Rising Heat, Mixing Air

Please help settle an argument

Setting: A two story home with stairway to second floor in the approximate middle of the floorplan. 1000 square feet/floor

The downstairs has air conditioning and is quite comfortable. Temperatures outside are in the 90s. Essentially still air (no breeze).

I maintain that the open windows upstairs are not an issue because it is much warmer up there, and given that heat rises the amount of cool downstairs air that could go “up and out” is negligible, perhaps zero. Therefore, there is no real reason to care that the upstairs windows are open (nor, to be fair, any reason not to close them since it’s so hot outside)

The other party maintains the “common sense above all” view, which is that one closes all the windows when running the AC.

In the current state (upstairs windows open), the AC can easily make the downstairs uncomfortably cool.

Even without breeze, if there’s a meaningful temperature difference between the upstairs air and what’s outside, you could expect some air circulation which could have some effect downstairs.

Nitpick: It’s more accurate to say that warmed air rises; it does so because it has become less dense.

Examples of heat descending are easy to find, e.g. hold a 4-ft long steel rod vertically and apply a torch to the upper end.

Why not simply get the numbers?

Thermometers are cheap; you can track the temperature both ways in both locations and you can read the electric meter and see if you’re using more electricity either way.

Would the “other party” be your spouse?

Heat flows from higher temperatures to lower, always. After you turn the A/C on and reach some steady state, there will be a steady heat flow from the upper floor to the lower, down the temperature gradient. That heat is supplied by heat entering the house on the upper floor (assuming for the sake of argument that the downstairs is perfectly insulated), and removed by the A/C on the lower floor. The only question is: does heat enter the upper floor of the house faster or slower if the windows are open? The answer is obvious: faster. Open windows allow the heat from the outside to enter the house faster, since it can be transported in by small air currents. (Transfer of heat by conduction or radiation is much slower.) So if you keep the windows open, the A/C will work harder to maintain the same temperature downstairs, or equivalently for the same electric bill will not be able to keep the house as cool. In principle you might be able to detect this: for the same downstairs temperature, the temperature upstairs will be higher if the heat flow is larger. Fourier’s Law says the rate of heat flow is directly proportional to the difference in temperature.

It is not true that “heat rises.” Heat flows from higher temperatures to lower, whether that be up, down or sideways. What is true is that portions of a fluid (e.g. air or water) that are hotter then their surroundings will usually* rise, and the rising fluid can transfer heat efficiently. For example, if you heat a pot of water, the hot water at the bottom of the pot becomes less dense than the cold water just above it. It rises up to the top of the water, where it transfers heat to the air above the pot (which is at a much lower temperature than the water).

  • I say “usually” because there are circumstances where heating causes substances to contract, which would make them sink. The most common example is water. Between 0 and 4F the density of water increases, meaning if you heat water at 0 up only a degree or so, it becomes more dense and sinks. The other interesting example is silica, which is the bulk of what makes up ordinary crustal rock. The density of molten silica also decreases slightly just above the melting point.

What’s the temperature upstairs? If it’s lower than the outside the open windows will lead to more heat transfering from outside to inside, so you should keep them shut. If it’s higher, then the open windows will help cool the upstairs.

It all depends on how the sun and the surrounding environment is heating your house.

The real problem here is that this 2-story home is lacking cold-air-return vents in the upstairs floor*.

If it had them, they would take cold air (in winter) from upstairs, and return it to the furnace for re-heating; and (in summer) take warm air from the upstairs and return it to the AC for re-cooling. Thus in both seasons, keeping both levels of the house at a more consistent, comfortable level.

Older houses commonly lack these return vents – they were built before AC was common, and they aren’t as needed for heating because the hot air rises to the 2nd floor. That means the furnace has to run more to keep the lower floor at the desired temperature, but when these houses were built, nobody cared because energy was so cheap.

  • Also probably has insufficient insulation in the attic, and poor attic venting.

So, is there an inexpensive way to remedy this? I have a similar problem (home built in 1913, originally with gravity furnace converted to forced-air). The lack of return ducts upstairs (plus the insufficient insulation) means that the upstairs stays substantially warmer than the main floor.

Depends on what you consider inexpensive. And just how your house is built.

In my house, we were able to find a vertical 16" cavity between 2x4 studs that goes uninterrupted from the basement to the ceiling of the 2nd floor. Both the Heating/AC company & the builder say that we can use this space as ‘ductwork’ without tearing out the plaster wall and actually installing metal ducts inside it. So we could cut holes near the ceiling on either side of that wall cavity, which would open into one bedroom and the upstairs hallway landing. Then down in the basement, we would just have to install ducts taking air from that wall cavity connecting to the furnace return air ductwork. If we can figure a way to fit that ductwork into the mess of plumbing, electrical, etc. through the basement, we’ll go ahead & do it.

The reason air conditioning is called “air conditioning” is because the air is … conditioned.

I.e., it is not just cooled but humidity is extracted. You don’t have to set the thermostat as low if the humidity is lowered. Leaving windows open if the outside air is at all humid while running the AC is very foolish.

The OP posts a complicated question. Several different things are going on and how they compete isn’t easy to say.

My guess is the upstairs windows being open does not make the downstairs warmer, and probably improves upstairs conditions by letting out the air getting heated through the sun-baked roof as well as creating a breeze.

But it depends. There is a chimney effect here (though a downward one). The cold air in the downstairs area is cooler and denser than the air outdoors, so it wants to run out, like water would if the house was full of that. If the downstairs doors and windows are leaky, but the upstairs is very tight, it might be that keeping upstairs windows closed will keep the cold air from falling out of the house, like in a siphon.

Carl Pham points out that heat flows from hot to cold areas, but that’s only in the case of the conduction mechanism of heat transfer. Big spaces with air in them transport a lot of heat on the basis of air flow. Whatever makes the air move also moves its heat content with it. You have to know how air flow throughout the house behaves to be sure what’s going to happen.

But if it were my house I’d go with the guess that open windows upstairs don’t make the downstairs warmer.

Thanks for all the thoughts.

This is what I was looking for, some thoughts on how the different dynamics play out. There is no meaningful spousal dispute here, if either of us really cared he or she would get his or her way. It just came from one of those “you know the windows upstairs are open?” “Yeah, so?” “Well, you’re running the A/C…” conversations.

My thought was that, based on the very noticeable and sharp gradient as one walks up the stairs, very little mixing is going on and therefore the windows don’t matter. So I wondered what others thought. Contrast that with the basement: When the basement door is opened, one can feel the cold air pouring down the stairs.

For the record, of course I know that heat moves towards cold, but warn air rises because it’s less dense. But I reject pedantry when it comes to common phrases like “heat rises.”

We don’t have enough extended hot spells to repeat the experiment with thermometers or compare electric bills.