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.
only part that is wrong is going up you need to be 1 pa less on the inside per every 4 feet, as the warmer air is lighter. The first one up from the bottom should be 310 on the outside and 309 on the inside. But you have the concept correct, it is the two stacks side by side that create the pressure between inside and out.
I told you I may be confused...did you notice that my "opening" is at the bottom similar to John Straube's Figure 3?
and when I get to the top the difference is 8?
You can actually start top, bottom or in the middle, that connection just ties that inside point to the outside pressure. Then add or subtract the difference in the weight of the air as you go up or down. I find top or bottom easier to visualize. The next step is where you create an opening at the 8pa pressure point. Air then flows in due to the pressure difference, increasing the pressure inside the entire building. As that pressure increases, air begins to flow out the other vent which was previously equal to the outside. Once the infiltration decreases to match the increasing exfiltration, the pressures stabilize and if the vents are equal in size, the pressure across them will be equal (roughly).
For me, this was an eye opener and has helped me to gain a better understanding on several issues that have long been fuzzy. Straube's math and diagrams are perfect, but they lack the simplicity I feel is needed to shine this light on our profession. And, this simplicity will spill over into the hvac and other areas as well.
air movement? temperature vs density ... I'll take density!
Hi Paul, I thought this was going to be easy, but some well positioned pros are determined to have me drawn and quartered for questioning the norm. Fortunately, density wins is correct and ultimately the message will get out.
One of my objectives with this subject has been an improved method of explaining air movement. Frankly, the current variety of descriptions borders on "inventive" and since this is supposed to be such a simple subject, then there should be a simple explanation, and there is.
Air, warm or cold, cannot go anywhere on its own. It can expand when heated and contract when cooled, and although that characteristic is an essential part of air movement, it does not by itself (in open spaces) create movement. Air movement, up, down, or sideways, requires a force from somewhere else. The two dominant forces we deal with in our homes are mechanical (as in fans and blowers) or as a result of gravity with the denser air moving to the lower levels and pushing the warmer air up.
The explanation can be much longer, but this is a good starting point for eliminating the mythical powers of hot air. This has been highly debated, but many are coming to understand that although we see many examples of hot or warm air moving up, the forces for that to occur are coming from somewhere else. When hot air goes up, something is pushing it.
Still not fully seeing the push thing. Can't get my head around negative pressure relative to outdoors in a basement, or at the bottom of a chimney relative to both. I see that as the air column rising faster than the air at the bottom can keep up causing vacuum, so how can that be pushing.
If you've ever watched a bubble ascend through water it appears if some force is pulling it, not pushing it. High altitude balloons appear to have the same thing occurring.
I get that the weight of gravity is causing cold air to fall. Maybe when hot air goes up something is pulling it.
Ted, our homes are open vessels. To add air at the bottom, we must force the entire column of inside air up, as there would otherwise be no extra room. It's like hydraulics, you push air in at one spot and air has to move out somewhere else as the house cannot sustain a closed vessel pressure.
Cold air pushes in the bottom and warm air is pushed out the top. The same occurs for attics, chimneys, and our homes. Note, I'm not forgetting that the incoming air is passing through a heater, as adding heat facilitates the ongoing nature of this process even though it is not the driving force.
The scientists have know all about this for a long time, it is just that they explain everything with equations, have fun: http://epb.lbl.gov/publications/lbl-30147.pdf
Bud, your description implies cold air pushing hot air. To me that implies compression. Compression implies an area of high pressure.
When an induced furnace kicks, the stack goes positive due to the fan. Eventually the stack heats and the stack goes negative. Even with the fan running the power of stack draws the flue negative.
I can't understand how a thing being pushed can have any vacuum, how it can have negative pressure. I think there must be a "Pull" force associated with buoyancy, not simply a push force.
I think these relative weights experience force of gravity and centrifugal force, not just gravity. Seems to me lighter objects experience more centrifugal force and heavier objects experience more gravity.
I am often frustrated by my inability to better express myself. I really would like to get a clearer picture of what is happening, but saying the same thing the same way isn't going to do it. If you can figure out a different way to say the same thing, maybe I'll get it.
I agree with Bud...that buoyancy is a pushing force
maybe this video will help
Hi John, great video, I do enjoy Professor Miller.
A buoy floats on water. It is not merely air and, as an object, it displaces air-at-large, pushing it out of the space it occupies. I'll grant you that sort of pushing.
When there is no such object, when there's nothing but air, something different happens. Under the influence of gravity, air of higher density must change places with air of lower density, eventually creating stratification in some cases and equilibrium in other cases.
When differences are strictly in (a volume of) air, we're looking neither at pushing nor pulling, but some combination of changing places and diffusion. Air has no ability to maintain differences within a volume. Until one introduces wind, no air pulls, no air pushes.