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What’s a pressure plan?? Where is it in my house? Does anybody know where the pressure is greater, if the outside of my house is 11 degrees F. and clear and the inside is a warm and toastee 68degrees F. I have a small bet with a friend ;-} on where I think it is.
Jack.
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Do you mean as in "neutral pressure plane"? It depends on how many holes of what size are in your house but somewhere below the top of the thermal envelope, the lower the better. But since you've got money riding on this here's a plan. Drill a row of 1" holes up and down one strip of your house, build a big fire in the fireplace, and close the damper. Now run outside and look for the lowest hole expelling smoke. The npp is between that hole and the next one below.
Sorry, but my first pass at this used a pot of beans as the fuel for the generator - silly night after shoveling snow all day!
*Bill,Now, I hope you don't think I'm gona go drill all those holes now, do you?;-} So, are you saying that there's this places were air will blow out? I'm not to clear on this. What about where the greater pressure is, that's the money question ;-}Jack
*The pressure should be highest at the uppermost thermal boundary. Think hot air balloon.Sorry for my silliness.
*Bill,No problem, I'm silly most of the times ;-} Ok, "should"? I need a little better than that Billy boy. The reason I ask is because back in the "school days" I remembered something like cold air moves to warm air. It had something to do with density?? Warm air is less dense cold air more dense?? Does any of this make sense?? Anyway, I remember that cold is "high pressure" so when I open my window, cold air comes in?? If the house is at a higher pressure, why doesn't the air go out??Anyway, I bet the inside but when I think back to school I'm not sure I bet right.Thanks ;-? Jack
*The highest pressure will be at the highest point within the heated portion of the house. Hot air rises within the house and acts like the draft of a chimney. Cold outside air is draw in low (especially between the concrete and the bottom plate) where the pressure in the house is low. Hot inside air flows out of the house up high where the inside pressure is highest. Take a candle or incense to the dog door - air is flowing in. Take it to a slightly cracked upstairs window - air will be flowing out. Then collect your money. How much? Just for the sake of completeness: air always flows from high (total) pressure to low pressure and the pressure outside your house is constant. Otherwise the unrestricted outside air would shift up or down to equilibrate. Now if I wanted to be real evil, I'd point out that while everyone (myself included) has been talking about altitude-corrected pressures, another perspective is absolute pressure. There are fewer air molecules in a cubic foot of second floor air than in a cubic foot of first floor air. There has to be because the first floor air has another 8 foot column of air pressing down on it. Walk up and down stairs with an excellent barometer/altimeter and it would show lower absolute pressure upstairs. My altimeter shows differences of 20 feet, so I have to be in a multi-story building to see it change. But it definitely moves on an elevator.When a pilot flies a plane, he/she's got an altimeter that basically measures the decreasing air pressure with increased altitude. But they also have an adjustment for that day's sea-level pressure (e.g. 29.92 inches of mercury) During a storm the atmospheric pressure is typically lower. During a spell of clear hot or clear cold weather, the atmospheric pressure is typically higher. And the pilot uses a sea-level pressure from a weather report to set his/her altimeter for that day's condition - even if at a high altitude. That altitude-corrected air pressure is what we had been talking about and what will win you the bet. But the absolute pressure prespective also seems valid. -David
*Jack: Uh oh. From the absolute pressure perspective mentioned above, the highest pressure is at the lowest elevation. (And I can definitely vouch that there is higher air pressure in Death Valley than on top of Mount Whitney). So between the ground inside and outside? Air clearly flows in at the bottom of the front door (in winter) from higher absolute presssure outside to lower absolute pressure inside. Sorry. -David
*Dave,Thanks, I kinda follow you about the valley and the mountain stuff. I thought that the air pressure was higher at night in the desert when the temperature was the lowest? ;-} Jack
*Bill, It was precisely "your silliness" that has caused me to reflect and conclude that the neutral pressure plane is dynamic, ie, a constantly moving elevation depending upon what's happening ambient-wise outside (air temp, vapor content and wind speed and direction) and what's going on inside (basicly the degree of energizing of the ambient that's happening, ie, number and activities of people as well as the operation of heating equipment. Your example of a big fire that then gets dampened actually results in a MEGA change in the location of the npp, ie, while the exhausting of combustion gases was unimpeded via the chimney, large quantities of combustion air was being sucked into the living space, but once the fire was dampened this flow changed to one where combustion gases now pressurized the living space until an equilibrium resulted in their finding a pathway out of it. Hence your example was that of a very specific point among the infinite variations of the npp for a given living space. Had you not made mention of that example, I highly doubt that I would have thought thru the concept of neutral-preesure plane. Thank you.Brian
*Joe,Uh, are you saying that the pressure could be greater on the outside on a cold day then the inside??;-?Jack
*I should have stuck with the kettke of beans.I was first exposed to npp in fire protection engineering and sitting through presentations at code hearings. So maybe the fireplaces was a poor example.I'd agree the npp is dynamic but I believe it's still a contiguous interface layer that could theoretically be measured and plotted in three dimensions - correct? It is perhaps easier to visualize in a structure fire - like a single large warehouse - where below the plane there is negative pressure (sucking in air) and above it a positive pressure (products of combustion).And these pressures are relative to the pressures on the other side of the wall - taking into account the specific elevation and atmospheric pressure.
