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Who Knew the Stack Effect Could Be So Controversial?

Argument Stack Effect Building Science Heat Rises Debate

Last week I wrote a little article about the stack effect to explain that the flow of air and heat is upward in winter but downward in summer. Turns out, the stack effect is a hot topic. My article here has gotten 25 comments so far. When I posted it to the RESNET BPI group on LinkedIn, it got another 22 comments. And John Brooks started a stack effect discussion over at Green Building Advisor that has 61 comments as I write. Based on all this discussion, I’d say that heat does rise!

So, my response is: Relax and repeat after me: Heat rises. Let’s say it again, just to help you get over any phobia you may have: Heat rises. What is it about that statement that’s wrong? Nothing! It doesn’t ascribe a cause. It doesn’t say the only thing heat can do is rise. It’s just stating an observation. Heat can move upward. It also can move downward, sideways, diagonally, or any direction at all.

In the winter stack effect, heat rises because it’s moving with the warm air that’s less dense than the colder surrounding air. In summer, it moves downward as it follows the cool, dense air. I described all this in the previous article. I also stated clearly at the top of the article that it’s the Second Law of Thermodynamics that drives heat flow, and that law says that the natural flow of heat is from hot to cold.

Yes, it’s true that a lot of people are confused about the nature of heat flow. It’s also true that many in the home performance community are quick to point out that, No, heat doesn’t rise – warm air does. Well, if the warm air is rising, isn’t the dang heat rising, too?

The other argument against my explanation was that not only does heat not rise but warm air doesn’t rise either. Nope. It gets pushed up by the cold air below, some say. Bud Poll and I exchanged several comments here, and he commented in the Green Building Advisor thread as well. His main beef is with the use of certain words to describe the process:

Avoid the words “pull” and “replacement” in conjunction with warm air moving up.

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.

As it turns out, a couple of other physics principles shed some light on what’s going on here. First, there’s Newton’s Third Law of Motion, which states, For every action, there’s an equal and opposite reaction. Then, another aspect of the Second Law of Thermodynamics says that air moves from high pressure to low pressure. The cold air and the warm air, in other words, work in concert with each other. Each does its thing because of the assistance of the other.

In the end, I agree with Martin Holladay, who said of Poll’s mission to change the way we talk about stack effect, “I’m skeptical that your new method of explanation gets us any closer to clarity and understanding.” Rather than arguing about how many angels can dance on the head of a pin, let’s focus on what’s important and try to help folks like the Hartfords in Maine, who had to resort to asking the fuel oil company to take their car in exchange for another tank of fuel oil.

If you agree, repeat after me, Heat rises. (Just don’t forget that it can fall and go sideways, too.) If you don’t agree, I’ll see you at Building Science Fight Club.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.


Photo by o5com from, used under a Creative Commons license.

This Post Has 20 Comments

  1. It could be this all comes
    It could be this all comes about because we are looking at heat with an understandable gravitational bias?

  2. Where are all the
    Where are all the displacement ventilation folks supposed to go in this fight? (maybe they are “laying low”, seeing as cold air falls….)

  3. Allison, I like the
    Allison, I like the illustrations in your “previous article”…I like John Straube’s Illustrations from his textbook (also reprinted at BSC BSD-014)…and I especially like Bud’s Illustrations from his “Energy Workshop”. 
    If anyone else has links to helpful “Stack Effect” Illustrations … I am interested… 
    I am also curious as to the origin of the term “stack effect” 

  4. andrew r.:
    andrew r.: Like a fish swimming in water, we could well be oblivious to how gravity affects our thinking. 
    Matthew C.: Who knows? Maybe they’ll rise to occasion. 
    John B.: It was reading Straube’s book that directly led to my first post. His diagrams and explanation are excellent. (Here’s the direct link for those who don’t know what John’s reference means: BSD-014: Air Flow Control in Buildings.) 

  5. …and I remember a
    …and I remember a Physics teacher who used to say “there is no reason to argue whether Gravity exists, all you need to know is your butt gets tired the longer you set in detention”.

