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.


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

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Here is one of the useful details that I have uncovered from my work on stack effect (SE).  To estimate the total SE available in a home at a given delta "t" simply multiply that temperature difference by the height of the house and by 0.007.

SE (pa) = .007 x ▲t (F) x ▲h (ft)

This gives you the total SE which gets divided in some proportion between high and low with that ever elusive neutral pressure plane (NPP) somewhere in between. 

Now, knowing where the NPP is located can give you a hint as to how the leakage is divided between high and low.  To get a rough position on it, substitute 1 ft for the height of the house and you get the SE pressure per foot inside the house.  Take your baseline number at the front door (or anywhere) and divide it by that pressure per foot you just calculated and your NPP is that many feet above or below the height of your manometer.

If your resulting NPP is up near the ceiling, then you have more high leakage than low.  If it closer to the floor, then you have more low leakage than high.  Since you already know the inside and outside temperatures and you are going to take a baseline reading anyway, one simple calculation will provide you with one more piece of information that at times can be very useful.



I think I have come up with a way might make almost everyone happy:

Hot air rises while denser cold air falls, but since gravity is the force that is causing it, it is the cold air that starts the cycle.

Ed MInch


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