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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I should have used "upward force" rather than "upward pressure" You're confusing gas pressure with buoyancy (delta-density).

Go back to your cork analogy. If you anchor a cork to the bottom of the ocean, it will strain at its anchor trying to rise.

A submarine works similarly - by reducing its average density, by evacuating its buoyancy tanks, it will rise (or strain against its moorings) even if it's sitting half submerged in the muck on the ocean floor and there is no water underneath to "lift" it.

Similarly, a vertical cylindrical helium balloon, anchored tightly to the ground with a gasketed bottom such that no air can get underneath, will strain at its anchor lines trying to elevate.

Stack effect pressure differentials determine the volume, rate and location of air exchange once one or more orifices are created. But it is different from density differentials, which determine buoyancy (which, unlike gravity which is an ill-defined field, is actually a simple vector force).

And you contradict yourself:

"In fact the pressure from anywhere inside that sealed home wrto (with reference to outside) will be zero."

Hence there is no pressure differential between inside and out.

"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."

If there is no pressure differential between inside and out, there is no driving force to "push" cold air in - air moves laterally only by pressure differential, and you already stated that the inside and outside pressures were the same and hence nothing to equalize.

And, we also know that no air flows into a container without an equal volume of air moving out. So either the warm and cold air moved simultaneously or there was no movement at all.

Robert said: "There is zero time lag between cold air falling into a warm container and the warm air rising"

Zero time lag? I have read that air pressure equalizes almost instantaneously....

....some say at the speed of sound...

But this isn't a case of increasing the air pressure in a container by pumping air into an entry port and having it propagate as a wave front across the container until pressure equilibrium is reached.

This is one mass of air displacing another.The cold air cannot fall into the space without an equal volume of warm air displacing upward.

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.

The fact is Heat does rise. Take a lighter and hold your hand beside and under, then above, ouch, that heat is rising, no cool air behind it pushing it upward. That is the rule inside, outside, on the top of a mountain and at sea level. The variable that is missing in the conversation is the reason it is so difficult to try to change the perception of a homeowner, there are many systems that are manipulated when you are in a home with different building techniques from one to the next. What we have created is getting closer and closer to a vacuum making the physics less likely to behave like they do in nature. 

I get why you are bringing this topic into light, however until the building industry adopts the same processes in every home built and every retrofit that takes place, meets and exceeds that process, it is not necessary to try to explain in layman's terms the concept that higher pressure moves to lower pressure since generally in a retrofit home, heat is actually rising due to the location of the pressure plain. 

Michael, thanks for joining the discussion.  If I may, and no disrespect intended, make a correction.  As Allison Bailes would say, and did in his blog,' heat can move in any direction, but we usually see hot air rising.'  But it is the cooler air around it that is pushing the hot air up.  If you have read a few of the recent posts then you would have seen my example of the cork and how it goes bobbing to the surface of the water.  The cork weighs less than the water it displaces and so does an area of hot air, although it isn't packaged as neatly as a cork.  Neither the cork nor the hot air have any internal moving force that moves them up, that force comes from the surrounding more dense fluid.

Some have called this just semantics, but it is a necessary aspect of our ventilation process.  We are still a long ways from a vacuum, but you are correct that homes are getting tighter and actions like turning on a fan are no longer just air going out, but "where?" is it coming in.  Think back to your energy training.  Did they tell you anything about how stack effect comes about or how to calculate it.  My courses all said the hot air rises and pulls the cold replacement air in behind it.  Should I want to find the NPP I should open a few windows and test with the back of my hand.  We deserve more than that and our customers do as well.

PS, introducing any of this to our customers would definitely be a later step if at all.   We need to get the energy practitioners on board first.


We can debate all day about whether hot air rises or cold air falls (the answer, of course, is both simultaneously with neither being the cause of the other).

But you repeated one of the most common misconceptions about heat. As we know, heat moves spontaneously only from warm to cold and it moves by three common mechanisms: conduction, radiation and convection (there's also evaporative heat transfer).

In conduction and radiation, when only the heat (kinetic molecular energy or photon energy) is moving, it moves isotropically - or equally in all directions. It is only in convection - the movement of relatively warm fluids (gas or liquid) that the heat moves with the substance and that substance moves upwards because it's less dense and less effected by gravity (more buoyant).

In a dynamic fluid displacement by buoyancy scenario, buoyancy can best be modeled as a vector force that is in opposition to the force of gravity, and in which a less dense object rises because its associated buoyancy force is greater than the force of gravity.


