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Community and Q&A

Yes, Hot Air Rises

homedesign | Posted in General Questions on

I stumbled on this ancient video recently and heard a sort-of contrarian statement:
“don’t EVER say that hot air rises..because that AIN’T so”
(just watch the first 1:30 minutes)

“What happens is that the less dense air is pushed up by the colder air”

I had never heard it put that way …hmmm…. very interesting

So yesterday Allison Bailes Posts a Blog on the same subject.

Heat Rises…and Falls — Stack Effect, Air Movement, & Heat Flow

A good the way

And this “Heretic” (Bud Poll) comes along saying almost the same thing as Professor Sumner Miller
and he posts a link to his “worksheet”

I like Bud’s words and illustrations … they are helping me to finally undserstand Stack Effect

Thank You ……Bud Poll

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  1. GBA Editor
    Martin Holladay | | #1

    The version of this old chestnut that I have heard is, "Don't say that heat rises, because heat moves in all directions, from hot to cold. Instead, say that hot air rises."

    I think that Julius Sumner Miller's distinction -- "you should say that less dense air rises, and denser air sinks" -- is also true, but in no way disallows the equally true statement, "hot air rises."

  2. homedesign | | #2

    Martin, I could not find your Julius Sumner Miller Quote in the video...I also listened to part 1
    Where did you find your quote?

  3. GBA Editor
    Martin Holladay | | #3

    I was supplying a paraphrase of the same statement that you used to open this discussion. His exact words are, “Don’t ever say that hot air rises, because that ain’t so …what happens is that the less dense air is pushed up by the colder air.”

    It's from this video:

    Commenting, I said that Julius Sumner Miller's distinction (between dense air and less dense air) is true, but in no way disallows the more common distinction between hot air and cold air.

  4. homedesign | | #4

    Martin, I thought you might be paraphrasing.
    Judging by the way you paraphrased ... I don't think you are grasping his point.

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

    I think it is intresting that you mentioned the word "chestnut"
    It took you quite a while to realize that J Chesnut did not have a "T" in the middle of his name and longer still to realize that there was no period after the J

    All I am saying is that we sometimes filter out some important details.

  5. GBA Editor
    Martin Holladay | | #5

    This is false erudition -- pretending that a rephrasing amounts to a deep revelation or truth, when really there is no fundamental difference being revealed.

    Some say hot air rises; some say dense air sinks. However, those of us who say "hot air rises" aren't being stupid, because there is no difference, really.

  6. wjrobinson | | #6

    I hang glide. Those of us with lots of air time soaring in lift late in the day when a whole valley seems to be rising experience this phenomena first hand.. What is actually happening is solar heat has shut down and cold denser air is flowing into the valley rapidly pushing up all the warm less dense air from the day. The valley fills up with this cold air and then the effect is over, no more lift.

    Same as this discussion

  7. homedesign | | #7

    AJ, I have been studying weather lately and I think Julius Sumner Miller's perspective falls right into line with many of the concepts in my Weather Book.

  8. wjrobinson | | #8

    Myself and most of my family scuba dive too. Buoyancy is everything as one changes depth which in turn changes the forces on one's lungs and wet suit and BC (bouyancy compensator).

  9. user-757117 | | #9

    There is lots of theory in meterology that I think is transferable to "building science".
    One of the best "met" books I've ever owned is called the "Air Command Weather Manual".
    It's an oldie but a goodie.

  10. homedesign | | #10

    Some say the Sun rises; some say the Earth rotates.

    I am not saying that Bud Poll and Julius Sumner Miller have made some earth shattering revelation....
    At least for me the concept makes it easier to visualize stack effect...
    And I will say again that Allison's Blog & Bud's illustrations are worth taking a look at.

  11. user-723121 | | #11

    Go into the attic of a house in the winter in a cold climate that is poorly insulated and has major bypasses and you will quickly understand the stack effect. Most of these older homes also had very little attic ventilation capability so the warm air and conductive losses from the living space below keep the attic quite toasty.

  12. user-788447 | | #12

    Was just speaking with some folks about stack effect today. While the stack effect during the winter (us being cold-climetians) seemed straight forward we quickly reached our limits in understanding when trying to explain how the stack effect reverses in the summer. This is due I think to our reliance on the notion of hot air rising, escaping and in turn drawing in replacement air at the bottom of the house. The idea of cold air falling, seeping into the ground and drawing down hot air didn't present itself as convincing.