*Jack: Air pressure is the weight of the column of air above you from your measuring point to the top of the atmosphere. Storm fronts and warming during the day over land areas cause air pressure changes over large areas.Air density is how many air molecules per cubic foot. It is a strong function of elevation, but also of temperature. On a hot day, the air is less dense (the whole air column is taller) and, for instance, airplanes have less lift and parachutists fall a bit faster.So the air density is higher at night due to lower temps. Air pressure (at a fixed loation/elevation) is most effected by weather systems moving through, not temperature. There is a tidal effect as well - two highs and two lows each day like the ocean - but that's a very small effect. -David
*ehhhhh - I'm not too sure about that Joe. Or at least the answer could also be nope. Back to a warm house on a cold night, there's some elevation in that house above which there is a positive pressure in relation to the outside and below which there is a negative pressure. This does assume at least a somewhat sealed house as might be typical of millions and no inverted temperature whatevers that Fred speaks of. Unless your building for NASA - and I doubt there vessels are even perfect - no house is that tight that there could be a constant one-way pressure differential between inside and outside.
*Joe,I have to tell you that after reading your post along with the others, I thought you we're just nuts. So I did what you suggested. I when out a got one of those barometers, it didn't cost much. Last night it was about 15 outside and about 72 inside the thing read 29.8 on the inside. I put it outside for a half hour, when I read it, it read 31 I just couldn't believe it so I did it again, same thing. The sad part is I lost my bet. Well at least I learned something.Jack
*Jack,And if you really want to play with the math here's some formulas:The difference in air pressure from outside-to-in will be P=8.2x10exp-5 x h x dT pounds/square ft where h = the bottom floor to top cieling ht. and dT is the diff in temperature inside from out (degrees F). It's neg pressure at the bottom, + pressure at the ceiling and the neutral pressure plain is in the middle (even temperatures with no fans running). For every hole (at the very top or bottom) a draft is coming in at about vel = sqrt(h x dT x .065) ft per sec. For a 40F dT and 17 feet ht this comes out to be about a 4 MPH draft. - Darrell
*Congratulations Jack! Now, I'm sure your house is not air tight and that the air outside is constantly flowing from that area of higher pressure to the inside, the area of lower pressure. So cut a hole, put a turbine in, and you'll have free energy! Perpetual motion! Fantastic!What else you taking book on?
*Dear Bill,I really don't know a lot about this stuff. My house is warm and I have avg. heating costs, compared with my neighbors. Maybe you could explain why my house is not air tight and what pressure has to do with it?. That's all this business with this turbine??Jack
*Darrell,Well I'm totally confused now! If I understand you correctly, the (- pressure low) the cold air is at the bottom (floor) and the (+ pressure high) the warm air is at the top (ceiling). I always thought that warm air was low pressure and cold air was high pressure, you know, like with the weather. Well it just goes to show you that you can learn something new everyday. My mother said I should have listened more in school ;-} I'm really interested in this, could you explain it a little further.Thanks Jack
*Hi Jack and Bill,Jack: What bill is saying is that since the pressure in the house is lower than the pressure ouside, than the vacuum created by the differential is indeed sucking cold air into the house. Interface this infiltration with a turbine and you can tap the energay of the wind generated by that "great sucking sound" that Ross Perot once talked about.Bill thinks, and so do I, that the lower pressure in the house is caused by warm air expanding and rising and escaping out the holes in the top of the house.Jack, would you mind running your Barometer test for me at the highlest interior level of the house for me? I wonder if you would get a higher pressure reading at the top of the house than at the bottom, and higher than the outside pressure as well. I observe on the several rooms in my house that have nothing but a sheet of poly for a ceiling, that the poly billows upward with great intensity on cold days, which would suggest to me that there is greater pressure on the inside of the poly ceiling than on outside of it. If the pressure at the top of the house is higher than outside, and the pressure at the bottom is lower than outside, than somewhere in between is the elusive Neutral Pressure Plane!Curiously,Steve Zerby
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Thanks, Steve! Somebody finally gave the common sense argument to explain the whole inside vs. outside pressure differential. Seems pretty obvious that where air is being sucked in the pressure is lower and where it's blowing out the pressure is higher and somewhere in between it's equal.
