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.
Reply to Ed Minch 12-4-11, I guess we are only allowed so many posts in a string of replies, so starting over.
Ed, I think the key I think is the word "measurable". Certainly none of the tools we have could detect a pressure change outside our homes from vented warm air. But you are correct that it is still a loop as there is a ▲p at the bottom of our homes pushing air in, a ▲p from bottom to top inside our homes, a ▲p from top to outside, and then a ▲p from top to bottom on the outside, and they all add up to form that loop. This is the very essence of why we have stack effect and it is something we can actually calculate for any given day using the outside and inside temperatures.
I just downloaded a graphics program that I hope will allow me to illustrate all of this, at least for myself, it has answered a lot of questions. Briefly, there is one set of numbers that is relatively predictable and that is the barometric pressure outside our homes. Inside our homes we have basically the same pressure, except that the last 20 or 30 feet of air is warmed or cooled as we condition our inside environment. For talking points, a ▲40° inside to outside temperature difference produces approximately 1/3 Pascal pressure difference per foot of elevation. In a 2 story home of 20' that is about 7 Pascals bottom to top. Assuming a balance of leakage bottom to top, the neutral plane will be in the middle and we will have 3.5 Pascals pushing air out the top and 3.5 Pascals pushing air in the bottom. I know this raises more questions than it answers, thus the diagrams will be necessary to make this digestible, but it finally puts some numbers on what moves our air.
Ed and all others,
I have completed my first run through the diagrams to help explain the forces behind the movement of air we know as stack effect, attic ventilation, chimney draft, and others. These figures deal with stack effect, but the same process can be used to evaluate any temperature difference that results in air flow, ie convection.
Comments and corrections welcome.
Good to see you sturing thoughts again. "Everything is pressure diven"...would be my comment.
The figures in the link above are just the first (second) run through how to present this information. In time it will evolve to be a simple graphics supported explanation that answers the questions instead of raising more. :). I'm hoping others will contribute as this evolves as it will be something we can all learn from. It doesn't take much searching to find totally wrong explanations involving hot air.
Thanks for the encouragement,
According to statistical mechanics, where air has greater kinetic energy it has a greater probability to occupy a higher gravitational potential than less energenic air. That is, the system tends toward maximal entropy.
I also found this reply.
It is, of course, just Archimedes Principle that the less dense hot air "floats" on the denser cold air. Both the hot air and the cold air are pulled down by gravity. However, the pressure at any depth is greater in the cold air than in the hot air and it is this difference in pressure that causes the net force that causes the hot air to rise.
This also explains why the chimney affect is less on shoulder temps. Less temp (pressure) less difference, less movement.
It also agrees with cold air falling which is also true. The hot air simply sits on top. But the net gravitational pull creates the lighter hot air to rise. Cold air moves in to fill the gap. does this mean it is pushing the hot air up? is less down (gravity) mean you are going up?
Hi Bob, I'm not sure about the statistical mechanics and actually prefer explanations that are easier to understand so I can explain this to a wider audience, myself included. Archimedes Principle better suits me :). April Holladay's explanation from the link in my initial post does a good job as well.
Your second post shows you are on target and given the volume of misunderstanding out there you have done well to select the right information. To illustrate the need, without embarrassing the author, one expert witness involving attic venting had air flow all wrong. Of all places that needs the exact "correct" wording, it would be in court.
"is less down (gravity) mean you are going up?" not by yourself, but if the surrounding air is denser, heavier, then the heavier will push the lighter out of the way. Hold a 10 pound rock in your hand and you can feel its weight. Move your hand and the rock under water and the rock feels lighter, even though it would still go down if you let it go. The water displaced is equal to the reduced weight.
In our world of energy science it is important to understand that warm air is pushed up, thus it does not pull its replacement air in behind it. The pulling concept would suggest a negative pressure, which isn't there. Chimneys don't draft in the sense that the hot air pulls the new combustion products from the fire behind it as it moves up. The hot air inside a chimney moves up because the chimney is now full of much hotter/lighter air and the effective air column outside that chimney is cooler and thus heavier and is pushing it up. The chimney is simply an escape route.