    Can't say I fully comprehend all the links I read just yet but I do see the content leading me to a better understanding of the stack effect particularly the 'reverse' dynamic during the summer.

  13. user-757117 | | #13

    The idea of cold air falling, seeping into the ground and drawing down hot air didn't present itself as convincing.

    In meterology, this is the process that drives the "katabatic wind".
    The bouyancy concept is important as relatively cold air is denser and therefore "falls" drawing relatively warm air in behind to replace the relatively cold air.

  14. user-1091688 | | #14

    Thanks for the invitation John.
    Talking points are just getting started as my efforts have been dominated by defending my position on the nonexistent powers of warm air, a task that has proved much more difficult than anticipated. But those talking points are evolving along with a consensus about the powers of warm air, so here are some starters.
    Avoid the words "pull" and "replacement" in conjunction with warm air moving up.
    Example of old and new for attics.
    Old: The warm air rises in the attic and exits the ridge vent, pulling in its replacement air from the soffits.
    New: The cold air pushes in the soffits forcing the warm air in the attic up and out the ridge vents.
    Here's old and new for stack effect, (up north).
    Old: Warm air rises inside our home and moves up and out those leaks in the upper portions of the building while pulling in its replacement air through leaks in the lower areas.
    New: Cold air pushes into the lower portions of our homes forcing the lighter warm air up and out through leaks at the top.

    The wording is just generic, but these represent just two examples where dropping words like "pull" and "replacement" combined with a change in the sequence of the two events, where the explanation remains clear yet we have avoided any implication that warm air is powering this flow of air.

    Now chimney draft is going to be a bit more challenging when I try to explain to the HVAC pros that the hot air in that chimney isn't pulling the combustion products behind it, but rather the colder air surrounding that appliance that is pushing everything up and out.

    More talking points, much improved over what I can propose on my own, will certainly evolve as this perception of air pressures and movement expands.

    For full disclosure, 6 months ago I was describing air flow just like everyone else, just the way it was explained to me. It was when I tried to put numbers on that flow that I discovered the missing perspective that has evaded me from the start. My Ahhh moment!


  15. GBA Editor
    Martin Holladay | | #15

    Good luck, Bud! As a method of describing the way the universe works, I'm skeptical that your new method of explanation gets us any closer to clarity and understanding.

    Even astronomers find themselves saying, occasionally, "When the sun has risen above the horizon..."

    If you were around in the 18th century, you could have explained to Joseph-Michel and Jacques-Étienne Montgolfier that they were mistaken -- the hot air in their balloons was not lifting them up. What was actually happening was that cold air was pushing the bottom of their silk contraption.

    And would your explanation have led to increased clarity? I don't think so.

  16. user-723121 | | #16

    Warm to cold does not change in building dynamics.

  17. user-788447 | | #17

    Today I took a BPI building analyst exam after a 5 day training. The language surrounding stack effect and air movement in the exam only uses the terms 'pressure' and 'density'. Stack effect and air movement are never described with terms of relative temperature. Just an observation on how BPI presents the topic through its test.

    This is a fun topic to reflect on I especially like gaining some insight to barometric pressure. So we've talked about the force of gravity, the properties of pressure and density, and the phenomena of buoyancy. In addition couldn't we also describe the dynamics in terms of changing volumes (expansion and contraction) when describing the stack effect and air movement in general?

    We understand from scientific theorems that higher temperatures translate to higher kinetic energies in gases which in turn translate to greater pressures or expansion in volume. As a gas increases in temperature it tends to expand in all directions. Because it isn't expanding in a vacuum it meets resistance to its expansion and we might speculate that this resistance differs significantly in orientation. When expanding in the direction of gravity (down) it meets air at higher densities, and upwards it meets air at lower densities. So as a mass of air increases in temperature couldn't we surmise a disproportionate (greater) amount of the expansion going against the trajectory of gravity ; )

  18. user-788447 | | #18

    As a method of describing the way the universe works, I'm skeptical that your new method of explanation gets us any closer to clarity and understanding.

    What then is a clear explanation of reverse stack effect using the notion that hot air rises?

    If you were around in the 18th century, you could have explained to Joseph-Michel and Jacques-Étienne Montgolfier that they were mistaken -- the hot air in their balloons was not lifting them up.