  6. My house (built in 1960) had
    My house (built in 1960) had Ceil Heat – radiant heat in the ceiling- before central air was put it. I wish someone would have explained basic physics to the makers of that nonsense (yes, I know radiant heat is different than what you’re talking about but guess what the ceiling was mostly heating up? Yep, air.). Before my gas turned on I spent a few horrible weeks having to use the Ceil Heat which made for a lot of hot crusty eyeballs and icicle feet.

  7. Allison, 

    Good Article as always! This issue as you point out is not what is happening. The issue is how it causes buildings to perform at less than optimum. A 2nd part of that is about how we explain it to the home owner or building manager. 
    Probably the only place that all air rises is a place that a majority of the population are politicians.!

  8. John B. While I work on a
    John B. While I work on a post, you ask if we had seen any other illustrations. This one is rather simple, but it gets the message across. Here’s the Link 

  9. Allen, 

    I’d like to refer to your referral to Newton’s third law of motion and Second Law of Thermodynamics and combine it into single one (please read it caferully): 
    Newton’s second law of motion (or momentum conservation law). 
    My deepest thanks to Mr. Newton who put this together long time ago. And it seems that everybody forgot such important thing (well not surprising for humans at all). Well, the law states that any change to momentum of particle (or its velocity) is equal to sum all forces acting on that particle. And thanks God the same law applies to fluid particles as well which slightly modified is well known as Navier Stokes equations. So much about theory. But remember one thing: 
    Fluid particle accelerates (or moves) due to action of forces on that fluid particle.  
    Which forces do we have here? In most cases those are:  
    1) Pressure forces acting on the boundaries of fluid particle (typical examples are pump or fan pushing the water/air through the hose pipe or duct)  
    2) viscous forces acting on the boundaries as well and between the fluid layers (pretty intuitive), and  
    3) gravity forces acting over the whole volume of fluid particle (this one is even more intuitive and everybody experiences it every day).  
    Here’s the trick. Buoyant force arises as the difference in weights of two columns of air at different temperatures. Warmer air is less dense than cold air, and therefore the pressure formed by stack of warm air is smaller than pressure formed by the cold air of same height. That difference in static pressures drives the air to move once these two stacks are connected. I say one more time: connected (or make a system, or interact). No movement would exist between these two stacks if there was perfect wall barrier(adiabatic-no heat exchange, and impermeable-no mass exchange). Yesssss, finaly some building science terms in this observation. In other words, if we had one container prefectly insulated and sealed with warm air inside in a cold room, no infiltration, no exfiltration. Period. With perfectly insulated container, there would be no heat loss through the container, and there would not be even movement of air inside of that container due to heat loss at the container walls (yeah yeah I know you all know now why that would happen). 
    So to summarize it briefly: 
    1) In order to have the air moving we need some forces to act on it (pressure differences or pressure gradient). 
    2) We need a flow path, or hole or connecting pipe for two air masses at two different temperature and consequently, two different static pressures. 
    3) we need a flowing medium, in this case air. 
    So depending where these holes are and where the locations of these two air stacks are, the air could move up and down, left and right carrying with itself the heat it contains (in physics called internal energy most commonly expressed through temperature). 
    I would stop my physics 101 lesson here, and I hope this will help people to form solid foundation and understanding how these phenomena occur in the nature. 
    If interested we may expand this further and start bringing up some more building science examples founded on these premises. Just a suggestion. 
    I noticed the message I sent above has huge blanks in it (I copied it from text editos). hope this one will be pasted in better shape so you may delete the previous one. 

  10. Armando:
    Armando: That would be another way to state Newton’s First Law of Motion, I guess: Bodies at rest tend to stay at rest. 
    ModernSauce: Good point! Heating the top of the room doesn’t help a lot because the stack effect drives the warm air upward. Now, if it’s acting like a good radiant heater, you might still be comfortable, but I have difficulty believing that an old system like that would be very effective, especially in the ceiling. 
    John N.: Thanks. Sometimes we get a little too hung up on language, and I’m guilty of that in some cases, too.

