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.
Ted, I'll start with yours, it's shorter. If you reviewed the link I created you should have seen that the negative pressures in the lower portion of a house, like a basement, are due to the column of warmer less dense air inside our homes. Above our homes, the weight of air is identical whether we are adding up the barometric pressure on the outside or on the inside. Once we look inside, the weight per foot is less because the air has expanded and thus per foot, adds slightly less weight to the total column of air. At the bottom of the two columns, outside and inside, the pressure difference is what pushes cold air into our homes and subsequently the warm air up and out as in stack effect and the warm air in a chimney as in draft.
As for cold air pushing warm air, I have difficulty explaining the vertical pressures, as they as they are simply a stack of one foot layers of air. Since our homes are not pressurized, any air pushed into the lower portion has to be matched with air exiting somewhere. When we look above the neutral plane we see positive pressure wrto so that is where the air exits.
What makes this more difficult to visualize, or test, is that our manometers cannot see Barometric pressure, they can only compare the weight of air between the reference port and the input. Any change in elevation is not registered due to the weight of the air inside the tube. I spoke directly with a technician at the Energy Conservatory to see if there was any way we could trick the manometers into seeing this drop in barometric pressure as we move up in elevation and he assured me there was not. And by the way, he totally agrees with the concept of warm being pushed up, not rising on its own. He is the one who gave me the equation for calculating stack effect.
Since we cannot measure the drop in barometric pressure as we go up, I had to calculate it and thus the diagrams in the link. We know the pressure on top of a mountain is less and the math does a good job of giving us an estimate of how much. Then, we know that warm air is lighter, so a little more math and suddenly we come up with the pressure differences that we already know are there. Only now we know why and where those pressure differences come from. For those who still think warm air can rise on its own, I'm willing to listen, but I need the math to be able to calculate the forces and no one so far has provided any numbers.
Keep reviewing the link I provided, despite the limits of my ability to explain things, the answer is there.
Hmmm. I think I'm seeing it and may be able to pull something really useful out. For me, providing analogies to homeowners so they understand the importance of these issues is really important.
My take on what you are saying:
Imagine warm air is air, cold air is water, and water is filling a glass pushing the air up. The warm air doesn't pull in the water (or cold air) at the bottom, its the water (cold air) that pushes up the warm.
That's it and even more, it is the thought of how we can improve our explanation of this process along with the understanding. I'm a poor choice to be rewriting this and I am hoping many others will contribute, directly and indirectly to this effort.
I have found a few places where this information affects our understanding of air flow, but once a thousand eyes are looking in this same direction, I believe many more will will be discovered. The move over into the HVAC field and another batch of questions will pop up.
Well, I'm going to work on the other post, looks much more difficult.
As I review this discussion, I am drawn back to a few physics definitions learned in high school:
1) Matter is anything that has mass and occupies space.
2) Matter is neither created nor destroyed it just changes form.
3) Energy is the ability to cause change or do work.
4) Matter will always strive to reach its lowest energy state.
Some forms of energy include light, heat, electrical and mechanical energy. For the purposes of this discussion, I'll focus on light and heat energy.
As light or heat energy is applied to matter, the atoms of that matter move faster. As the atoms move faster, they occupy more space or -- expand. The mass (weight) of the matter has not changed but, the space that it now occupies is larger, meaning that its density has changed (decreased).
Applying Archimedes principle to the same matter (i.e. water vapor) but with different energy levels (light or heat) and thus, different densities, leads to stratification. That is, the warmer water vapor (with greater energy and occupying more space) will float above the cooler water vapor (with less energy and occupying less space).
Now, lets add the rule that energy always strives to reach its lowest state (sometimes stated that hot always flows to cold). Absent any other influences, the warm water vapor will raise the temperature of the surrounding cold water vapor until the two reach equilibrium -- their lowest energy state, absent other influences.
We can observe all of this at work with rainfall and clouds. A rain puddle on the ground is warmed by the sun. The water molecules absorb energy, move more rapidly and evaporate to water vapor. The warm water vapor rises in the sky to form clouds at a point at which it is no longer lighter than the air above it. The clouds accumulate, blocking sunlight that might warm other water vapor below. As the the water vapor cools, the clouds fall. As the water vapor cools further, it condenses to liquid. Now heavier than the air, the rain falls from the sky to the ground. Then the whole process repeats itself.
You can apply these principles to air or any other matter.
You can also use these principles to explain air pressure changes... as energy is applied to matter, the matter occupies more space (expands). As matter expands, it exerts more pressure.
Think of a steam locomotive: Coal heats water in a massive boiler. The water absorbs the energy, expands, and becomes steam. As more water is converted to steam, pressure builds and pushes the pistons that drive the locomotive.
