Evan, I'm hoping you will comment on this.

Since energy myths are front and center at the moment I would like to discuss a single myth, the science of hot air rising.  Actually, I have for my own purposes upgraded this one to an energy legend, as it has proved extremely difficult to correct.  I'm assuming it is wrong but I'm certainly open to all opinions.

There are several areas in our energy business where this is important, attic venting, stack effect, convection, and chimney draft to name the obvious.  The legend as I see it is that people have observed warm air moving up for so long that there is a belief that warm air has some inert power of its own.  Statements like "the warm air will rise and exit the upper vents and pull the cold air in the lower vents", implies that the warm air initiated that process and as a result not only pulled the cold air in through the soffits, but additionally air from the house as well.  My belief is, the opposite is true.  The cold air pushes its way into the attic and forces the warm air up and out the upper vents based upon the principles of buoyancy.  Here is a simple article by April Holladay that explains this invading cold air process very well is:http://www.usatoday.com/tech/columnist/aprilholladay/2005-02-18-won...

At first glance this appears to be just a simple statement of what everyone sees in the real world.  But the concept that cold air is the driving force becomes important in properly explaining the other, above, modes of air movement.  As energy professionals I believe it is important that we determine the truth about this legend and learn to state it correctly so future generations will not be led to believe that there is magic in warm air.


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Well, you are dancing nicely, but your arguments are losing steam.  If I open a window and the cold air comes flooding in, the warm air on the other side of the room doesn't get the message until after the cold air has entered.

You also missed the cork analogy, as it requires no extra heat and is moved by the same principle, buoyancy.  The forces that move the cork up come from outside the cork and the same is true for the hot air.


Sorry, Bud, but you're the one dancing on hot coals and at some point are going to have to realize that your feet are burning.

The cork in the ocean is not an appropriate analogy, as that is two dissimilar masses at different densities at the same temperature. What makes air become more buoyant than the surrounding air mass is the energy added to it, which increases its kinetic activity (heat), and expands it to become less dense.

When you fire up the burners on a hot-air balloon, nothing has changed in the surrounding air mass, but the air in the balloon is made lighter by adding heat energy, so the only objective explanation is that the heat made the balloon rise. The cause of a motion can only be a change in the system. The only thing that changed was the addition of heat energy.

No scientist, from Newton to Einstein, has ever found a force of gravity. Newton's formulas approximate what we observe to be true, but he repeated over and again that he never found a theory to explain gravity as an independent force and that instantaneous force over distance defied all the laws of physics (Einstein also refused to accept "spooky action at a distance" such as in Quantum entanglement). To this day, no one else has come up with a force theory for gravity. So if gravity is not a force, but merely an observation, then it can't "force" anything to fall or rise, and any objective description of the observed reality is equally correct.

If one description makes you happier than the other, that's perfectly fine, but don't try to tell the rest of us that we're wrong and you're right - you're not.

Do you really believe that a discussing Einstein's difficulty explaining gravity adds anything to this discussion.  Sorry, it is just a smoke screen. 

The cork displaces a volume of water that has a greater mass.

The expanded warn air displaces a volume of colder air that has a greater mass.

They look the same to me in relation to buoyant forces.

And I'm not alone.  The number of people taking an interest in this improved view of air pressures is growing and I'm patient.

Most of the energy auditors out there do not have an advanced degree, but they can certainly follow what is being presented and I'm perfectly willing to send them here to see what kind of dissenting views the professionals have.


If all you have is a hammer...

They look the same to you because you insist on looking at them from one direction only.

Again, the density of a confined air mass changes toward the negative only when heat energy is added causing increased molecular motion and moving the molecules farther apart. Less mass per unit volume means that, according to Newton's Law of Universal Gravitation, that the Earth's gravity exerts less action upon it, and hence the warmed air is let loose by the Earth to rise.

Similarly, because a cork has less volumetric mass than water, the Earth's gravity exerts less of a "pull" on it relative to the surrounding water and so releases the cork to rise away from the center of the Earth's mass in the same way that a helium balloon is let loose to rise.

How you explain the exact same phenomenon depends on whether you're the water or the cork.

And Einstein didn't have "difficulty explaining gravity" (he's the only one who ever has), but no scientist who has ever lived has offered a theory for gravity as an independent force. It's merely something (no one knows what) that is given a name in order to explain why some things fall down and other things rise up (and why both often happen at the exact same time).

Wow. After several month's hiatus, this dormant thread surges back to life!  :)

Bud, for your illustration here, I can't help but note that as soon as the window is opened and cold air enters, warm air directly adjacent to the window is displaced. Warm air clear across the room will be affected as well, but not immediately. But, as I see it, displacement occurs immediately for warm air adjacent to the window.

I think Robert is correct regarding his explanations of displacement. For cold air to enter, such as in your illustration above, warm air must be displaced. This must occur as soon as any equilibrium that may have been present in the room was upset by the window's opening. The cold air entering will of course sink until it reaches the lowest point it can go, where it will remain until warmed. For a typical house in winter enjoying central heating, one must assume that these lowest region surfaces are warmer than the cold air entering the house through the window. And because they are, the cold air that sank will warm up, become less dense, and more buoyant. For simplicity, it begins to rise. No, it does not pull up. No, there is no magic force it contains by itself that creates levitation. Yes, colder air is displacing it. Greater pressure to lesser pressure, always. Greater density to lesser density, yes?