    According to my physics textbook through the end of the 18th century into the 19th century the nature of heat transfer was understood with the concept of the 'caloric' which was supposed to be a material substance transferred from one body to another. When experimental observation suggested that this heat could not be a material transfer because in certain scenarios the caloric was never exhausted, theories suggested that heat then was a form of motion.

    'Increased clarity' (or at least today's conventional understanding) happened in the middle of the 19th century when the subject of heat and mechanics where unified and resulted in the law of conservation of energy i.e., there are several types of energies that can transform from one to another.

  19. BobHr | | #19

    J Chesnut

    The reserve of the stack effect is part of hot to cold, high pressure to low pressure. The stack and its reserve is the relative relationship between indoor and outdoor temps. In the winter we are heating and expaning the air. The more boyount air rises to the top and leaks to the colder attic. In the heating season the outside air is cold inside is warm.

    Now in the summer you have warm outside and cool inside. We cool and condense the indoor air. Thus making it heavier and denser. The denser air sinks to the lowest level and pulls in air from the attic.

  20. user-788447 | | #20

    OK Robert I think my brain just got less dense ; ) (maybe)

    If we popularly understand the stack effect as hot air rising and escaping out of the house the reverse in similar terms would be cold air sinking and escaping out the basement (but pulling down hot air into the upper living spaces as it does ((but hot air rises?)))

    But now I see the light. Due to the temperature differential between inside and outside the weights of air are different and when redistributed in the inside set up the pressure differentials at the bottom and top of the structure. (Thank you Bud).

    So the Stack Effect just "stacks" the dense and less dense air (in a sense this doesn't reverse at all) and creates pressure differentials. The Stack Effect itself doesn't drive the infiltration/ exfiltration dynamic. It just sets up the positive pressure at the top of the house (where the air is less dense) and the negative pressure at the bottom of the house (where the air is more dense). The pressure differentials then in tandem with holes in the air barrier start the infiltration/exfiltration wheels going.

    Am I correct to say that in relationship to outside pressure the air at the top of the house always sets up as positive due to the stack effect and the bottom of the house always sets up as negative whether summer or winter? If so is 'reverse' stack effect a misleading term? Or am I just blowing hot air?

  21. BobHr | | #21

    The hot air being pulled down from the attic is controlled by air pressure. You can make a heavy liquid go up a straw by creating negative air pressure. The liquid is heavier but travels up as you suck on the straw but take your mouth away and the liquid falls back in to the glass.

    Air flow is affected by pressure differences and not temperature differences. Now temp will play a role in the air pressure. When we heat or cool the air we affect is pressure and density.

    I see this affect pretty frequently. I have a 2 gallon plastic gas can that I keep in the bed of my truck . In the afternoon when its empty it will blow up like a ballon. In the morning before I stop to refill the side are all sucked in. The expansion and contraction affect the air pressure in a sealed container.

    There are many interacting influences that affect the dynamic of a building.. Wind, blowers/fans, heating/cooling that all affect leaks. These can work to intensify or cancel out the stack affect.

  22. user-757117 | | #22

    I think Martin is correct.
    The nomenclature is maybe not-so-important so much as the dynamics - I'm not sure we "must say" things one way or another except that it is good for everyone to be on the same page.
    Relatively warm air DOES rise by convection - whether it's "pulling" relatively cold air or being "pushed" by it.

    I also think Robert Hronek is right that pressure gradients drive airflow.

    But I think what's most important is the energy flow - without an energy gradient there can be no pressure gradient.

    Energy input can enhance the natural tendancies of bouyancy - like latent heat release when a moist rising air mass cools and condenses, or as in "stack effect".
    But energy input can also overcome the natural tendancies of bouyancy - like a cold air mass being forced up a mountainside by wind, or warm air being pulled out of an attic by the cooling effect of an A/C unit.

    Like yin yang - one must follow the other or there is vacuum.

  23. GBA Editor
    Martin Holladay | | #23

    A couple of points:

    1. There are all kinds of examples of common expressions that explain the universe well but aren't strictly true from a scientific perspective -- including our description of a slab of granite as a solid object, even though the electrons and protons and neutrons that make up the granite are tiny, and most of the space filled by the granite is empty. That doesn't mean that our common methods of description aren't useful. We say that "hot air rises" not because we believe in "Hindu levitation" -- an unpleasant phrase, by the way, that ignorantly ridicules the Hindu religion -- but because hot air does, indeed, rise with respect to adjacent volumes of cooler air.

