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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John, good questions.... where are your answer?  Location of fresh air intakes?   we are usually restricted by the feasibility of access.

Hi Bob,

My take on John's list is not that stack effect is the starting point for determining where a fresh air intake should be, but rather another piece of data we should have as we choose.

I don't know how your training went, but mine gave me the standard plus up top, minus down bottom, and a NPP in the middle with only "hot air rising" to explain why.  I come from an electronics background, so we often had to zero out our meters.  When they discussed zeroing out the manometer, I asked why.  I never got a real answer.  When I compared the allowable draft pressures to what we were zeroing out (remember I was new) it looked like there wasn't much margin for safety.  But my instructor only had what was in the book and had never seen a full explanation as to where these pressures come from.  Yes, in a one or two week course there is only so much they can cover, but there wasn't even a book to direct me to and certainly no explanation that excluded hot air as the moving force.

Back to your question.  If you want natural venting, stack pressures and the NPP are critical, as in attic venting.  I'm even looking at what might work as a passive HRV.  Even if it wouldn't recover much, it would at least provide holes that would help tame the extreme swings of the NPP.  In our future super tight homes, I think "holes" are going to be desirable and where they are located will be important.

If I missed your question, take another shot.


Bud, I agree with your take

Stack Effect is only ONE of several important parameters



you asked: "where are your answers? Location of fresh air intakes?"


It might be easier to start with where NOT to locate the fresh air intakes....

Here is a link to an example from a recent discussion (RESNET BPI)



the location is Western Washington

Note: My illustration may not be an accurate depiction... it is only my interpretation of the System...


I am guessing that the Intention of this System was to filter and distribute the make-up air(fresh air, relief air)

I agree with the Original Poster (Tyson) ...... there is "Something Wrong"


Hi John,

I also agree that the description or the design is wrong.  I suspect the powered damper is supposed to be open when the furnace is running, although I don't like that method of providing ventilation (personal preference).  He mentions the manual damper is set for 25 cfm, which would only occur when the furnace is running.  And, as mentioned, nothing is going to "flow in" otherwise.  There was also one general comment, and typical, that the NPP is located in the middle third.  Although that may be common, it is not an accurate way to account for its effects on venting. 

We will see if the op comes back with some corrections.


I just stumbled on this older discussion thread of a current issue. Bud might have had an "AHA" moment when he realized that cold air falls and pushes and he's wanting to share it with others, but that is just as "wrong" as saying "hot air rises and pulls" without context.

As Building Scientist extraordinaire, John Straube, puts is: "hot air rises and cold air sinks". A more accurate phrasing would be that relatively warm (and hence less dense) air within a body of relative colder (and more dense) air will rise relative to the cold air mass and the cold air will sink relative to the hot air mass.

The governing principle here is Newton's Third Law: for every motion there is an equal and opposite reaction. If it's true that cold air falls and pushes, then it's equally try that warm air rises and pulls - equal and opposite simultaneous forces, neither of which could occur without the other.

A way to appreciate the both common sense and scientifically accurate proposition that "hot air rises" is to consider a 70° two-storey house in a 0° environment. If the house was perfectly air-sealed, there would be no air movement, but nevertheless the warm air would be attempting to lift the house into the sky just like a hot air balloon. But, with a 1000 SF ceiling, the warm air would be exerting a buoyancy, or uplift, force of almost 200 lbs, which is not nearly enough to lift a house.

This thread has gotten a bit oversized, but we can add to the bottom.

My "AHA" moment actually came when I searched the internet and saw how many people were referring to hot air as if it had some internal moving force that moved it up.  Which it doesn't.  Gravity is pulling hot air down just like cold air. Why would so many have it wrong?  No flame intended, but because respected people like yourself keep using the simple explanation without adding that it is not the cause, but the effect.

You said

<If it's true that cold air falls and pushes, then it's equally try that warm air rises and pulls>

Consider.  The warn air rises because the heavier cold air displaces it.  If a large sumo wrestler pushes a smaller opponent off the mat, we do not say the smaller wrestler pulled him in that direction, maybe not the best example.  In our world, the heavier cold air is pushing against the lighter warm air that is pushing back.  The resulting direction of motion is credited to the heavier cold air, and not a pulling force from the lighter warm air.

Our negative stack pressure (wrto) in a basement is due to a positive pressure outside.

I'll stop here and apologize in advance because every time I try to defend this simple understanding I seem to offend people.  I wasn't raised on a diet of newton et al so my wording is poor and my arguments weak.  But keep in mind, that lots of people out there have it wrong and we owe them a better explanation and that ex[explanation should come from better educated people than Bud.


You're assuming that because you "had it wrong" that a lot of other people do. I've never met anyone who believed that warm air rising was due to "Hindu levitation".

You just don't get it. Negative stack pressure in a basement is NOT due to positive outside pressure - it's due to a pressure differential CAUSED by the heating of air inside that makes it less dense and more buoyant. If the inside air were not heated, there would be no stack effect pressure, so it's the heating that - in the most logical and intelligible sense - caused the air to expand, become less dense and more buoyant and hence to rise. When air rises out of a space - just like with a suction pump - it creates a relative negative pressure which draws, sucks, pulls in the colder denser replacement air.

