One of the objectives of my many discussions on "why hot air rises" has been to change the way we describe that process.  The reason I believe that change is needed is the current simplified description has inadvertently credited hot air with its own mystical power to levitate itself, which of course it doesn't have.

In place of phrases like "the warm air rises" it is better to say "the warm air is pushed up".  Saying "it rises" implies it does so on its own, where saying "it is pushed up" clearly states that another force is responsible for the upward air flow.  In most cases, that other force is the colder air to which it is being compared.

So, why the fuss, the wording doesn't seem to be all that different?  When talking to home owners, simplicity is often the better choice.  But when discussing air flow with those trained in home performance along with the HVAC and other related technical people, it is desirable to perpetuate the correct explanation.  Here are some examples of what has evolved from the current poor choice of wording.

1.  The "short circuit" issue often discussed in relation to gable vents "short circuiting" the air flow between soffit and ridge vents is incorrect.  Air will continue to flow into the soffit vents based upon the pressure difference between inside and outside.   There are reasons to close off the old gable vents, weather, lack of air sealing, but the short circuit reasoning is wrong.  If they have to be removed, do it for the right reason.  Otherwise, save the homeowner some money and gain the benefits of extra ventilation.

2.  The warm/hot air flowing upward through a chimney is not pulling its replacement air into the combustion appliance from which it originated.  It is the heavier air surrounding that appliance that is pushing the warm air up the chimney and providing the necessary supply of combustion air.

3.  Advising someone to open a window to facilitate starting a fire in a wood stove or fireplace can be dangerous advice.  Understanding the effect of opening that window (shifting the neutral plane) will help explain why they should be looking for another solution, like a dedicated direct air source. 

4.  If hot air is not rising by itself to exit through high leaks and thus not pulling in colder air through lower leaks, how would you explain stack effect?  I'll let you work on that one.

Why hot air moves up is a simple principle and one that can help all of us sort out some very complex problems. 

The selected examples above come with longer explanations which we can cover as needed.

Bud

Tags: air, circuit, effect, hot, short, stack

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Bud

Hot air DOES rise, but cool are also falls.  You don't get one without the other.  It sounds like you are giving cool air the power to mystically de-levitate itself.

I'll keep saying "hot air rises" because it is simple and intuitive - "see that big balloon over there?"

Hi Ed,

I know you have followed some of my discussions so I have to ask.  If the more popular "simple and convenient" description has led to a significant misinterpretation of air flow, and there are many examples, isn't it appropriate that we seek a better way to explain the process to at least begin to reverse the years of oversimplification.  I recognize that we need to avoid confusing our consumers, but these forums offer a great opportunity to pass this message along to the many newly minted energy professionals and even the training programs that are still following the simple and convenient approach.

As for the damage the "hot air rises" phrase has done, a quick search on any of the topics I have mentioned above will return a variety of explanations.  I hesitate to quote examples as those explaining it poorly would probably prefer to not be singled out.  I did read one description that I found very interesting.  It was written by a building inspector acting as a professional witness in a law suit discussing mold issues in an attic and pointing blame at the contractor for all of the wrong reasons.  Simple and convenient may not be enough in all situations. 

Bud

Bud

"Hot air rises" is true, and I think there is nothing confusing about it.

1) What we want is a blanket of cool(er) air coming through the soffit vents and rising along the rake of the roof to go out the ridge, this takes the warmest air, the air closest to the hot sheathing, out of the attic.  The gable vent can supply a "short circuit" in that it will cool unevenly.  There is still plenty of hot air rising here.

2) Again - you can't have one without the other, so which is the chicken and which is the egg

3) If there is no backdrafting when the customer opens the window until things heat up, the problem is solved.

4) Not sure I understand the question - hot air is NOT rising by itself, and it IS pulling cold air in.  It is just as true to say that the cold air is pushing in the low leaks so that the hot air is pushed out the high leaks - hot air is interacting with cold air to form a system of air movement - see 2 above. 

I am surprised no one has a comment here.

Ed- I didn't comment because you nailed it in your first comment (mystically de-levitate!).

Sorry Bud, but hot air rises and cold air falls. It's fine if you want to push people to say both instead of just the hot air part, but I've never really seen this as a big problem.

Ed, Michael, to be clear, I certainly agree that hot air goes up while cold air goes down. My objective is to clarify that hot air does not go up under its own power and it is my belief that the long established "hot air rises" phrase has inadvertently led to many wrong conclusion by far too many people.

All air, hot and cold, is subject to gravity and thus being pulled down. The cold air ends up at the bottom because it is heavier. The hot air moves to the top because the cold air displaces it.

