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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Let's see if we can stir some interest with the conclusions that can be drawn from the above.

1. Attic venting:  When considering attic venting without the effects of the wind, static, the question of removing or leaving gable vents in the presence of ridge and soffit vents says leave them in place.  The discussion can extend to include wind driven effects, but first we must understand that hot air rising is the effect, not the cause of this process.

2. Stack effect is not initiated by warm air rising, but by colder air invading the lower air leaks in our homes.  Gravity is the engine, buoyancy is the science, and cold air wins the bottom.

3. Convection is also powered by cold air falling and pushing warm air up, not the other way around. 

4. Chimney draft is not powered by all of those hot combustion byproducts, but by the cooler air surrounding our combustion appliances pushing those hot gasses up.

I searched on stack effect and didn't find a single correct explanation.  Boats float because of buoyancy.  Hot air balloons go up because of buoyancy.  How is it that so many feel that warm air can rise and pull in its replacement air?




Yes, I agree that buouancy is they key here.  (But wouldn't it be handy if those "heat molecules" had invisible little propellers in them that helped power them--for unknown reasons--upwards?)

Perhaps the perception among lay people is fueled by the way they experience warm air being propelled (by invisible fans, or course) upwards from heating supply registers. Dunno.

Sometimes these "folk perceptions" (a.k.a. Legends) are safe enough (e.g. the sun "rising" in the morning and "setting" in the evening), but other times they are a hindrance. In any case, I agree with you that HP experts (and sometimes consumers) need to understand this just as an astronomer (or the informed lay person) needs to know that the Earth goes around the Sun. 



Good morning Evan and thank you for weighing in on this issue.  As I had hoped, your explanation is short and very clear.  Like your example of the sun rising, warm air rising on its own has become the accepted way of stating the process.  Unfortunately, explaining it correctly creates confusion and that confusion makes it difficult to discuss other areas of our energy science, this will help.  I'm sure I will be back looking for more clarification as I re-write the way we explain all things related to warm air, without those little propellers. 

What I thought was going to be a discussion of the modest forces involved in static attic ventilation has turned into a much bigger project, but from the searching I have done, we have let this stand for far too long and although it will be difficult, the correction process needs to begin.

Thanks again





The article you cited has this interesting tidbit:

"The air parcel, however, has mass and therefore weight. Gravity pulls it down. If gravity's pull is less than the buoyant upward push, the parcel rises. If gravity's pull is greater than the buoyant push, it falls."

So it is not the cold air pushing the warm air up, it is the reduced mass of each cubic foot (or inch) being less affected by gravity that allows it rise, allowing the colder air to fill the void.

It seems like you can't have cold air falling without warm air rising, and vice-versa, so they are intimately connected.  It does not strike me as unscientific to think of warm air rising  as long as you also think of cool air falling along with it.  

Ed Minch


Hi Ed,

I've been trying to come up with a short answer, always difficult for me, but here goes.

Gravity provides the downward force for both the warm and cold air.  The weight of the missing colder air (now displaced by the parcel of warm air) is the upward buoyant force.  That number minus the weight of the parcel of warm air yields the net upward force.  Reducing the weight of our air parcel would increase the net upward force, but it would not be fair to credit the lighter parcel with any form of lifting ability.

Convention has described warm air rising, as if under its own power, for so long it has become an energy legend and is repeated everywhere.  It is that "everywhere" and the need for our profession to properly understand this energy science that is fueling my efforts.  Now, I am certain we will never change the perception that warm air rises, but as energy professionals we need to recognize the issues like stack effect don't follow that often stated perception.  Cold air invades the lower portions of our homes and pushes the warm air up and out.  Nowhere is it being described that way and IMO that needs to be corrected, even if it is only within our profession.  It does make a difference, along with our understanding of convection, draft, and ventilation.

I realize it will take time to digest this, but rely on the old sciences, so far they have not changed.