Hi Cameron, probably a poor example as it wasn't well thought out.  I was reacting to the exaggeration that warm air rises by itself and pulls the cold air in after it, which, well I'll be kind, I disagree with.  Apparently you have your doubts as well :).

<For simplicity, it begins to rise.>  It is the best articulation we have, hot air rises, as long as those using it and reading or listening to it understand, not all by itself.  

I'm pleased to hear your understanding, whether it came from my efforts or through your training/education, with homes becoming tighter and tighter, we all need to understand the driving forces behind natural ventilation.  Until everyone can afford mechanical ventilation, we need to know how tight we can go and how much mother nature is providing.

This thread is far too long so once my supporting web pages are ready we will start some more. 

Thanks for jumping in. 


You return again to your straw man: nobody has claimed that "warm air rises by itself and pulls" - though you insist that cold air falls by itself and pushes as the "cause" of warm air rising.

There is no "by itself" in this universe - there are only equal and opposite sides to every coin.

And, since you think that bringing this discussion into the broader context of theories of gravity (which you insist is the driving force) is nothing but "smokescreen", you'll certainly think the same of this from a physics forum:

"According to statistical mechanics, where air has greater kinetic energy it has a greater probability to occupy a higher gravitational potential than less energetic air. That is, the system tends toward maximal entropy."

This is another valid explanation of the phenomenon which eliminates gravity. And since even Einstein believed that the Law of Entropy (the Second Law of Thermodynamics) was the most universal law that will never be overturned,and since it's the primary driving principle of the entire universe since the Big Bang, it is more fundamental than gravity and hence should be considered as a primary explanation.

The idea that, once the window opens, the rising warm air pulls in the cold air, I believe is not quite right. What I'm seeing here is a chicken and egg thing. As for chickens, who knows which came first. But as for which entity caused a change within the house once the window was opened, I'm not sure that focusing on "first cause" is as useful as seeing all of the aspects simultaneously. Our natural tendency is to break it down into a linear sequence of events, but all of the players were already there before we arrived on the scene, and none of them care about our linear thought patterns. Warm air was already in the house, cold air outside. True, you did set a linear chain of events in motion when you opened the window...or would it be better to say you merely accelerated what the house was already experiencing prior to the window opening event? A window is just a big hole when open, lots of small ones when shut.

Regardless, all of the players obey the laws of physics, which in essence is the relentless pursuit of equilibrium in this case. Stack effect is an inequality seeking equilibrium, and as long as the building leaks and a temperature and pressure delta remains across the envelope, equilbrium is never achieved.

But if one could build a hermetically sealed house with no leaks and filled it with warmed (expanded) air, there would be an upward pressure on the ceiling of the house without any inside/out air exchange (the stack effect is caused by height and temperature differential).

In other words, the warm, less dense inside air is trying to lift the house off its foundation without any help from the cold air outside.

Robert said  "In other words, the warm, less dense inside air is trying to lift the house off its foundation without any help from the cold air outside."

Let me remind you what Julius Sumner Miller said

"There ain't no hindu levitation in this business"

That's the point. Miller is a crackpot. Hindu levitation is not involved in this phenomenon, but the house is straining at its anchor bolts without any cold air underneath to lift it up.

Hi Robert, you are moving backwards, but we can work on that :).  The bold is just to make this easier to read, not a flame.

I like your example so let's go over it.

<But if one could build a hermetically sealed house with no leaks and filled it with warmed (expanded) air, there would be an upward pressure on the ceiling of the house without any inside/out air exchange (the stack effect is caused by height and temperature differential).>

When filled with expanded warm air and sealed, there is no pressurization and no upward force at the ceiling.  The ceiling to outside pressure will be zero.  In fact the pressure from anywhere inside that sealed home wrto (with reference to outside) will be zero.  It is a zero pressure container and there is no force from inside trying to push it anywhere.

<In other words, the warm, less dense inside air is trying to lift the house off its foundation without any help from the cold air outside.>

The stack effect number we calculate for a home is an absolute number.  We give it a positive or negative value once we establish the home's connection to the outside.  A single hole at the bottom and it becomes the maximum positive pressure at the top.  A single hole at the top and it becomes the maximum negative pressure at the bottom.  Equal holes top and bottom and we get half that value top and half bottom, positive and negative respectively.

So, is a positive stack effect pressure at the top pushing the house upwards, yes.  But that is not the buoyant force determined by the volume of cold air displaced.  With a single small hole at the top of a house, there would be zero stack effect at the ceiling, but still a large displacement.

<In a super-tight house, an air-lock entry is not necessary to prevent cold air displacing the warm air volume of the house (though there might be some local mixing at the door). In a container that was hermetically sealed except for a small opening near the bottom, there will be no air displacement with the "door" open, as cold air cannot enter unless the hot air moves out of the way and "allows" it to.>

With a small opening at the bottom, the pressure inside has adjusted to locate the NPP at that opening.  However, once you open the front door, a large bucket full (about 5 gallons) of warm air exits due to a positive stack effect pressure inside to allow the NPP to relocate to the middle of the door.  Once the new NPP is established (very quickly) you will then have a positive pressure at the top of the door and a negative pressure at the bottom.  Instead of just local mixing, you will have cold air flowing in at the bottom of the door and warm air flowing out at the top.  Here they will both move at the same time as they both have a pressure differential to move them.

Note:  when you opened a small hole at the bottom, cold air pushed in and pressurized the home until the pressure at the opening equalized with the outside.  The positive stack effect pressure at the front door was due to the cold air that already pushed in.



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