Your turn Paul.
At my first reading I thought I was going to have to dig out my physics books, but upon a second read, you are basically correct. There are just two areas where I would suggest changes.
1. From your post: "Now, lets add the rule that energy always strives to reach its lowest state (sometimes stated that hot always flows to cold)." Agreed, except that the problem at hand is only dealing with different density of air. The fact that we are using heat to create those densities should not be confused with heat flow. Our problem could just as easily be explained using different gasses at the same temperature, but different densities.
2. Again, from your post: "The warm water vapor rises in the sky ". That is indeed what happens, as in many examples of warm air rising. However, one of the pro-active steps I believe we need to take is to state these occurrences in a way that minimizes the inference that the warm air is rising on its own, just because it is warm. This is not an easy task as we, meaning almost everyone, are so used to the describing what we see without concern for why it is happening. This particular subject is so common, that any changes we make need to be carefully worded so as to not create more confusion than we/I hope to eliminate. I have suggested before that this may actually result in two languages, one we share with our customers and another we use within our technical community, like those acronyms mentioned in another post :).
Thanks for the input,
If we said "warm air from the end of the flue rises when the heater is on because it is less dense and therefore lighter than the surrounding cooler air and gravity has less of a pull on it, and cold air falls out of the open door of the freezer because it is more dense and therefore heavier than the surrounding warmer air and gravity has more of a pull on it" would that be incorrect? I am just trying to state this in a way that is accurate, succinct, and does not intimidate those that are physics-averse. We don't go into detailed descriptions of the shape of an air leak, the chemical changes that occur during combustion, or how to determine emmisivity so you can alter the setting in your scanner, yet auditors are able to function perfectly well in the field.
I think the wording we use with our customers will be the last to change. Among the HVAC and energy related community, the change in thinking will be first, followed by better wording, and then somewhere along the line we will get comfortable with a couple of key phrases that make sense to everyone. I'm an optimist.
For now, that wording sounds fine to me when talking to the home owner. If I were talking to this forum, I would use the more uncomfortable wording like "the warm air is pushed up and out the chimney and up into the atmosphere by the colder surrounding air". Maybe a bit different, but in essence, at every chance we get within our technical community, we should be making the point, or not continuing the wrong message, that warm air does not rise on its own.
One of the problems that this change faces is that everyone these days references the internet when there is something they need clarified. However, the internet is far from the Encyclopedia Britannica which was reviewed and corrected extensively prior to print as once published, it would be impossible to change. With all of its advantages, the information on the internet is all too often taken for the absolute truth, this topic being a good example. If I were to take a WAG (wild axx guess) I would say that 80% of the references to this subject are either dead out wrong or misleading. I was thinking 90% but I compromised. Looking back on my own education, and that goes back many years, warm air has always gone up, even though I knew different from my physics classes. A chimney always pulled the smoke from a fire up the flue and I never questioned it. So, conservatively speaking, we have 50 or more years of saying and thinking about this the wrong way to correct. I may not be here when that final wording is deemed acceptable.
To give you an example of how this new thinking can influence our work, here is a post I just put up on the EnergyAuditorTalk forum. I will include it over here eventually, but I want to test it over there first. http://www.energyauditortalk.org/index.php?topic=1534.0
This might be a useful illustration, thanks to another member of HEP.
Hi Evan, I always love IR images. Even though the description is currently acceptable for discussions with our home owners, it lacks the distinction I think we need to be making when talking to other energy professionals. Here is an example using a description of convection. "The warm air rises and the cold air flows in behind it." Stated in many different ways this does describe what people see. But in the technical world, be it the IR lantern or any other simple explanation of convection, we need to begin reversing the sequence so that everyone who has forgotten their high school physics can be reminded what comes first, "the cold air pushes in forcing the warm air to rise." Despite being somewhat uncomfortable to describe the process this way, the change is necessary to begin the correction.
As an example of the damage that has been done by allowing the convenient wording to prevail over the technically correct, consider the common perception of gable vents short circuiting the air flow between soffits and ridge vents. In my search for the testing that produced this conclusion, all I have found is other people searching for the same testing and, some statements concluding that this short circuit concept is an obvious conclusion. What I'm coming to believe is, the short circuit conclusion was a wrong one based upon a wrong understanding of convention. If one believes that warm air can rise and thus pull its replacement air in behind it, then the gable vents would provide an alternate path and thus disrupt the desired ventilation flow. However, if one understands that the venting process is powered by cold air pushing in the soffits then, where the warm air exits is independent of that air flow. Leaving the gable vents in place does not change the pressure outside our soffit vents and therefore that flow will continue as it should and where it should.
