstairs open window to replace the lost air upstairs.

Yes, it’s an imperfect example, largely because the difference in density between water and air is much bigger than the difference between warm air and cold air. Air mixing as others have posted does contribute to things, but a lot of the overall issue is hot air rising up and out of your house, assisted by an opening at the bottom of that warm air. The same effect is what makes tall smokestacks more efficient than just a plain opening.

Hot fluid rises and cool fluid falls… assuming there are no other forces acting on them but gravity.

In addition to gravity, hot fluid has a higher pressure than cold fluid, so hot pushes out into cold. If this was the only factor it would eventually cause homogeny – every bit of the fluid would eventually be an identical mix of hot and cold particles.

Of course, hot and cold particles that touch each other will share heat and become two in-the-middle warm particles.

Also, most fluid has current. In the case of air we call that the wind. Winds are produced whenever anybody moves, opens doors, etc. The wind mixes up the hot and cold like stirring while cooking food.

When all the windows are closed, your home is largely sealed-off from all the outside air. This slows air flow, because no air can escape this confined area, so when pressed on it can only press back. As soon as even one window is open, this pressure can go out of the house, so air flow dramatically increases.