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And Jack, while you're at it, would you mind climbing to the top of the tallest tree that you can find and run your barometer test, there....but first, please sign the attached waiver.......
*Wait a second. Could your house with stand that kind of pressure difference? 1.2 inches of mercury is a lot, it equals 0.59 psi. (And it had to be inches mercury your barometer was reading, average atmospheric pressure is 29.92 inches mercury). 0.59 psi is 85 psf and that more than the snow load the ROOF of my Alaskan house is rated for (55 psf snow load). 85 psf over a 40 foot x 8 foot wall would be 27,000 pounds. If you turned your house 90 degrees, would you feel comfortable parking 10 volkswagens on the (now horizontal) wall? If the 2x4's in your wall could handle that, why do I have 2 x 12's as my rafters to handle less load?I don't doubt that you read the gauge accurately, but I suspect an instrument error due to the big temperature change. My mountaineering altimeter/barometer is temperature compensated, but costs about $350. -David
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Mr Thomas,
I wonder why you neglected to tell Jack that, at normal atomospheric pressure, the total pressure on this same area is (((40*8)*144)*14.7) = 677,376 lbs. That a 27,000lb increase is only a 4% change in total pressure??
Inquiring Minds want to KNOW..........
*It is only the pressure differential that matters in this case. It would require a vacuum on one side of the wall for the entire 677,000 lb. force to be applied. I think Mr. Thomas is right about the temperature effect. Otherwise the walls would burst.John
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Jack, It helps to visualise it as a lot of cold, heavy air trying to get in through a bottom door. The pressure of this forces the hot air up and out through the top. Oops, I just read my old post. The draft comes IN at the bottom and OUT at the top.
Hot air has a lower change in pressure per altitude change (pressure gradient) than cold air does. This explains the positive indoor pressure at the ceiling than at the floor. Darrell
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JohnK,
I guess by "pressure differential" you wish to imply the "resultant force" Because, I'm sure, you would not presume to state that there isn't 677,376 lbs for force acting on both sides?? You could not also mean to say that this wall would be under only 27,000 lbs of force due to the pressure differential?
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Hi Joe,
Thoughtful hypothoses, both.
The poly doesn't rise up suddenly. As the temperature differential increases, the poly strains upward with greater intensity. I can just about tell what the temperature is outside by looking at the ceiling. When it is moderate outside, it strains upward less than when it's really cold.
The ceiling bows up whether the heat is on or off. I'm running hot-water baseboard heat in this room and a woodstove several rooms removed from it. My smoke sticks reveal the convective currents to be pretty weak even when the baseboards are warm and circulating, but when they've been off for awhile, the air is quite still, and the ceiling still bulges up with the same intensity.
As to the airfoil effect, I doubt it, as it doesn't billow on a windy summer day. The poly ceiling is below the upstairs floorboards of an eclosed, gutted, unheated second story, and is not subject to direct high speed horizantal winds blowing over it.
This whole concept seems like such simple common sense, I don't understand all the confusion about/resistance to it. Whether it's due to convection, to kinetic energy, to gravity or whatever, the warm air rises.
Think of the hot air balloon. Something is pressing on the top of that balloon pretty darn hard in order to lift Richard Branson, Steve Fossett and Per Lindstrom and thousands of pounds of equipment. I'd wager the pressure on the underside of the top of that balloon is pretty high. Convection? Kinetic energy, whatever (but I doubt it's airfoil effect), the warm air presses on the top, even with a huge hole at the bottom of the balloon that will allow for equalization of the system as a whole.