    2. The stack effect in winter is driven by delta-T. That's why the stack effect is so strong when the outdoor temperature is -20°F, but relatively weak when it is 55°F. The stack effect in winter can be overwhelmed by other driving forces, including HVAC fans, exhaust fans, and wind, but the greater the delta-T, the harder it is for these other driving forces to counteract the stack effect.

    3. The delta-T in summer tends to be small. Even on a hot day the delta-T may only be 20 F°. That's why the stack effect is summer is weak, and is more easily overwhelmed by other forces, like HVAC fans and exhaust fans.

  24. homedesign | | #24

    Bud, You and Julius Sumner Miller are Heretics....
    What you are proposing does not jive with what we usually read.

    Here is a more traditional explanation of stack effect
    John Straube.... Pages 4,5,6 & 7
    It's a good explanation...but Professor Sumner Miller would not agree with the phrase "hot air rises"

  25. homedesign | | #25

    The Push of the Atmosphere

    Ah so... It's a Cold Climate bias
    Maybe that's why I found Julius & Bud's Perspective more satisfying.

    The professor may be goofy...but he is no dummy.
    I am still extremely curious why he(JSM) would say such a thing.
    I have never heard it anywhere else until Bud Poll said the same thing.

    This business is related to the atmosphere ... Bud seems to have a good handle on the atmosphere (I can tell by his sketch)...and so does Julius Sumner Miller

    Concerning Robert's gas can anology ....
    here is another JSM quote (not a paraphrase)
    "I MUST advise you to put the word suction out of your vocabulary!"

  26. user-757117 | | #26

    Again I agree with Martin.

    "Turning up the heat" increases the energy gradient across the enclosure - Delta T is the measure of that energy gradient.

    But Delta T does not move air - pressure gradients do.
    Pressure gradients are caused by energy gradients.

    This is how wind blows.

  27. user-1091688 | | #27

    Martin, "Hot Air Rises" perfectly describes what we see. But it leaves the door open to why and how and that is where the problem comes from. My next answer to John will illustrate.

    John, you asked: "What you are proposing does not jive with what we usually read.
    Here is a more traditional explanation of stack effect
    John Straube.... Pages 4,5,6 & 7"

    Those building science pages are perfect but the title of the article is "Air Flow Control in Buildings" which isn't the hot button like challenging "Hot Air Rises". And, I would not consider that an example of "what we usually read". Here's a typical quote:
    "As heat escapes a roof, cold air is sucked in through the basement and first floor windows."
    The caption under the picture.

    I selected the GBA article because you are respected and credible, where not everything we read on the internet can be considered so. But that quote is far from the only one out there, as I could fill pages with examples of how poorly the topic at hand is understood.

    Once a larger population hears this explanation of what is moving our air around, and now I know I'm not alone, bsd-104, maybe we will be able to leave that oh so simple and easy to understand wording "Hot Air Rises" alone. I would be happy with that. And given the speed at which this group has picked up on this confusing twist we have been caught in, I have hope.


  28. GBA Editor
    Martin Holladay | | #28

    You wrote, "I could fill pages with examples of how poorly the topic at hand is understood." That may be, but your attitude is a little arrogant. Just because many authors write that "hot air rises" doesn't mean that we don't understand the topic at hand. I assure you we do.

  29. homedesign | | #29

    Bud, I am glad you were not brushed away

    I appreciate the different perspectives of all who have commented here.
    I strive to visualize this stuff ...
    and it frustrates me that many times I can't comprehend it.

    Lucas, You have a much better understanding of Physics and meteorology than I do ...
    and you can visualize in 3D
    Your description sounds OK to me....and you have avoided making "commandments".
    What do you think of Bud's Illustrations? ... I think they are very helpful
    What would you suggest that he change?

  30. user-1091688 | | #30

    Martin, my communication skills are minimal at best and if you detected some arrogance, it certainly was not intended and you do have my fullest apology. I am humbled to be here.
    "Understood" was definitely too broad of a word, but was intended to point to the wording and not the science. But even the wording is not the author's fault, as this has been the accepted wording for a long time and anything written in the past would have been considered out of line stating it any other way.

