First of all......
* As we upgrade and build new....
I think it is time to eliminate atmospherically vented appliances from the breathing zones of our homes
* I realize that the effects of wind and mechanical equipment can and often do overwhelm Stack Effect.
My current favorite online resources for visualizing "Stack Effect" are.....
*John Straube's Article
*John Klote's Article
*Bud Poll's Worksheet
I plan to post some Illustrations and see if you folks agree with my current thinking about the location of Neutral Pressure Planes
Does anyone else have suggestions for online links concerning "Stack Effect"?
Not true. As long as there is enough leakage to make up that forced air volume, then a 60 cfm intake or exhaust fan would push that much air (less static duct losses).
But what you say brings up an important point. A balanced HRV won't change the natural air-flow dynamics of a building, so there will still be the same amount of natural infiltration and exfiltration (actually a bit more exfiltration in winter since the incoming cold air will expand and create more positive pressure).
With an exhaust-only system, if the fan pressure exceeds the stack effect pressure then natural exfiltration will be eliminated and, along with it, condensation problems.
With an intake-only system, this will prevent infiltration and condensation problems in the summer cooling season.
I hate to keep being the bearer of bad news, but the barometric pressure is irrelevant to the stack effect. Please see my article at Green Building Advisor: Who Knew the Stack Effect Could Be So Controversial?
In one of the comments, I wrote:
Let's take a step back and clarify our terms. Barometric pressure is the absolute pressure we read with a barometer. With no wind, stack effect, or mechanical systems inducing pressure differences, the barometric pressure will increase as you go down from the top of a house to the bottom. It changes about 3.4 Pascals per foot of height.
A house that is 20' tall will have a barometric pressure at the top that is about 68 Pa lower than the pressure at the bottom. That's more pressure difference than we apply to a home during a Blower Door test (typically 50 Pa in a single point test). Yet why, if the Second Law of Thermodynamics says that air moves from high pressure to low pressure, will the air in the bottom of the house not push its way upward?
Because of gravity, that's why. That fundamental force of nature completely cancels out the effect of that pressure difference that it created in the first place.
So again I say the only thing you need to worry about are the relative pressures created by wind, stack effect, and mechanical systems. You're making this more complicated than it really is.
Hi Allison, we are getting so close on this issue, if I can just continue one more step. The 3.4 Pascals per foot is correct, except, that number is directly tied to the temperature of the air. When you compare inside and outside barometric pressures, the numbers differ by the weight of the last 20'. If, for example the outside is 35° and the inside is 70°, the difference in the weight of air becomes about 0.25 Pascals per foot of elevation. Above the house both columns would be the same. For the last 20' the inside air is lighter, so the barometric pressure ends up being less. Uniquely less, by exactly the pressure we call stack effect, 5 Pascals for this 20' house. You can calculate it with John Straube's equation shown here: http://www.buildingscience.com/documents/digests/bsd-014-air-flow-c...
Or an English version: Total Stack = .007 ▲T x ▲H
Or you can follow my graphics at: http://myenergyworkshop.homestead.com/hot-air.html
I understand that the vertical differences per foot for barometric pressure do not result in vertical air movement. But when two columns are placed adjacent to each other with different temperature air, say one inside and the other outside, the resulting horizontal pressure differences WILL move air. That pressure is what we call stack effect.
Sorry for drifting John, but this is an important part of the understanding that is evolving.
And thanks Allison for joining the discussion as I know you appreciate a good discussion.
A firm understanding of the source of stack pressure is beneficial to discussing the wanderings of our NPP, so we are actually still on topic.
Barometric Pressure (PB) is a function of the weight of the air above the point being measured. If we compare two columns of air, one outside our house and another that passes down through our house and both inside and outside are 35°, then the BP readings will be the same. Lets apply a number for this example, 101,200 Pascals, (14.7 psi = 101,325 pa so we are somewhere just above sea level). Now lets seal our 20' house completely except for a small hole at the top where that column of air passes down through. Now heat the inside air to 70°. Any expansion will vent out the top and our column of air that passes down through our house will now be lighter by virtue of the last 20' being warmer. If my math is close, then the inside air will be approximately 0.25 Pascals per foot lighter than the outside air. Over 20' that would accumulate to 5 pascals, so outside would still be 101,200 pa and now inside will be 101,195 pa.