The confusion surfaces when people assume that the hot air moving up is responsible for that negative pressure below it that contributes to the corresponding infiltration. The reality is that negative pressure below is a result of the lower atmospheric pressure inside the house when compared to the outside. The actual negative number (wrto) will vary with temperatures and as the infiltration and exfiltration seek a balance by adding to or subtracting from the pressure within the house.

It is the heavier cold air from outside pushing in through the lower leaks that pressurizes the house to force the lighter hot air up and out.

Again, when cold air moves down there will be a corresponding flow of warm air moving up, but the only force exerted by that hot air is down.

Bud

Bud

What you are saying makes sense:

"The cold air ends up at the bottom because it is heavier. The hot air moves to the top because the cold air displaces it."

However, if there were no warm air to get out of the way, the cold air would have nothing to react with.  It's a totally symbiotic relationship.  

Going back to the 4 responses:

The first part of #1 is correct for a cooling climate, but the second part needs more discussion.

"The gable vent can supply a "short circuit" in that it will cool unevenly.  There is still plenty of hot air rising here."

Since the air flow entering the soffits will actually be increased by leaving the gable vents in place, the neutral plane moves up, what portion of the attic will "cool unevenly"?  And yes, there will still be plenty of hot air being pushed out both gable and ridge vents.

#2. Done

#3. " If there is no backdrafting when the customer opens the window until things heat up, the problem is solved."

Actually, the problem has just been temporarily avoided.  The more dangerous phase of the fire cycle comes as the fire is dying out and the home owner shuts the window and heads off to bed.  With an outside chimney and a cold night, the draft can reverse before the last coals buried in the ashes die out.  The switch from draft to backdraft can also be hastened by one or more exhaust appliances which will fill the chimney with cold air allowing it to remain in the backdraft mode after the appliance cycles off.  Relying on homeowners to do things right fails to consider, it may not always be the home owner controlling the fire.

#4. This has been covered, but to restate, the warm air is being pushed up and out.  It is an unwilling participant in this process as it is always being pulled down by gravity.

Bud

B: http://www.physicsforums.com/showthread.php?t=32331  unless you've taken thermodynamics(engineering, physics, chemical physics) five times, and taught it once, there's no way to understand it! That's why the old wives tales never die! enjoy, t

Hi Tom,

If there is one thing I have learned from the physics forum it is that they can add confusion to any topic.  I did read some of the thread you linked and surprisingly some of them managed to get it right.

1. Hot air does not "rise". It is pushed up by denser (cooler and/or drier) air underneath it. Stop the pushing and the air stops rising. Nothing moves against the pull of the force of gravity unless pushed by a stronger force.

http://www.physicsforums.com/showthread.php?t=32331&page=4

Now, if anyone brings some of their "thermodynamics" questions back here I will need your help answering them. :)

Bud

Hello all

Doing some research for our trip to Kauai next week when these two paragraphs rang a bell. Always the provocateur, and not wanting to miss an opportunity to announce my vacation plans (hey, I'm retired and the wife and I earned this over many years of hard work :-), thought I'd toss this into the conversation:

Kauai Trade Winds

In Kauai, the northeast trade winds, averaging 12 mph, occur about 90% of the time in the summer and about 50% of the time in the winter. They keep humidity at a minimum and ensure moderate temperatures, especially on the windward, unlike other tropical islands closer to the equator. These cooling winds are created because warm air rises near the equator, flows northward through the upper atmosphere, and cools. Because it becomes heavier as it cools, it falls back to the earth’s surface at about 30 degrees latitude, where it flows back toward the equator to replace more rising warm air. This creates cool breezes moving from the northeast to the southwest along the ocean’s surface, and over the Hawaiian Islands.

Now, if someone could tell me if my HVAC fan pushes or pulls air through my home, I'd be very appreciative :-)

Best wishes.

Hi Steve, I'm retired as well and thought I would get into energy auditing to keep busy.  20/20 hind sight tells me I should have taken up plumbing, but it has been fun.

Your example is a typical one where we have grown accustom to describing warm air moving up as rising and for the most part it does no harm.  If only they had started 50 years ago telling people why.  Without the why people have been left to their own imaginations and "hot air" has been credited with all kinds of powers, from lifting balloons to the smoke that exits our chimneys.

While you enjoy Kauai, remember, it was the cold air that started it all.

Have a great vacation,

OH, and mechanical fans can do both, push and pull.

Bud.  

Thanks for your reply, Bud.

"While you enjoy Kauai, remember, it was the cold air that started it all."

I'll be sure to educate any locals who try to persuade me otherwise :-).

Best wishes

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