    The only reason I posted the suggested new wording is that was what the title of the thread was asking. I certainly don't feel like the right person to be dictating any changes in our language and hope that my suggestions will simply be viewed as examples of what could be done.

    Given the conversation here, can I ask what direction you would suggest? When this thread dies out, I will be returning to the two small forums I haunt to discuss the information posted. I was hoping to improve my worksheet and include summer stack effect along with chimney draft and others, but if you feel I'm doing more harm than good, I will reconsider.


  31. GBA Editor
    Martin Holladay | | #31

    Thanks for the clarification. I think all of this discussion is good. There's nothing wrong with teaching anyone who doesn't know that cold air is denser than warm air, cold water is denser than warm water, air expands in volume as it is heated, cold air flows downhill, and so on. These are all good things to know.

    But I think it's unnecessary to try to tell people not to say that "Hot air rises." I'll repeat what I said before: hot air does rise with respect to nearby volumes of colder air. And cold air sinks.

  32. homedesign | | #32

    I apologize too
    I don't want to misquote Bud
    The title of this thread is a Quote from Julius Sumner Miller

  33. BobHr | | #33

    In the professors experiment with the candle he placed a cylinder that did not have a path. The candle was extingushing because they was not a path.

    Had there been a gap at the bottom you would have seen an updraft. Or if cylinder was wider there would have been an updraft.

    I think in many respects people have the idea that heat rises but dont consider that air moves due to pressure differences. You can always think of high to low, that is hot to cold, high pressure to low pressure and high humidity to low.

    Heat will have an effect on pressure. If the cooler denser air can fill in the lighter hotter air will rise.

  34. user-757117 | | #34

    What do you think of Bud's Illustrations? ... I think they are very helpful
    What would you suggest that he change?

    I'm not sure I disagree with anything shown on Bud's illustrations...

    But I might expand a bit to get at the root cause that drives the process he goes on to describe.
    That root cause is that little part where he says "I have increased the inside air temperature to 70 degrees."

    In fig. 1, nothing is happening.
    Once he applies an energy input to heat the air, things happen.

  35. user-788447 | | #35

    What I would like to see is Bud's diagrams reconfigured for conditions where the air is cooled in the interior with respect to hot air outside.

  36. homedesign | | #36

    J, I like your suggestion

    Bud, about a year ago there was a good discussion going here about air barriers....
    I thought Lucas was asking some very good questions...and Michael Blasnik was giving some good answers

  37. primitivelamps | | #37

    So just to get this clear in my head. Is the following correct.
    In my New England climate during the winter the denser cold air surrounds the house. Inside we add energy (by various means) which makes the air inside warmer than outside and less dense. This difference causes a pressure drive from outside to inside. Colder denser air enters through gaps and cracks pushing the warmer air up inside the house creating a stacking effect. At the same time the attic has equal pressure (if air tight) as outside ( in the ideal world being the same temperature. If this is actually what is going on would it not make more sense to air seal the lower levels to stop pushing the warmer air up? I suppose it is the same effect as air sealing the attic floor preventing the less dense air from escaping. In theory should not the same effect happen by doing one or the other. Based on this with out the denser air forcing the warmer air up the "cap" of denser air in the attic should hold the warmer air in right? I realize heat seeks cold I am just referring to air and movement.

    Not sure if I am starting to confuse myself.

  38. GBA Editor
    Martin Holladay | | #38

    You have it right. The two most important areas for air sealing are the top of the house (usually the attic floor) and the bottom of the house (usually the basement).

    Comprehensive air sealing at either location will greatly reduce a home's air leakage rate. Of course, air sealing at both locations will produce better results than just air sealing at one of these locations.

  39. user-1091688 | | #39

    Terry, to add to Martin's reply, the inside to outside temperatures define a total delta pressure for a given home. Part of that will be above the neutral pressure plane (NPP) and part will be below. That neutral plane will shift to where the pressure above combined with the leakage above produces an exfiltration rate to match the infiltration created by the pressure below the NPP and the leakage below. Sealing high or low, as Martin stated, will effectively reduce your total air leakage. Defining where that NPP will move and what net effect it would have would be most difficult, however there are some factors that can be useful to discuss. I will include some text on my thoughts when I add the winter version of my diagrams to my work page and post here when ready.