Beyond Archimedes' principle

Archimedes' principle is a fluid statics concept. In its simple form, it applies when the object is not accelerating relative to the fluid. To examine the case when the object is accelerated by buoyancy and gravity, the fact that the displaced fluid itself has inertia as well must be considered.

This means that both the buoyant object and a parcel of fluid (equal in volume to the object) will experience the same magnitude of buoyancy force because of Newton's third law, and will experience the same acceleration, but in opposite directions, since the total volume of the system is unchanged. In each case, the difference between magnitudes of the buoyancy force and the force of gravity is the net force, and when divided by the relevant mass, it will yield the respective acceleration through Newton's second law. All acceleration measures are relative to the reference frame of the undisturbed background fluid.


Atwood's machine analogy

Atwood machine analogy for dynamics of buoyant objects in vertical motion. The displaced parcel of fluid is indicated as the dark blue rectangle, and the buoyant solid object is indicated as the gray object. The acceleration vectors (a) in this visual depict a positively buoyant object which naturally accelerates upward, and upward acceleration of the object is our sign convention.

The system can be understood by analogy with a suitable modification of Atwood's machine, to represent the mechanical coupling of the displaced fluid and the buoyant object, as shown in the diagram right.

The solid object is represented by the gray object

The fluid being displaced is represented by dark blue object

Undisturbed background fluid is analogous to the inextensible massless cord

The force of buoyancy is analogous to the tension in the cord

The solid floor of the body of fluid is analogous to the pulley, and reverses the direction of the buoyancy force, such that both the solid object and the displaced fluid experience their buoyancy force upward.

As can be plainly seen in this descriptive analogy, the displaced water volume has neutral buoyancy (does not move on its own) and is pulled downward by the less dense object which has positive buoyancy moving it upward (greater upward vector force than downward vector force).

There are many ways to objectively, accurately, appropriately and scientifically describe any given phenomenon. None of them are right or wrong - just different ways of looking at the same thing.

Robert, just wanted you to know I didn't go away, just pondering and it's 3 am so it is affecting my sleep.

Just a quick thought and I haven't even gone over your last post, but, when a submarine empties its ballasts it changes it buoyancy, we agree.  But, if it were to use it's compressed air system to increase the atmospheric pressure within the vessel there would be a greater outward pressure on the shell, but no change in buoyancy.  The total air would weigh the same compressed or released.  The sailors might not appreciate it, but it wouldn't get them off the bottom.  As for how the buoyancy forces pressing in on the sides of a vessel, but can't get under it, how does it result in an upward force, that is part of my pondering.


It seems that they way you're looking at this is part or your problem. Buoyancy force is a vector force that has only one direction: up. There are no "buoyancy forces pressing in on the sides of a vessel".

You may also be confused by the vernacular notion of pressure. From Wikipedia:

Pressure is a scalar quantity. It relates the vector surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors: P = F / A.

It is incorrect (although rather usual) to say "the pressure is directed in such or such direction". The pressure, as a scalar ratio, has no direction.

In other words, when any mass rises or sinks due to gravity and differential gravitational (inertial) mass (buoyancy), the only force acting on it is one pointing either towards or away from the center of the earth (down or up). There are no other forces. Fluid pressure is not a force.

I was doing some unrelated research and stumbled upon this graphic from the National Institute for Building Sciences:

Stack effect in cold and hot climates

I guess they must be confused as well ;-)


Just another example of a convenient shortcut with the wording and there are many of them.  Another example is your statement "Buoyancy force is a vector force that has only one direction: up. There are no "buoyancy forces pressing in on the sides of a vessel"."

The single vector representation is just the resulting sum of all of the vectors as stated by:
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

"the center of buoyancy is at the center of volume because it is essentially the average location. If you add up all the little bits of force on all the little bits of surface area, the effect is the same as applying all these bits of force at the center of volume."


Note, he stated "surface area", as buoyancy forces are applied from the outside, not internally.

There are definitely forces pressing on the all sides of any submerged object.  That someone chooses to label the up component of these forces as the buoyancy force does not mean it is the only force being applied.

As for "National Institute for Building Sciences" there is nothing wrong with their labels for their representation per se, it is just without the qualifiers, people like you take them literally and suddenly believe warm air can rise all by itself.  Oops, did someone just throw a cup of coffee at me? :) :)



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