Steve
*As I read through the postings, at the 4.0 mark I wondered how long it would take for "vents" to be mentioned!Rich
*Hi Joe,I'm glad to think this through with anyone who will take the time to engage the conversation in a thoughtful manner.I'm no rocket scientist, I just play one on the Web ;-) No, really, my last and only physics class was as a junior in high school 24 years ago. I don't remember much of it, but I do remember however that warm air rises. Is this because it is of lower molecular density (mass) than the cooler air and therefore gravity acts more strongly on the cool air? Anyway, the warm air does rise and exert some sort of force on the ceiling.I guess I don't really know what's going on at the molecular level, but I can make the following emperical observations:1) the warm air is at the top of the room. 2) the plastic bows up with ever greater intensity, the greater the temperature differential. 3) the cold air is at the bottom of the room. My feet are cold. 4) if I open the top sash, air rushes out of it. If I open the bottom sash air rushes in. 5) if I cut a big hole in the plastic, warm air rushes out of it, and it must be replaced by more heated air if I want to keep warm.All this leads me to believe that whatever is pushing on the plastic ceiling is pushing harder from the inside than the outside, and whatever is pushing on the walls high up in the room is pushing harder on the inside than the outside, and whatever is pushing on the walls lower down in the room is pushing harder from the outside than the inside. Somewere in that room is the point where the air inside the room is pushing out at the same force that the outside air is pushing in. That point is some sort of demarcation. Let's call it the neutral pressure plane. If I open a hole in the ceiling, that plane moves higher up in the room (as long as I am trying to generate some heat in the room to keep it warmer than outside.)I guess what it all boils down to is what are the practical implications of the air's observed behavior. They seem to me to be that the most cost-effective way to keep the expensive warm air in the room the longest is to apply your air-sealing efforts from the top down. Locating the point of demarcation we are talking about is merely an indicator of how sucessful the air sealing is.I don't think of the house as a pressure cooker. It's an open system for the most part. I do think of it like a hot air balloon. It's closed at the top, open at the bottom, and I generate heat in it, which rises to the top and would like to keep rising if I didn't plug the holes. Thank goodness it's too heavy for the air to lift off the ground!Now my brain is strained, so I'll stop babbling.Steve
*I feel compelled to point out (because I still remember this tidbit from thermodynamics) that while Joe is right, expanding air does cool (like the exhaust of your air tools), it is an exceedingly minor effect over a range of 20 feet. The adiabatic expansion of air works out to 3.5 degrees F per 1,000 feet of elevation. So if it's 100F in Sacramento then it's 69F (100 - 8,800*3.5/1000) on top of Half Dome in Yosemite. And that has been my experience, repeatedly. So over a 20-foot rise, air would cool 0.07 degrees F. Not much. Adiabatically, (without the loss or gain of heat from outside the system)David
*And another, less destructive student went up to the doorman of the buliding (in another version of this joke) and said, "I'll give you this nice barometer if you tell me how high this biulding is." Barometrically, David
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Steve, Joe, et al
Not wishing to muddy the waters I offer the following facts, and an observation. The second law of thermal dynamics says that, unhindered, heat will always move to a colder area regardless of the direction:up,down, inside or out. All forms of heat loss take place at a faster rate when the temp. difference between the two areas is greater. Air is fluid (not meaning liquid). When a fluid is heated the distance between the molecules increases, and a given volume of the fluid will become lighter and rise. As it cools, the distance between the molecules decreases, the fluid becomes heavier and gravity pulls it down. This action is called convection. It would seem that poorly insulated/sealed dwellings with heat sources and cold exteriors would have constant convection. . . convection loops, and these convection loops would cause the hypothetical Neutral Pressure Plane to be constantly, or at least frequently shifting. Also, in Steve's case at any given time on a windy day a vacumn effect could be exerted on that gutted, open to the weather 2nd floor which further exascerbates pinpoint explanations for the phenomenom of the upwardly bulging plastic.
Resisting the urge to quip "hope this helps". . .
-Patrick
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Hi Joe,
Does it matter if the warm air is being pushed, pulled or rocketing along under its own propulsion? Whatever the method of propulsion, it is rising. The ceiling stops it from rising further, and if there were holes in the ceiling it would rise through them, and I would have to pay to heat some more air.
Beside my entry door I have two sidelights under construction that are currently covered with sheets of poly. They are slightly loose, so they telegraph pressure (force?) in or out if you look closely. Last night it was still and cool and I observed the following condition: The plastic from its top at about seven feet off the floor down to about four feet off the floor was bowing out. At the four foot level it was slack, and from there down to the bottom of it (about 30" off the floor) it was bowing in. This was true for both sidelights.
Also, when I crack the door about and inch and hold a smoke stick next to it, from the top down to about four feet the smoke blows out. From four feet down to about the bottom, it blows in. at the transition point it does neither.
This all leads me to believe there is greater pressure or force than the pressure or force of the exterior atmosphere above four feet off the floor, and below that point there is greater pressure or force from the exterior atmosphere than there is from the air in the house. The place where this condition reverses itself I'll continue to think of as the neutral pressure plane, unless you can suggest a more technically correct name for it.