There is no positive pressure in our universe without an equal and opposite negative pressure. There is no rising without an equal falling. There is no uplift without an equal downwelling. There is no one cause and the other effect - they exist only because the other exists.

But if one part of the equation was changed (interior air made warmer) then it's logically and scientifically correct to say that the warm air rising is causing the cold air to replace it by pressure differential and flow.

We need a chalkboard and a pot of coffee, but I guess that will never happen.  But I do appreciate the input, I'll add it to my collection.  You can be sure I will invite you when I open another thread on this topic.


Chalkboard? That dates you. They're all white these days. And I learned recently that there is a difference between a chalkboard or blackboard and a slate. The poor schools all used black-painted boards and only the wealthier schools could afford real slate (I lived for a time in a former one-room schoolhouse in VT that had the original black boards - no slate).

I believe you live in Maine. When I was up in Orland (just downeast of Bucksport) in 1980-81 at HOME, Inc. living with four nuns and a priest out at Toddy Pond, we hosted a state-wide organizers convention and there was an elderly woman from Aroostook County who said her "secret" to community organizing was coffee. When she saw a problem that needed addressing, she would invite a few local women to sit around her kitchen table and drink coffee until a solution emerged - and then they would just go out and do it.

So I believe that coffee is the answer to everything.

Bangor, just north of Toddy.  Good friends had a camp there and I spent some time in the late 50's.  I also started in a one room school in Salina KS.  Military brat and dad retired here in Maine.  I have spanned a few generations, from that one room school to working with the engineer that developed the electronic blackboard, Gabe Torok (sp), been awhile.  You could write on it in one conference room and it would show up at the other end on a Picturephone.  What we worked hard to do way back then, 2nd graders do daily now, talk and see each other.

A quick story that few have ever heard.  The technician that worked with me got the assignment to install Picturephones for all of Nixon's staff.  Someone had the idea that this technology would be something they would love, a camera and microphone sitting in front of them all day long, LOL.  A week later he got a call to get down there immediately and remove them all.  Seems they were not comfortable with cameras they did not install.  Little did we know how infamous they would become.  Sure wish I had got that assignment, and saved the work orders.

It has been a long journey, and my ending choice has been energy efficiency, so far very rewarding.  Where it is fun discussing net zero, the reality for many up here is just the zero, as in no money to buy oil and no money to make improvements.  So I help them find the quick and easy fixes that can save what it cost in one season.  It's a short list. 

I'm drifting, they just took me out for coffee and blueberry pie.  Coffee at 9:30 at night isn't going to go well.


Robert, I just can't let these whoppers go unchallenged.

<When air rises out of a space - just like with a suction pump - it creates a relative negative pressure which draws, sucks, pulls in the colder denser replacement air.>

That is so untrue.  For the warm air to rise and create a negative pressure behind it, there would have to be some internal moving force within the warm air and there isn't.  Heating it made it expand in all directions, that's all. 

<The governing principle here is Newton's Third Law: for every motion there is an equal and opposite reaction.>  This has been presented several times, saying that these events happen simultaneously.  Well, it depends upon the scale of your event chart.  But the cold air falls and pushes the warm air up.  Before you yell, here is the rest of the story.  "for every motion there is an equal and opposite reaction", yes, and when the cold air falls, the earth moves in the other direction.  It is gravity that connects them, so they are the simultaneous actions.  Only after the cold air moves can it bump into the warmer air and create a positive pressure that motivates the warm air to move.  Once the net forces are in motion, the warm air is seen rising and the cold air falling, but it was started by the cold air and there was no negative pressure pulling the cold air along.  There is a negative pressure however, it is above the cold air where indeed replacement air is pulled in to replace the vacating cold air.

One more detail.  The net moving buoyancy forces come from outside the warm air volume, not from within.  A cork rises from the bottom of a pool, but it is just a cork.  Our warm air has had energy added to it, but the analysis could just as well be about a cork.  The warn air is just a lighter fluid being pushed around by a heavier fluid.

If my explanation lacks the proper scientific wording, then that is why I'm talking to people like you.  But you have to take a cup half full approach.


Your perspective is not "a cup half full" it's half a cup. You can say that a cup is half full because there's water filling up half the space. Or you can say that the same cup is half full because there's air taking up the other half. It's the same thing described in two different ways, but both are equally true and one cannot exist without the other.

There is no such thing in our universe as "positive pressure" - there are only measurable pressure differentials, with one measurement being less (more negative) than the other (which is more positive). They're both relative measurements and neither exists without the other.

You've identified the mechanism which is lifting the warm air mass - the energy transferred to it in order to raise it's temperature, or average molecular kinetic energy, which also results (if it's not confined) in a lowered density and a buoyancy (which can be modeled as a force).

While Newton was also the one who mathematically modeled gravity as a force of attraction, we still have no clue as to the nature of gravity or whether it's a force at all - current physics uses Einstein's General Theory of Relativity to "explain" gravity in terms of space/time curvature.

There is zero time lag between cold air falling into a warm container and the warm air rising - so one cannot state that one causes the other (which requires that one happens first). They are simultaneous because they are one and the same phenomenon, which can be accurately described either in terms of the cold air falling or the warm air rising.


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