    Lucas, you are correct that heating our homes is a major contributor to the air movement we see, but I have not emphasized it for now as it is the difference in densities that propels our air movement and I was trying to avoid the hot moves to cold energy transfer that is different from warm air and cold air. My intent with producing that page was to obtain input beyond my thinking, and that is exactly what you/all are providing and I thank you.

    John, I read that link when it came out, but have not followed to get the full contribution, but I will.

    J, as mentioned above I will rework a set of diagrams to illustrate a summer configuration. As has been stated, summer forces will be considerably less than winter, probably less than half, but the diagram will give us a picture, and I'm a picture person.


  40. homedesign | | #40

    It may not seem like it...
    The nutty professor reminds us that we are living in a fluid.

    and I say that's a wonderous thing to contemplate

  41. primitivelamps | | #41

    Couple observations related to this. I see constantly (during winter in New England) duct work installed outside the "thermal envelope" (not where we want mechanicals or ducts I know). When blowers are not running or it is an AC only system I see colder denser air pouring out these ducts. Obviously some is related to duct leakage at any number of spots or even from leakage at the handler. I see this even in systems that have passed duct leakage tests. My guess is that warm air is pushed up into the ducts which is now outside the thermal envelop and is just inside an R6-8 duct. Conduction takes place,the energy leaves the air which is now cooler and denser and it flows back the same way into the room. This process appears like a single pipe steam system that delivers heat then returns the condensate to be reheated in the same pipe. Would this seem like an accurate description?
    Recessed lights for example are almost always leaky (between conditioned and unconditioned spaces). Thermally it appears now accurate to say colder denser air is dropping down through or conducting/convecting through the housing. Verse the warm air rising and leaving. I realize the warm air contains energy but then there is that "heat seeks cold thing" that is independent of the air movement part.
    I guess what I am wondering is the "most" accurate way to state the findings. The results really have not changed but I want to make sure my statements are accurate.
    I may have made this more complicated than it needs to be.

  42. user-1091688 | | #42

    Hi Terry, there are variables, ranch, 2-story, leaky home, tight home, temperatures, but in general, a can light in a ceiling below the attic, the air pressure from the stack effect up there would be positive with respect to outside. You should expect the warm air to be flowing out around the leaky can. I've seen this with IR without the BD running and the can actually looks warmer than the ceiling. But I have also seen where the insulation has been held back from the can, per the instructions, and the surrounding ceiling appears very cold, that would be conduction.

    My take would be, if you are experiencing cold around a can in a ceiling below an attic, you are experiencing conduction or even wind but the net air flow should be out.


  43. user-757117 | | #43 are correct that heating our homes is a major contributor to the air movement we see, but I have not emphasized it for now as it is the difference in densities that propels our air movement and I was trying to avoid the hot moves to cold energy transfer that is different from warm air and cold air.

    I think it is good that you want to teach people about buoyancy - that is, the mechanics of relative densities.

    I think your illustrations do a good job of illustrating how relative densities set up pressure gradients - and that pressure gradients imply the potential for air movement.

    However, I would like to suggest that excluding the role of energy input into the system is a bit like excluding the role of gasoline in an explanation of how an engine moves a car.
    To someone reading such an explanation, the drivetrain mechanics can make perfect sense but the engine may appear as a "magic box".

    If people are not to "continue to believe that hot air has some magical power to move up when heated" it may be a good idea to include an explanation that energy input can cause air to expand, thereby increasing its buoyancy.
    Or that energy input can cause air to become more dense, thereby decreasing its buoyancy.

  44. user-1091688 | | #44

    Lucas, I do agree and my intent is to create multiple pages where I can take a phrase like buoyancy and link it to another internal page for a more complete explanation. The need for heat is now on my list as will be many others. I suffer from being too close to my subject and thus will overlook items where I have become comfortable with their contribution. My last physics class was over 45 years ago, so this has been quite a refresher for me.


  45. homedesign | | #45

    Bud, you say you're a picture person...
    Have you taken a look at KhanAcademy?
    lots of good refresher stuff and new to me stuff there
    The Physics section may be obvious for Building Science ...but Chemistry and Biology sections are relevant also

    another portal

  46. stuccofirst | | #46

    Instead of saying "hot air rises", we should just say hot air moves through the path of least resistance. The only reason hot air moves "up and out" is because we give it a place to go.

  47. homedesign | | #47

    Well, this is interesting...
    now it looks like Holladay agrees with Julius Sumner Miller and Bud Poll

  48. GBA Editor
    Martin Holladay | | #48

    As far as I know, I'm not related to April Holladay. Perhaps she is a long-lost 3rd cousin one removed.