By the way, I find difficult to read all the italics in your posts.
Floating along in Westford,
Steve
*Hi Joe,I have to continue our discussion in a fresh hierarchy of responses because when I hit reply to your last one all I get is a blank screen. Maybe we hit the max on the number of levels deep the software can handle, what do you think?I think you are splitting hairs over semantics. When you heat air, it rises. Websters defines the verb "rise" as follows: "...to go to a higher place or position; to ascend." Is that not what happens, even if it is pushed? What exactly is it that you are arguing about, other than that it irks you to call the upward motion/force of the heated air "rising?"Tomorrow I'm going to pick up a physics book. It's been a long time, but as they say, it's not rocket science. I did manage a 4.0 in graduate school, so I think I can handle a little physicsYou are correct that it does matter why it happens, inasmuch as we want to understand how to have the desired effect on the process through our construction practices. Of course it does. I'm not trying to run from understanding what's happening here. I just think you are making the whole thing more complicated than it has to be.So in your view, what is causing the warm air to move to the top of the building is that the denser cold air is pushing it up, Right? Ok. Now what? What are we all trying to accomplish here?Here's what I want to be able to do:1) to keep the warm air in the house for 3 hours in the safest, most cost-effective way.2) to evacuate the moist air from the house in a controlled manner at the desired rate so as to prevent it from condensing where it can cause damage.What might be your suggestions for accomplishing these objectives?Oh, one other question? What is your explanation of the behavior of the poly sheeting on the windows that I talked about in my last post? Bear in mind it was very still outside--none of it was wind-induced, as far as I can tell. No fans or blowers going. By the way, my living space is actually pretty well air sealed now, if only with poly in a lot of places. I think I'm losing a lot of my heat via conduction through the uninsulated areas, though.I'm really interested in what you think, I'm not trying to argue for argument's sake. I'm trying to learn.Steve
*If the warmed air expands, isn't it then less dense (us non scientests might say lighter) than the cold air? So the cold air - which is denser - is pulled by gravity downward? Forcing the warm air upward? (I know - the warm air going up isn't rising - its just going up by the force of the cold air displacing it.)Now, put warm air in a house. Cold exterior air finds openings in the house, infiltrates trying to get below the warm air by gravity, and pushes the lighter warm air up and out other openings.So if I put a piece of plastic over an opening up high in the house why isn't the warm air pushing it outward?I won't even ask why if there are vents and unenclosed fibreglass that the cold air won't enter the vent and fall to and through the fibreglass and displace the warm air upward.
*Joe, Bill, Fred, et al,I haven't been scared off here. Max Planck just makes for a some slow reading.I'll jump back in on this one when I feel better equipped to contribute.In the meantime, I would be interested in hearing your answers to Bill's Questions.Steve
*Joe? You still there? I was hoping for your comments on this. Thanks in advance.
*I'm sure I'll regret this but in the context of your statement above: "Gravity acts on everything equally. It doesn't act more on cold air then warm air. Galileo proved this in the early seventeen century. Due to gravity, cold air can accumulate thus become more dense. It is the pressure of this accumulation that would drive a convection current if a low pressure zone should exist or form." why do balloonists fill their balloon with warm air instead of cold? Why does heating the air in ballon make it rise if gravity is acting the same on the warm air inside as the cold air outside? What is the meaning of the word "is"?