I understand what you are trying to do - get the science right.  This is obviously not an easy concept if we are discussing it here, and we throw enough concepts at our customers already. We are paying for hot air, so it is easy for a homeowner to visualize his money going out the roof.  And it would not make the fact that the warm air IS rising out of the building any more palpable.

So how would you rather we describe it?  Put it in one paragraph that I can train my auditors on, and maybe even talk to the more receptive customers about.

Ed Minch


When I started this I thought what you are asking would be possible, but I'm having trouble explaining this to energy professionals, so changing the language for the home owners would probably introduce an unnecessary level of confusion.  For the time being, we will need to learn two languages and the one I'm suggesting is more of an understanding between professionals than a spoken language. 

Example, the flue gas that goes up and out our chimneys is being pushed out, not rising on its own.  Therefore it is not contributing to the negative pressure in our combustion zone by creating a vacuum behind it.  The same applies to stack effect.  The negative pressures we see in our basements WRT outside are due to a lower atmospheric pressure, the column of air passing down through our homes, when compared to the column of air outside.  This is something we can discuss, but I would not do so when talking to our home owners.

There are however small changes we can make and I will be working on them as support grows, like avoiding the vacuum word.  Currently, any suggestions I make will fall on many deaf ears as most still want to believe in those little propellers Dr Mills mentioned.  If we can get more people to comment, pro or con, we can clear the air and work on that new language.

As for anyone considering adding comments, don't be shy if you disagree with my explanation, many do.  But without discussion nothing will change.  If you agree, even better :).

And those talking points, they will come.




Still having a problem with this.  Here is something we can all try - it is kind of cool.  Find a single pane window on a cold day, no sun hitting the window, no ductwork nearby (at least the heater off).  With your smoke gun, stick or pencil, puff a little smoke at the top of the window, wait 10 seconds and puff again, wait ten seconds and puff again.  In a couple of minutes you will have a donut of smoke slowly moving around and around.  It falls down the window, hits the floor and goes out into the room, travels a foot or so across the floor, then starts to go up until it gets near the ceiling where it heads back to the wall where it joins itself at the top of the window.

So what is happening?  The room air at the top of the window is starting to cool off which reduces its volume making it heavier per unit volume.  As it falls down the window it gains speed as it gets a little cooler.  Once it goes all the way down it travels level away from the window, but then it starts to heat again from the room air, which increases its volume making it lighter per unit volume so gravity has less of an effect on it.

But notice that nothing would go up if nothing went down - you can't have one without the other.  This is, of course, a convective loop, but it is the same thing as stack effect.

I was taught by the Princeton Research team that came up with this, that there are two kinds of convective loops - open and closed.  A closed convective loop would be a top floor stud cavity in an old victorian house.  There is no top plate, so from the attic I can look straight down into this cavity.  There are no air leaks into the house so there is no blower door component to this.  However, there is still energy loss as the cold attic air falls into the hole, where it is heated and rises out again.  This is a chicken and egg situation where the falling and the rising act together.  And this is the problem that caused them to get the NASA-purchased infrared scanner out of the closet as they knew something was going on but couldn't figure it out.

An open convective loop is the same situation as above, but now we have installed electrical wiring in this stud cavity and have a hole where the box is cut into the wall. We still have the same internal convective loss, but now it is accelerated by the room air being (pushed or pulled - do we really know?) forced through the hole and the losses are much greater.  Convection plus stack.

So in technical terms can you really isolate the force that causes the warm air to rise and the cold air to fall so that you know which is doing the work?

Sorry for the lengthy reply - got going on it

Ed Minch

Good morning Ed, I took you to breakfast this morning to work up an answer.  As for the lengthy reply, I appreciate of the reply and the effort behind it.  This topic has been pushed aside for many years because simply describing what we see has been easier for all.  Changing this thinking is going to take more than my shoulder to the wheel to make the air movements we see fit the science behind them.   