Let's test the equation above: Total Stack = .007 ▲T x ▲H
TS = .007 x 35 (F) x 20 = 4.9 pa
That is the negative pressure at the bottom of this 20' house (wrto) when there is a 35° temperature difference between inside and outside. And that 5 pa is the difference between the BP inside and the BP outside.
Now, in a typical home this might adjust to +2.5 pa high and -2.5 pa low with a NPP in the middle. How that adjustment occurs is what this thread will answer.
Allison, I'm hoping you will reply and tell us where our stack effect pressures come from, if not as described above. The equations tell us how large those pressures are and derive their answer from the ▲T and the ▲H involved. What else is involved?
Bud, I admire your determination and your thoroughness, but by continuing to focus on barometric pressure, you continue to confuse people. The answer comes most simply from Archimedes' Principle, which says the buoyant force on a body in a fluid, which in this case is a less dense fluid, is equal to the weight of the fluid displaced. That's all there is to it really.
We also can call on Sir Isaac Newton and Herr Gottfried Wilhelm Leibniz and the calculus to eliminate height as a factor, if you like. Take your bubble of air inside the house. I believe you used a height of 20'. Now make it a smaller bubble. And still smaller. The smaller you make it, the less height it has.
Ah, some might say, but it still has a height, no matter how small you make it. That's where the calculus comes in. (For every epsilon, there exists a delta...) We can shrink that bubble down to an infinitesimal point particle, and the buoyant force still pushes it up.
Perhaps you can still show me something I haven't thought of, but barring that, I'm done discussing stack effect (until I find something else interesting about it anyway).
Allison, I'm working on a framing technique using the space between joists or studs as duct-work for circulation and insulate on the outside [pvc pipes of water as thermal-mass]. So this needs to use pressure & flows to get it to work with a solar wall on a mt. ridge w/o power. But, when you tighten up a building and then put in a pot-belly it'll deplete the air of O2 if you don't have a way for it to balance the needed input & turn-over rate for fresh-air.
This requires having some way of getting this to balance and have the fire draw OK & the air to turn over enough w/o losing a lot of heat. From my experience in construction getting this right wasn't always the case.
In my case it forced using vents that mix outside air, for the cabin this at the base of the solar wall to enter the room 8-ft off the ground so it won't be cooling the floor, the floor is heated by cooling air being moved from the ceiling back below the floor to return to the solar wall using density.
For homes with a furnace & more like the standard diagram, getting this right is beyond my skill atm. I did figure out I get about 64cf/hr flow from the solar, hardly the 60cfm for the example but for a closed-up cabin may work fine to keep it drier & warmer till visited.
2012 IECC prohibits use of wall and floor cavities as supplies or returns. Only true ducts installed in cavities will pass. originally prohibited 2009 IECC
Just found this, thanks o' ton for the info Randy, for sure it's for fire reg's, will just add the ductwork into the budget, have always had a hunch it'd need them since there's no fire-blocking.
Changed my mind, what I'm doing deserves a variance due to the pvc pipes full of water used in these walls or floors as thermal-mass.
With the water there it's a basic fire-suppression system, as the pipes melt the water is released and there's a lot of it, 4-tons for one 10hx12w wall and a 12x12 floor, why would ductwork make it safer?
Then why is atmospheric pressure part of the stack effect pressure formula?
ΔP available pressure difference, in psi
a atmospheric pressure, in psia
h height or distance, in ft
To absolute outside temperature, in °R
Ti absolute inside temperature, in °R
And, I will disagree with your assertion in the article you link to that "heat rises" is a meaningful statement. You justify is as an observation. But it's stated, and commonly understood, as a physical principle (which it is not).
To conclude that heat rises because one can observe hot air rising is like concluding that money rises because one can observe that the money in one's pocket goes up the stair as the person goes up the stairs.
Heat, following the second law, is isotropic in its movement. Warm air rises because it is less dense - not because it's hot, just as helium rises in air because it's less dense.