*Bill and Joe,My head is really starting to ache now.I've been inching my way through Max Planck's treatise on thermodynamics, plus a nifty little book called instant physics. I really need something in between the two to fill in the holes in my ability to comprehend what Planck is saying. What I really need is to take physics again. I think we should all just wait till we are 35 to start any such classes. When you are a junior in high school it all seems so remote and abstract. Now it all seems real to me, and vitally important as well.Anyway, I had just finished reading the part about Galileo when I read your post mentioning it, Joe. And it hit me like a ton of bricks. Of course gravity effects warm and and cold air equally! But I get lost in the leap to the sentence, "due to gravity cold air can accumulate." Why? Gravity is acting on things irrespective of mass, right?So now I really want to know what makes the cold air and the warm air stratify in the first place? How come the denser cold air and the less dense warm air don't just diffuse evenly like mixing any two gasses?What forces are at play in convection currents?Also, what causes the stack effect? Why does a chimney draw?More confused than ever,Steve
*Steve - Sure, given a equal distance from the center of the object whose gravity you may be interested in - the Earth's in this case - the force of garvity is the same per unit mass. For instance, the feather and steel cannon ball in a vacuum fall and accellerate at the same rate. But since the mass is greater for the cannon ball, the force is greater - ie: its heavier. Just like a cubic foot of cold air is heavier than a cubic foot of warm air and the cold air sinks forcing the warm air up.You know, I work on tall buildings - theatres - and if you walk from the bottom to top - like the catwalks or the grid above the stage - you know that air does stratify and only very rarely is it other than warm at the top and cold at the bottom. Up high in the audience chamber cool air (from the air conditioning) is dumped in at very slow speed - 200 fpm or so at the terminus of the duct (for noise purposes) and unless great care is taken on distributing the return intakes around the room, the cold air will fall and puddle on a few seats. Same thing happens below the front of the balcony - cool air moves down the balcony and spills over the balcony fascia onto the row or two below. It can be very noticable in larger facilities if the design fails to get returns distributed along the first couple rows of the balcony.Now why they don't mix - instantly - and sort of average out is a whole other issue for me - because I know they don't else I'd never feel a blast of cold aire when someone opens the door. I suppose it's like some of those fancy after dinner drinks with the various colored liquors all layered - but I never waited long enough to figure it out. (Would specific gravity be involved here? Stay tuned -don't touch that dial - someone will surely offer an explanation.)Now I've got to pull out my fire protection engineering texts and refresh my memory on the neutral pressure plane. I know its related to pressure relative to the outside of the compartment or envelope, and that above the NPP the pressure inside is greater and below the inside pressure is less than the exterior. In a big fire (like on a theatre stage) that can cause a door at stage level to buckle inwards. I believe that for pressure dynamics in a house that the principals as used for a fire may apply.Simply, Bill
*Thanks Bill,As I was outside shoveling some very heavy dense snow I was thinking about all this some more and feeling very stupid about not realizing that there is a difference between accelleration due to gravity vs. Weight.F=ma, right?When acceleration due to gravity is the same for two objects (9.8m per second squared, as per Galileo), The resultant force between them is proportional to their mass. The force due to gravity between the earth and any object is proportional to its mass. Objects with a greater mass apply a greater downward force, due to gravity (they are heavier). Is this correct?If so, then the air stratifies simply due to gravity. Heavier cold air, is pushing down harder, and the lighter, warmer air gets displaced upward. Right?So is the stack effect simply due to gravity as well, or is there more to it than that? I have an idea what contributes to it, Let me throw it out and see what you think.When you generate heat at the base of a chimney, the air expands rapidy (expands by 1/273 for every 1 degree centigrade rise in temperature as per Planck) and the particles move faster, with greater kinetic energy. The air underneath is denser, so the expanding air wants expand upward and sideways. When the sides of the combustion chamber are closed, and the bottom of it is already denser, hense effectively closed, the air expands only upward. This quickly creates a large low pressure zone all the way up the chimney, which the dense air all around it is readily available to displace through the open bottom.So when you warm air in a vertical tube that is open at the top and the bottom, the heated air will rise up and out of the tube with greater force than if it is unconstrained. Yes?Not only that, if you have actual combustion going on towards the base of the tube, the oxygen flowing in the bottom to displace the lower density warm air will "add fuel to the fire," which in turn creates more heat to replace the hot air that has gone up the stack, keeping the whole thing going.No?Time to stop for lunch and a reality check from those of you who are more versed in this stuff.Steve
*Steve - I guess so. But stay away from combustion - it complicates matters.I'm still trying to imagine a house where the pressure everywhere inside the house is lower than all around it and its a steady state. Got to be away to harness that potential energy.Regarding the "air stratifies simply due to gravity", you're prophetic - or at least clever in writing - because I believe the air can sometimes stratify due to other conditions - but then it's not "simply".Have a good lunch! Bill
