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

Views: 687

Reply to This

Replies to This Discussion

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


Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network.

Latest Activity

Adrienne DeAngelo posted an event

GreenPoint Rated CORE Training at Best Western Plus Marina Shores Hotel

October 15, 2015 to October 17, 2015
Increase your certifications or get to know the GreenPoint Rated system so that you can work more…See More
20 minutes ago
Colin Genge replied to John Carton's discussion Smoke Puffer stick
"Retrotec Smoke Puffer is available for pickup from Energy Federation. They will not ship them due…"
25 minutes ago
Colin Genge updated an event

Retrotec is exhibiting at the ACI National Home Performance Conference & Trade Show at Austin Hilton

April 4, 2016 at 8am to April 7, 2016 at 5pm
The 2016 ACI National Home Performance Conference and Trade Show is coming to Austin, TX April…See More
28 minutes ago
Colin Genge updated an event

Retrotec is exhibiting at the NEBB Annual Conference at Hyatt Regency Albuquerque

April 14, 2016 at 8am to April 16, 2016 at 5pm
Save the dates for the 2016 NEBB Annual Conference.See More
29 minutes ago
Colin Genge updated an event

Retrotec is exhibiting at the CxEnergy 2016 Conference at Hilton Anatole

April 11, 2016 at 8am to April 13, 2016 at 5pm
Top Reasons to Participate in CxEnergy 2016 Premier Conference & Exposition for Building…See More
29 minutes ago
Colin Genge updated an event

Retrotec is exhibiting at the ABAA-Air Barrier Association of America Conference & Trade Show at Renaissance Baltimore Harborplace Hotel

March 22, 2016 at 8am to March 24, 2016 at 5pm
The Air Barrier Association of America is hosting its fifth annual conference and trade show in…See More
30 minutes ago
Colin Genge updated an event

Retrotec is exhibiting at the Midwest Residential Energy Conference at Hyatt Regency Lexington

April 4, 2016 at 8am to April 6, 2016 at 5pm
The Midwest Residential Energy Conference promotes energy efficient home performance through the…See More
30 minutes ago
Colin Genge updated an event

Retrotec is exhibiting at the JLC Live New England 2016 Residential Construction Show at Rhode Island Convention Center

March 16, 2016 at 8am to March 19, 2016 at 5pm
Welcome to JLC LIVE New England!JLC LIVE attracts thousands of residential construction…See More
30 minutes ago

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service