I have observed convective loops as you describe and they make for a great demonstration, but to confess, at the time I would have described them as the warm air rising, because that is the way I had always heard it.  As for identifying the initiating force, we have to get down to the molecular density.  When that cold air first starts to move down the glass, there is a wave of slightly compressed air in front of it and a zone of slightly decompressed air behind it.  So, at the start, there is an increased pressure pushing air at the bottom out of the way, while there is a decrease in pressure above pulling in replacement air, simultaneously.  However, since the increase in pressure below and the decreased pressure above are still in the process of moving air molecules, the air farther out into the room doesn't know anything has happened yet.  As the process continues, that increase in pressure at the bottom and the decrease in pressure above extends to a column of air a few feet from the window and results in an upward flow within that column and thus begins our convective loop.  But it was gravity pulling the denser/heavier air down at the window that started the process.  We can repeat the demonstration with a hot glass window and a cold room and again it will be the greater weight of the cold air in the room that now pushes the lighter warm air next to the window up to initiate our new convective loop.  As the glass adds energy to the air next to it (heats it up) the air becomes warmer, less dense, and lighter, but it goes nowhere, until we define the temperature, density, weight of the air around it.  Once we know the density of our air sample and the density of the air around it, the denser/heavier air will initiate the movement, through buoyancy.

There is another way of looking at open loop convection.  If we look at stack effect where the incoming air and outgoing air are supplied by and absorbed by the great outdoors, where neither has any measurable effect on the pressures surrounding our homes, then the convection process can be considered as an open loop.  In other words, there is no continuation of the process once the air leaves the home.  This fact, that the outside pressures remain the same, is important in determining the boundary pressures that move our stack effect air in and out.  I will have a neat chart soon to illustrate just what those numbers are. 

I hope that makes sense. 



Still having a problem with this as the rising and falling do not happen unless in conjunction. 

You say:

"If we look at stack effect where the incoming air and outgoing air are supplied by and absorbed by the great outdoors, where neither has any measurable effect on the pressures surrounding our homes, then the convection process can be considered as an open loop."

This is the "butterfly in China" - of course there is an effect outside the house, it is a very small effect, but an effect nonethless.  Does opening the door of the refrigerator and watching the cold air fall onto the floor - to be replaces with room air sucked into the refrigerator - have an effect on the second floor of my house? Yes it does.  It is masked by the sun shining in the windows making little loops of their own and by the duct system forcing air around (and in and out of the envelope), but with the right instruments it should be measurable.

Stack and convection are the same thing - it's just a matter of scale.  And still, there cannot be one without the other.  This is starting to sound like my college calculus class - my head is hurting

Ed Minch

However, I am enjoying the interplay.

Just a quick reply, little new to the site and this is maybe a little simplistic but, at least for clients to grasp, the opener"heat will always go to cold" helps themto get their heads around the idea. To this point as it pertains to flue gasses, isnt it possible the hot combustion gasses are not being pushed at all but 'pulled' by the cooler outdoor temperatures surrounding the flue pipe? Whis is why drafting problems can occur more often when it is not that cold outside, like in shoulder seasons? I just wanted to throw it out there, have a good day all.

Hi Ernest, hot will always go to cold, but when describing air movements, hot and cold are just defining the density/weight of our air parcels.  We could shift to different gasses with different densities with all at the same temperature and the science moving them around would be the same.  As for drafting problems being more common in the shoulder seasons I would guess that might be due to more cold starts.  When it is cold outside we always kept the fire burning, I love a fire.  But in shoulder seasons, it wasn't needed as much so we would let it die and then have to cold start it again.  Just guessing, but stay tuned as I'm optimistic that with some help we will have some user friendly talking points that will follow the science and improve everyone's understanding of air flow. 

PS, the sad part about all of this, is the geeks in the labs already know all of this, I just haven't been able to find it written in plain English (no calculus equations) so we can all apply it.  If anyone is at fault, it is not the people describing what they see, but the scientific community that has stood by all these years and allowed the public's understanding to erode to its current level.

Re-read Dr Mills post above, he helped, and I hope he goes digging for some good material.



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