  49. user-659915 | | #49

    I've generally tried to stay away from commenting on this angels-on-a-pinhead thread but this comment of Terry's caught my eye and it seemed to go unchallenged:

    In my New England climate during the winter the denser cold air surrounds the house. Inside we add energy (by various means) which makes the air inside warmer than outside and less dense. This difference causes a pressure drive from outside to inside.

    No. The heat causes the interior air to expand (that's what becoming less dense means - same number of molecules occupying a larger space). This will cause an overall pressure drive from inside to outside, not the reverse. Stack effect forces will overcome that pressure differential at the lower end of the stack, allowing cold air ingress to the extent that leaks in the enclosure permit, so Martin's subsequent comment that sealing 'tops and bottoms' is key to reducing air movement through the enclosure is of course correct. But the OVERALL pressure drive from heating up the air in the enclosure will be from inside out to outside.

  50. primitivelamps | | #50

    Hi James
    Actually reading my question again I agree with you. I did not word that very well.

  51. homedesign | | #51

    James, I always like your comments....and there is a very, very good chance that you, Martin and Lucas have a better perspective than Sumner Miller, April Holladay and Bud Poll.
    After all, you debunked William Rose's wacky idea for a "dehumidifying slab"
    I concede that the dehumidifying slab idea was probably goofy.

    However... it is good to know that even William Rose can have some half-baked wacky thoughts too.

    I don't know why you and Lucas are having trouble getting "past" the fact that most of us will heat (and cool) our homes.
    Bud was able to jump right past it...

    I agree with Martin .. "Discussion is good"

  52. user-757117 | | #52

    It's not hat I can't get past that people heat and cool their homes.
    It's more a concern that by jumping "right past it" something important is being taken for granted.

    Here's a few quotes from the comments in Allison's blog post:

    Warm air moving up is indeed what we see, but warm air has no internal energy that makes it move up or down.

    I'm not sure what "internal energy" means exactly, but there is definately energy embodied in the warm air... otherwise it would not be warm.

    The objective here is to dispel the notion that warm air is leading the way in the process of convection, when in fact it is the cold air that initiates the process.

    It is the application of energy that initiates the process. Otherwise, the interior would remain in thermal quasi-equilibrium with the exterior and there would be nothing interesting happening.

    It is obvious that both must move, but the warm air cannot rise by itself and therefore cannot be credited with pulling the cold air in behind it.

    No, air can't rise all by itself, it needs to be heated - using some form of energy.
    Whether its pulling cold air or being pushed by cold air...
    You can't have one without the other.
    Gravity does some pushing while the energy it took to expand the warm air does some pulling.
    Hence my earlier toungue-in-cheek comment about yin yang.

    People heat or cool their homes for comfort but we maybe don't always appreciate the fact that by doing so we set in motion a chain of events and "unintended consequences".
    "Stack effect" is just an "unintended consequence" of heating for comfort.
    Likewise, "reverse stack effect" is an unintended consequence of cooling for comfort.

    And I also agree that "discussion is good".

  53. user-1091688 | | #53

    I'm falling behind, but I will catch up. A couple of quick comments, it's half time.
    If a high density gas and a low density gas are side by side and being pulled down by gravity, It is my perspective that the higher density gas will initiate the movement as the resulting direction is that of the heavier gas. The lower density gas was also pushing down, but ended up moving up. The consequences of this denser gas initiating the downward motion is that it does indeed create a pulling effect behind it, where as the motion of the less dense gas moving up is a result of an increase in pressure at the boundary between the two gasses, that pushing force.

    I have tried to shift this statement to two gasses of different densities to separate the warm air, cold air issue from the fluid dynamics. With stack effect, heat is a contributing element, but only in expanding the cold air, not in determining the direction of motion. I realize that needs more clarification as the less dense gas is part of this motion, but the expanding process doesn't create the push that creates the motion.

    I have more diagrams coming.


  54. user-659915 | | #54

    Are we still talking about the same OP? 'Hot air rises' is simply an observation, and as such it is clearly neutral as to causative influence. It does not pretend to offer an explanation of the observed phenomenon, and explorations of what 'initiates' the motion, whether it's 'pulling' or 'pushing' that's at work, while interesting and possibly entertaining, do nothing to invalidate it. Just saying.

