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.


Views: 8134

Reply to This

Replies to This Discussion

Oh, and Dr. Ken Mellendorf's language wasn't also just "a convenient shortcut with the wording"? This becomes obvious in his highly strained attempt to explain why the upward buoyancy of a column on the bottom is "caused" by water pressure:

As for the pillar on the bottom of a lake, there is still water within the material at the bottom. Even a pillar submerged in the sand has water pressing against its bottom. Water also presses on the sand from all sides, which then "transfers" the effect to the bottom of the pillar. Although sand molecules do not exert water pressure in the strictest sense, they do exert a normal force that has the same effect. 

Your problem is that you have latched onto one "convenient shortcut" which gave you some new insight and insist that it's the only "true" description while all the others are wrong.

All of science is mere metaphor to describe in abstract terms what we experience directly in life. And, as in all metaphorically or even "objective" descriptions, the truth of a description depends upon the perspective.


"When it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images..."

- Niels Henrik David Bohr (1885 – 1962) Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922


"The outstanding achievement of twentieth century physics is not the theory of relativity with its welding together of space and time, or the theory of quanta with its present apparent negation of the laws of causation, or the dissection of the atom with the resultant discovery that things are not what they seem; it is the general recognition that we are not yet in contact with the ultimate reality. We are still imprisoned in our cave, with our backs to the light, and can only watch the shadows on the wall."

- Sir James Hopwood Jeans (1877 – 1946) English physicist, astronomer and mathematician


"What we observe is not nature itself, but nature exposed to our method of questioning."

- Werner Karl Heisenberg (1901 – 1976) German theoretical physicist and philosopher who discovered in 1925 a way to formulate quantum mechanics in terms of matrices, for which he was awarded the Nobel Prize for Physics for 1932, and who developed the uncertainty principle in 1927


"Science is the belief in the ignorance of experts…A great deal more is known than has been proved…Scientific knowledge is a body of statements of varying degrees of certainty – some most unsure, some nearly sure, but none absolutely certain."

- Richard Phillips Feynman (1918 – 1988) American theoretical physicist and Nobel prize recipient for his contribution to quantum electrodynamics, named one of the ten greatest physicists of all time


"So much of science consists of things we can never see: light 'waves' and charged 'particles'; magnetic 'fields' and gravitational 'forces'; quantum 'jumps' and electron 'orbits.' In fact, none of these phenomena is literally what we say it is…The words we use are merely metaphors."

- K. C. Cole (b. 1946) science writer and professor at USC Annenberg's School of Journalism (described as “the Leonardo da Vinci of science writing”)


From Archimedes, On Floating Bodies, Book I:

"Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."

He did not relate this buoyant force to water pressure.


Proposition 6: If a solid lighter than a fluid be forcibly immersed in it, the solid will be driven upwards by a force equal to the difference between its weight and the weight of the fluid displaced.”

He related the buoyant force only to the differential weight, i.e. the differential result of the force of gravity.


Proposition 7: A solid heavier than a fluid will, if placed in it, descend to the bottom of the fluid, and the solid will, when weighed in the fluid, be lighter than its true weight by the weight of the fluid displaced.”

If its density is less than that of the fluid, the apparent weight would be negative. Archimedes, again, defined buoyancy as the difference in the "true weight" and what we today call the "apparent weight" of a submerged object.

Buoyancy is a function of the effective weight, or result of a gravitational field, on an object within a fluid (this is just as true in air as in water).


What is the buoyant force on a block at the bottom of a beaker of water?

Carl E. Mungan, Assistant Professor of Physics at the U.S. Naval Academy in Annapolis,

Maryland. http://www.usna.edu/Users/physics/mungan/Publications/FEd3.pdf

"I propose that buoyant force be generally defined as the negative of the total weight of the fluids that are displaced, rather than as the net force exerted by fluid pressures on the surface of an object. In the case of a body fully surrounded by fluids, these two definitions are equivalent. However, if the object makes contact with a solid surface (such as the bottom of a beaker of liquid), only the first, volumetric definition is well defined while the second definition ambiguously depends on how much fluid penetrates between the object and the solid surface."

You're also misunderstanding your good professor's description. While it's true that it's only the resultant buoyancy vector that is at the center of mass, all buoyancy vectors act in the same direction: up. We use the same center of inertial mass to mathematically describe gravity, even though gravity effects every part of a mass.

But both gravity and buoyancy (the opposite of gravity) operate on all parts of the mass, not on surfaces. The professor contradicted himself when he acknowledged that buoyancy can also create rotational movement, which depends on the distribution of mass/density within the object.

He was straining to explain the buoyancy of a column resting on the bottom of a lake because his (and your) theory don't apply. A more comprehensive explanation is that buoyancy is a relative gravitational effect. A less dense object resting on the bottom of a body of more dense water (which would, obviously, require some form of restraint) will rise when unrestrained because, relative to the surrounding water, gravity has less effect (less "hold") on it.

In other words, buoyancy is to gravity as centrifugal force is to centripetal force: they are mirror images, neither exists without the other, and they are opposite vectors: one down (towards the center) and the other up (away from center).

Let me know when you are done.


I'll be done when you admit that you're wrong in accusing everyone else of being wrong.

But I'm not holding my breath. On this issue, you are coming across as someone with a newly-discovered faith who is determined to convert the rest of the world over to it.

I'm glad that your epiphany helped you understand the stack effect, but you've stumbled upon one path to understanding and are insisting that it's the ONE AND ONLY TRUE PATH.

I'm not going to fight back.  I think you have demonstrated your objection sufficiently and any more would border on stalking.  I will start another thread to continue my discussion and despite your declaration that you will continue until I give up, I hope you don't follow.  I am disappointed in your obsessive lecturing and name calling and will let others judge your actions.  Please stop.



I'm even more disappointed in your obsessive compulsion to declare anyone "wrong" who sees a different perspective or a broader picture than you're capable of.

And you have no right to ask me or anyone else not to participate in any public discussion here.

If you start another thread to propagate this falsehood and slander, I will continue to counter it, as intellectual honesty requires.


Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

Casey Gesell posted a video

Home Comfort Pays for Itself | Ideas Worth Sharing

http://www.myhomescience.com There are 88 million homes in the U.S. and Canada alone that burn twice as much fuel for heating and electricity for cooling tha...
12 minutes ago
Profile IconRob Madden, Solar Home Broker, Alicia Hobbs, Rachel Scott and 2 more joined Home Energy Pros
1 hour ago
Don Fitchett posted a video

HVAC AC Compressors Training

This HVAC training sample is from the HVAC section of the Online Industrial Training Video Library (http://bin95.com/Industrial-Training-Videos/HVAC-Training...
1 hour ago
Sean Lintow Sr replied to Robert Cameron's discussion Air duct testing questions.
"#1 - outside "conditioned" space & really should not be allowed #2 - up to the AHJ if…"
3 hours ago
Susan E. Buchan posted a status
"EEBA's Conference in St. Louis last week was a great success! Presentations at http://www.eeba.org/conference/2014-presentations."
18 hours ago
Jan Green replied to George Kopf's discussion Real Estate Professionals need our support - but how?
"It helps for companies to understand what the public wants and to educate them so that realtors are…"
18 hours ago
Hal Skinner replied to Hal Skinner's discussion An example of what an RCC can do with no R- insulation in the walls.
"Hi Bret and thans for the offer.   I sent you a friend request so I can send you a…"
18 hours ago
Kevin Jordan shared George Kopf's discussion on Facebook
18 hours ago

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service