*I think the fire is a red herring. I remember visiting the non-winterized Michigan cottage in January a couple of times. If you started a fire in the fire place, you filled the house with smoke. There was no stack effect, no draft. We had to use the space heater (and fill the room with odorless carbon monoxide, which may explain a few things about my present condition :)) to warm the room. THEN, the stack effect was present and a fire could be started.It didn't matter how big a blaze we got going, nothing went up the chimney until the room was warm.Rich
*Rich,Yes, I've heard of having trouble getting a draft going in a cold chimney. I haven't figured out why that is (maybe warm air doesn't rise, but needs a push after all), but I think I can figure out why heating the room gets the draft started. If the damper is open and every thing else is closed, as you heat the air in the room with the space heater, perhaps the easiest place for it to expand to is up the flue, dropping the density of the air in the chimney, thus allowing the denser air to displace it from below, thus starting your draft.Steve
*Rich,Yes, I've heard of having trouble getting a draft going in a cold chimney.Let me preface the following by saying it is all speculation. I know nothing for a fact. I don't know why a cold chimney inhibits a draft. (maybe warm air doesn't rise, but needs a push after all), but I think I can figure out why heating the room gets the draft started. If the damper is open and every thing else is closed, as you heat the air in the room with the space heater, perhaps the easiest place for it to expand to is up the flue, dropping the density of the air in the chimney, thus allowing the denser air to displace it from below, thus starting your draft.Now I just had another thought--When the chimney and house are really cold, and you walk into it, the heat from your body begins to warm some of the air in the room. This air begins to stratify, with the warmer air getting pushed to the top of the room. So you have created a low-density area in the room which might be the first place the expanding products of combustion from the fire you started will want to expand to. This only heats the room further, while the chimney stays full of cold dense air, further discouraging the start of a draft.On another discussion group I frequent this topic came up. The person who had trouble found that by lighting a rolled up newspaper and sticking it up into the flue for a few seconds, he could get the draft going easily. That might save you waiting for the space heaters next time.Steve
*My brother claimed that the trick with the newspaper would work, but I didn't see any improvement myself. This was a few years ago, and the cottage has long been sold, winterized, and renovated to the point that it is a completely different house.Rich
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Actually, Joe, The baloon fills with hot combustion gasses from the large propane burners which rise to mix with and displace much of the cooler air in the lower part of the baloon. And the pressure is the throbbing in my head after reading your ridiculous claptrap.
*Joe, Webster's dictionary will tell you that "deification" means to make a god of so from now I will respect myself and proudly use your pet name.Divinely, HDO
*HDOYou know, and I know that Joe made a little slip with his fingers and inserted an extra "i" in Defication. He was, to further enlighten you, politely telling you that he thinks your an A**hole. I think I recognize your writing style. . . please tell me it's not true. . . are we back to this again?-Patrick
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Well, Steve, I too fell that neither Joe nor anyone else has answered a number of my questions very satisfactorily or even clearly. At the heart of the issue for this board seems to be "Why does warm air blow out every hole and gap at the top of a heated house and cold exterior air rush in every hole and gap at or near the bottom on cold still nights with no forced air? I believe its pressure but I don't believe the pressure inside can be lower throughout the interior relative to the exterior. It must be lower relative to the exterior at the bottom and greater at the top - else the air - which happens to be warm - would flow out.
Which means somewhere between the pressure inside will be equal to the pressure outside.
The same thing happens in a fire in a room - the pressure is greater relative to the exterior high - and smoke and such rushes out - while lower down air is sucked in by the negative pressure.
Now, I think gravity and mass and the relative densities of different air masses have a role in this but what is important for the purposes of comfort and economy is the exfiltration of the warm air and the infiltration of the cold.
Steve I'm sorry you didn't take my wager offered by email about Fred showing up here - but it was a no brainer.
Warm regards without pressure for a response, Bill
*Hi Joe.I printed out the 69 messages this thread had last night. I have read them.Respectfully, I ask you to take your time now, and define once more please, in most simple terms, what you are trying to say.I get the feeling that you are very well versed in this matter. I seek to tap some of that knowledge, because these are the answers I sought in my cold roof questions topic.Please dont be impatient with me here. It is very important for me to learn this.Thank you.
*Joe - First - I never ever claimed expertise on thes issues. I have reread this entire thred several times and have not seen a clear explanation of the observations I mentione in the last post.I have seen a lot of impolite, crass, and just plain belligerent comments.Regarding passing judgement or finding your comments invalid, not my place. You've made some statements that seem contradictory to what I have observed and I have asked - politely I believe - for an explanation or some how to explain how they reconcile. Warm air blowing out the top and cold air rushing in the bottom is a basic example that all your posts don't seem to explain.I have no interest in making you or any one go away. It was not me who disinvited Brian from this board. It is not in my upbringing and education to be so rude. I am interested in learning and sharing what I have learned in a friendly manner.So make whatever assumptions you would like, cast aspersions, and generally attack the messenger - it seems to be in vogue. I will refrain, thank you.