  11. Danko:
    Danko: Thanks for filling in those details. Rather than call out only Newton’s Third Law of Motion, I should have referred to all three of them. As you point out, any time particles accelerate, the Second Law is relevant because there’s a net force acting. If they remain at rest (in the adiabatic, airtight container you mentioned, for example), that’s Newton’s First Law. 
    The really interesting thing is that when you have systems with lots of particles, like the air in a building, you could have really weird results that obey Newton’s Laws as well as the Law of Conservation of Energy. For example, all of the air in the building could suddenly end up on the top floor, leaving a vacuum below. In statistical mechanics, we find that the probability of that state is extremely low, though not zero. That’s where the Second Law of Thermodynamics comes in, telling us that although air could obey the other laws while going from low pressure to high pressure, it doesn’t.

  12. My first comment is directed
    My first comment is directed to John B. I believe that the term stack effect comes from the fact that hot air and smoke rises in chimney stacks. 
    In regards to ceiling heat, in Canada we take advantage of ceiling fans to drive the warm air down in the winter and reverse the fan to pull cool air up in the summer. 
    Jim Jennings 
    Canadian Academy of Building Sciences Inc.

  13. Allison, you’re right.
    Allison, you’re right. Everything is possible in world of statistical physics (although with extremely low probability). It seems that buoyancy itself is so confusing to people so let’s rather stay within the scope of macroscopic physics and continuous matter (I wouldn’t mind though if you invite me for a drink to discuss this other very interesting topic). I spent quite a bit of time of my life studying fluids, and it seems that notion of pressure in fluids is still vague derived mainly from statistical physics, and no wonder it creates a lot of confusion among people on topics such as buoyancy effect. It is hard to bridge discrete and continuous worlds…

  14. Jim J.: I
    Jim J.: I don’t know the origin either, but that connection with chimney stacks makes a lot of sense. 
    Danko: “It is hard to bridge discrete and continuous worlds.” Indeed! It’s also hard to go from systems with 2 particles to systems with gazillions of particles, which is why we have statistical mechanics. And of course, we definitely need to discuss this over drinks. How about the Brick Store Pub? It’ll be nice to catch up with what you’re doing at Huber these days.

  15. Everyone has seen that photo
    Everyone has seen that photo of the building under construction that shows the plastic sheathing sucked in at the bottom of the scaffolding but billowing outwards at the top. 
    I was so lucky to have found my own version of this on a building in Lakeland Florida recently. 
    I did get a few strange looks as i ran out of the car to take a few photos of the outside of a nondescript building. But, hey, it was almost like hitting a tri-fecta at the track!

  16. I am assuming that the
    I am assuming that the referenced Canadian practice of seasonally pushing and pulling conditioned air is a joke. Right? 
    By the way, in a very tight building, it shouldn’t matter where the heat comes from. A radiant ceiling will perform as well as, if not better than a floor. Why better? We tend not to place furniture and rugs on the ceiling.

  17. A short two cents: this post
    A short two cents: this post points out why a square ranch home is much more energy efficient than a two story colonial !

  18. Thank you once again, Allison
    Thank you once again, Allison. All too often we get hung up on things that we could just keep simple for our students and our clients. After all sometimes, a cigar is just a cigar.

  19. admit I haven’t read every
    admit I haven’t read every comment here but at first blush, it seems there’s a key point missing. It’s warm FLUIDS that rise. Heat will go in any direction, on its way to outer space

  20. Loma, 

    One of the distinctions I’m trying to make, not very successfully, is that a warm fluid does not act on its own, but rather moves up when surrounded by a denser fluid which pushes it up with buoyant forces. Where that distinction becomes important to me is the in places like an attic where it is often described that the warm air exits the ridge vent and pulls in its replacement air. In fact, the incoming air is being pushed into the attic which then forces the warm air up and out the upper vents. Discussion of issues related to the short-circuit theory, for example, require this understanding. 

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