Did you find my workshop link above any help in understanding this? Any suggestions to improve what I'm trying to say?
Your link is an excellent explanation of what is happening when warm air rises. I might remove the large atmospheric pressure reading from the diagram as it only is important when you start talking about differing atmospheric pressures and how they would affect the stack effect (that sentence really challenged my understanding of affect/effect!).
This very good explanation would work in a classroom for non-beginners, but is not something I would ever attempt to give a homeowner (with the exception of the local DuPont engineers that I run into on a regular basis - they are fun to deal with). Our explanation of how warm air rises and cool air falls and they force each other to move around is accurate for what it is and seems to work.
Consider: when the system fan on my forced air heater starts up, is it pushing air out the supply side or pulling air in the return side? If I restrict the supply, I reduce the return, and if I restrict the return, I reduce the supply. The air going out is exactly the same as the air going into the box in the basement (in a perfect world) and one doesn't happen without the other. In this case, the driving force is the fan, not gravity, but I would be hard pressed to tell an engineer that the fan is pushing out the supply and the return is just responding, although that is how we phrase it to a homeowner as it makes it easier to visualize the manometer that is showing the positive pressure on the other side of the bedroom door.
Likewise, I am hesitant to attribute the force behind warm/cold air movement to either of the two components as they play an equal role in this little drama. Warm air rises because it is lighter and has less of a gravitational pull than the cool air that replaces the hole it leaves behind as it rises OR cool air falls because it is heavier and has more gravitational pull than the warm air that replaces the hole it leaves behind as it falls. You say potato, I say warm air rises AND cool air falls.
Thanks Ed, this is just the first generation of this corrected education and as people like yourself recognize what is actually happening and work with both the technical side and the consumer side I'm hoping a new language will evolve that can explain this process without misrepresenting the facts. I know I'm optimistic, but time is on our side.
As for the atmospheric pressure, Pascals works for us, but means nothing to the public so I'm hoping to convert that to a term they can connect with, even if not entirely accurate. Convert 14.7 to feathers or something like that.
For now, I will use the existing diagram to support further discussions on attic venting, chimney draft, and other energy related air flow issues. You are familiar, I'm sure, with the reference to short circuiting when gable vents are left in place after a ridge vent is installed along with existing soffit vents. I have a feeling this short circuit thinking was a specific application and has now been adopted as the science for all ridge installations. When my diagrams are ready I will start another thread.
I think that ANY reference to the barometric pressure (regardless of the barometric metric) muddies the waters, because, again, it only has meaning when referring to changes in that pressure and how they might affect the stack effect. The stack effect can more easily be described as a stand alone item. YMMV.
The gable vents become a problem only when there is a wind out of the right (wrong) direction. Without a wind, the stack effect is still the biggest difference between high and low - think of opening a window in a house with a basement window open and the attic hatch missing. As for attic vents, throw a fan in there and what happens to stack effect in the attic? Throw a BIG fan in there and what happens to the stack effect in the house?
Fans will require a separate discussion. But, "think of opening a window in a house with a basement window open and the attic hatch missing." Until the new window changes the temperature inside the home, opening it up will increase the air flow into the basement window, not because it is pulling air out, but because it will shift the neutral plane to increase the delta p at the basement window and decrease the delta p at the top. The top to bottom pressure inside the home will remain the same as long as the temperature remains the same and the air exiting will always match the air entering. Similarly, leaving the gable vents in place will increase the air flow into the soffits until the attic cools down and since that is the end objective, leaving the gables in place simply helped us get to where we want.
The pulling concept would suggest a negative pressure, which isn't there. Chimneys don't draft in the sense that the hot air pulls the new combustion products from the fire behind it as it moves up.
The hot air inside a chimney moves up because the chimney is now full of much hotter/lighter air and the effective air column outside that chimney is cooler and thus heavier and is pushing it up.
If cold air is pushing the hot air to me that implies positive pressure. If hot air isn't pulling combustion, how does an induced draft appliance work? Why is my manometer telling me there is negative pressure at the flue, even after turning combustion off?
Maybe I'm not understanding something you are trying to say. I think these issues are connected like a chain and you are trying to say one link is the one that matter and another link does not.