*I have been trying to understand the thread myself. Not with complete success.Now I find that maybe I have something to add. I have a brochure for Cor-A-Vent. It says:"Ventilation air will move into the attic through vents located within the positive pressure (intake) areas and will exhaust throught the vent opening at the negative pressure areas, the ridge. Wind moving over the ridge literally "siphons" the air out of the attic, by the same aerodynamic principle that lifts an airplane off the ground."The aerodynamic principle being that moving air has a lower air pressure than still air. This much makes sense to me.So, I guess the vent system is dependant on the usual situation of the wind being stronger as one gets above the ground.Rich "don't really know s**t" Beckman
*This has been a very interesting thread, and many good points have been made, but I think, not all neatly tied together. The point that warm air does not rise of it's own accord is a good one, as is the statement regarding density because density and the directed force of gravity are the keys to what happens. As has already been stated, cool air is denser than warm air and therefore weighs more. Archimedes' principal states that an object immersed, or partly immersed in a fluid will be forced (buoyed) upwards (upwards because gravity is our driving force) with a force equal to the weight of the displaced fluid. The "object" may be a hot air ballon, an air bubble in water, or just unrestrained warm air. Permuting the equation mentioned by Steve, F=ma, to a=F/m it should be obvious why the warmer the air (immersed in cool air) the faster it rises. Of course, Boyle's law (which states that for a given mass of gas at constant temperature, pressure and volume are inversely related) also applies so the warm air also want'so expand and will do so as the pressure around it lessens. It should also be pointed out that the changes in fluid density around a heat source and the action of gravity on that fluid are responsible for natural convection.
*Jt... I think you're hitting the nail d*mn near on the head! There has been more hot air created due to global wide attention to this thread than air changes in all our houses combined. Quite enjoyable too.
*Joe - I did as you asked regarding rereading these posts. My post 16 expressed my frustration - shared with Steve. It did not cast aspersions on anyone. I would submit that the basic question has still not been answered clearly. If warm air is at a lower pressure than cool air, then why does warm air blow out of the holes? If you cut a hole in the top of house - warm interior air; still, cold, exterior air - the warm air goes out. The air moves from higher pressure inside to lower pressure outside. Your statements that all the warm air inside is at a lower pressure than all the cold air outside deny this - which I have observed - can happen.
*So - JT or anyone else -In the warm house - reasonably well sealed - cold, still exterior example, is the pressure at the bottom lower relative to the exterior and at the top higher?Your post would suggest that the cold air pushing in the bottom displaces the warm, bouyant air exiting the top. Is that accurate?Thanks for jumping in. Bill
*I'll bump this back to the top in hopes someone clipped and saved JT's and Fred's 3 or 4 posts that seemd to be to the point on these issues. If someone can delete the rest, please do.
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Yes, especially what Bill Conner's had to offer, and that's nothing.
Damage Control
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I haven't followed this thread completely and now a lot is lost in the crash, but I see that boys will be boys. I was taught in physics 101 that there really is no such thing as "suction" (escept in Washington) but gas/fluid always move from high pressure to low pressure, in other words the air is not sucked out but pushed out by higher pressure air. And colder air will exert more pressure that warmer less dense air but the pressure of the colder air will apply itself thru any number of locations whether it is thru the basement window crack of under a door or even thru a hole in the roof. Think of a totally sealed box withwarm air inside and only one hole in the top-the cold dense air will slowly displace the less warm air thru that hole-the cold will flow in and the warm will flow outboth thru the same hole and to some extent at the same time! And it might follow the senerio of the cold dense air flowing in for a while, compressing the warm air in the box until the pressure inside the box can over come the cold air weight and then the warm air might flow out for a while and the process then might repeat. Just like pouring water out of a jug that goes glug, glug, glug.
The basic theories aren't that complicated but there are an infinite number of parameters to each senerio which all can be reduced to mathematics...and I sometimes can't keep checkbook balanced, so there.
*Hi George,Yes to all of the above. So your box is glug-glugging. Now drill another hole in the bottom of the box and whoosh, no more glugging, no more warm air. Now pick one of the two holes to plug. Yes, ideally you would plug them both, but say you had to chose because you could only afford to plug one. Plugging which one will be the most effective at keeping the now-replenished heat in?I vote for the hole in the top.Or how about this. You are standing outside in zero-degree weather with nothing but a barrel (minus top and bottom, like the circus clown) hanging from your shoulders by suspenders to shelter you. You have one lid that you can add to it. Are you going to cut two holes for your legs and put it on the bottom of the barrel, or cut one for your head and put it on the top of the barrel?I vote to go indoors! Steve
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What's a pressure plan?? Where is it in my house? Does anybody know where the pressure is greater, if the outside of my house is 11 degrees F. and clear and the inside is a warm and toastee 68degrees F. I have a small bet with a friend ;-} on where I think it is.
Jack.