Evan, if you have a source for some testing that supports the short circuit warning, I'm interested.
And thank you for the IR, input from all directions on this issue will be necessary for us to develop and become comfortable with our new language.
Ah...I think I've found someone beside me who has ventured past the "myth" curtain, and the verbal harrangue from the little man behind it to ignore him. I just joined this forum, and finding this thread is a good sign I've joined a good group of knowledgeable folks.
Basic law of physics; the greater force is the driving component. Greater heat to lesser heat, higher moisture content to lower moisture content, higher pressure to lower pressure. That's primarily what interests those dealing with building envelopes.
Looking at wintertime stack effect within a house. What is the prime force driving stack effect: warm air "rising" in the house, thereby "sucking cold air in" down low to make up the difference? No? Well, how about atmospheric barometric pressure, then? Yes. All one needs to do to verify this is pull up barometric pressure trend data for a given location over the course of a year. Wintertime - barometric pressure levels trend higher than summertime levels. Meaning in winter the atmospheric air mass is sinking and in summer it is rising. Oh yeah, there's that other law of physics: gravity, which acts "down".
Does air buoyancy have any play in this scenario? Of course. Air expands as it is warmed, thereby decreasing in density. Gravity can't act as "down" as it could with the air colder and more dense. So the air mass rises. Which would explain summer air masses conveying upward and winter air masses conveying downward.
For winter indoor stack effect, the sinking air mass outdoors is the greater force, seeking to enter the building. As it does so, it displaces the ligher (less dense), warmer air mass within the house, causing it to escape the structure toward the top. The rising air mass within the home finds resistance at the top; all of it can't escape at once. Therefore it is slightly compressed as the cold air mass from outdoors continues pressing inward from the structure's low regions, creating enough of a pressure delta for the displaced, warm air to squirt through any hole or gap in the upper building envelope, seeking equilibrium with the outdoor air mass.
For summer reverse stack effect, the opposite from above is true. The outdoor air mass is less dense and therefore conveys upward. Since humans like to cool their building spaces, this creates a more dense air mass within the structure, which gravity acts down upon. It collects down low and builds pressure against the lower components of the envelope (baseboards, windows, etc). The outdoor barometric pressure is less than the indoor air mass pressing against its boundaries, so it exfiltrates here, to be replaced by infiltrating air from overheated attics and other leakage up high in the building.
In both cases, the fact that we keep our indoor air temperatures within a fairly tight range is the agent that plays against the primary force; our atmosphere, and the incredible ocean of air that floats above the ground.
Hi Cameron and welcome to the discussion on a topic that should have been easy. Your explanation is long with some rough edges, but basically you have a handle on the topic. My challenge is several fold. First, bring enough of the technical world on board with the correct thinking. Second, condense the explanation down to something simple that becomes obvious so more will follow. And third, correct the language we use when describing this natural happening so we don't continue to imply that warm air can rise on its own.
In the process of all of this I have the distinct feeling many will identify errors that need correcting and processes that can be improved. As mentioned in the thread, I feel that the "short circuit" concerns have evolved from some wind blown scenario into the belief that leaving a mid zone vent area like a gable vent in place will short circuit the function of the soffit venting rendering it ineffective. In reality, the pressure pushing cooler air into our soffits is unchanged by having the extra gable vent.
I have created the one link to illustrate how the barometric differences between inside and outside air result in the driving force for stack effect. The same can be applied to attic venting and chimney draft. Soon I will have additional pages to take each supporting example for hot air being pushed up to further explain this process. any and all advice is welcome, as this legend is well embedded into our thinking and training.
I'll move back to the top to keep this going, but you may need to read all of the above to pick up where we are at:
Gravity pulls both warm and cold air down. Cold air, being heavier, will move to occupy the lower level, thus pushing the warm air up. Warm air is always pushed up, it never rises on its own. The problem that we face today is that we have observed and described warm air as "rising" for so long that we now read where it has the ability to pull in its replacement air as it leaves. Since warm air cannot rise, it is always pushed, it cannot pull anything in behind it.
The ultimate goal here is to gradually modify our Energy Language to be more, if you will, Energy Correct. If we can communicate the same message without implying that hot air moves by some new science of its own, those who write about this process, teach this process, or work with this process, will eventually drop the old language in favor of the new.
For starters, when describing hot air rising, avoid the "pull" word. Then, instead of describing the flow of cold air behind it as replacement air, call it displacement air. So, the warm air exits the upper vents as the cooler displacement air flows in behind it. Not perfect, but it conveys the same message and avoids implying that warm air has some unique powers all its own.
That's the first of the many corrections that will be needed to purge our energy profession of this confusion and I hope, without creating more confusion.