  55. user-757117 | | #55

    'Hot air rises' is simply an observation, and as such it is clearly neutral as to causative influence.

    I agree.
    I also think I am probably being pedantic.

    As to whether we are on-topic for the OP...
    I believe so.
    John opened the door pretty wide - which I don't think is a bad thing.

  56. homedesign | | #56

    The Thread Title and the "OQ" Original Quote were from Julius Sumner Miller.
    He was a very passionate Student and Teacher.
    I would rather watch his demonstrations than attend KahnAcademy.
    He was a Student and longtime friend of Albert Einstein.
    He spoke to all ages....
    and he was very passionate about "words".
    In my mind he is VERY credible......
    There must be a reason for him to make such an outrageous statement???

    Do we just "know better now"..... 50 years later?
    Or was JSM "right" about most things and a little half-baked with some of his stuff?

    Maybe he (JSM) is quoting his friend and teacher "Albert"... who knows??

  57. GBA Editor
    Martin Holladay | | #57

    Don't get so stuck in either-or thinking. There's always more than one way to describe a phenomenon.

    I think a "word police" attempt to ban the phrase "hot air rises" is the silliest proposal to come down the pike in some time...

  58. homedesign | | #58

    Bud, I am looking forward to seeing your new illustrations.
    I don't think that you or Dr. Sumner Miller are "Silly"

    John Straube's Illustrations are good... but I think yours are better

  59. user-1091688 | | #59

    Hi John and thank you, I'm just a guest here so I'm trying to skip over those little jabs..
    You said "There must be a reason for him to make such an outrageous statement???". In regards to JSM's, I have an opinion (a guess) as to why he might have said that. As a professor, a teacher, he would want to convey the "why" as well as the "what" is happening. There is nothing wrong with what "hot air rises" says, it is what it doesn't say that has left the public to fill in the blank. As I sit here watching the smoke rise from a chimney across the street, who could possible guess, beyond the well informed, that it is the cold air infiltrating the basement that is pushing that smoke up and out the chimney. Certainly not an obvious conclusion, but lacking even an occasional explanation, a foot note or some different wording, the general public has filled in that blank, but with the wrong information. The suggestion that attempting to correct this misunderstanding would only create confusion, I would suggest that the confusion is already in place and that an occasional foot note or change in wording would only add clarity to this process, a correction that is long overdue. If I can take the liberty to modify the JSM quote, the title to this thread, "What Else Must We Say?

    John, I am making rather major changes to my web site with the diagrams on stack effect to try to be more complete. I will include a contact path to where I can be reached should I not make it back here.


  60. homedesign | | #60

    Bud & Others,
    I am curious if you have found any references for the origin of the term "Stack Effect"?
    Is it named after Chimney Stack or is Chimney Stack named after Stack Effect?

  61. user-1091688 | | #61

    Hi John, I have watched as I have searched for related information and I have seen "stack" referred to in old structures where each floor would have its own fireplace, thus one stacked above the other. I have also seen references to the layers of our atmosphere stacked up as clouds form. But nowhere have I seen either take credit as the origin of the term.
    But I will keep watching :).


  62. maine_tyler | | #62

    My high school physics teacher always said, 'nothing sucks in physics!'
    Semantics, yes, but a point that is not novel nor without lesson.

    And a top-side pump head can only pump water around sea-level to about 33.9ft. It doesn't suck. But the water does rise...
    What caused the water to rise? The vacuum (lack of air), the atmospheric pressure, or the pump?

  63. Prutuja | | #63

    I'm curious if you've uncovered any sources for the word "Stack Effect's" origin.
    Is Chimney Stack named after Stack Effect or Stack Effect named after Chimney Stack?
    CCNA Course

    1. charlie_sullivan | | #64

      The OED traces "stack" meaning a chimney back a 1825:

      1825 ‘J. Nicholson’ Operative Mechanic 357 "In smelting by the reverberatory-furnace..the flame passes over the hearth, and enters into an oblique chimney, which terminates in a perpendicular one, called a stack, of considerable height."

      But even much earlier, i.e. 1667, it was used to me a set of many chimneys exiting the roof together.

      On the other hand, "stack effect" didn't become common until the 1920s, and didn't really take off until after 1970, based on a Google n-grams search. The earliest I found was a few references in the 1890s.

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