Greetings

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

http://www.buildingscience.com/documents/digests/bsd-014-air-flow-c...

 

*John Klote's Article

http://fire.nist.gov/bfrlpubs/fire91/PDF/f91013.pdf

 

*Bud Poll's Worksheet

http://myenergyworkshop.homestead.com/hot-air.html

 

I plan to post some Illustrations and see if you folks agree with my current thinking about the location of Neutral Pressure Planes

Meanwhile...

Does anyone else have suggestions for online links concerning  "Stack Effect"?

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Ted, good questions

The profile thru an opening above the NPP is not exactly a mirror image of an opening below the Neutral Pressure Plane.

For one thing it depends on the distance from the NPP

but even if the openings were the same size and the same distance from the NPP...they are not exactly mirror images.

I think these "contours" of equal pressure are "happy" to stay "stacked"....

For instance contour 100,009 will always be above contour 100,010 and below 100,008

 

the JPG is not high resolution

I am attaching a high resolution PDF and I believe you will be able to zoom-in and follow the contours.

Attachments:

I think the Physics of stack effect within enclosures/containers is connected to Pascal's Principle

and because Gravity IS inolved....Hydrostatic Pressure

There is some good on-topic food for thought in this video

(Professor Walter Lewin....Lecture 27)

http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechani...

 

John, I will admit my understanding, or ability to explain the irrelevance Allison refers to is weak.  I need a good analogy for how the vertical stacks of pressure are busy holding up what is above them and thus aren't moving air upwards.  Yet, when two stacks of air have different pressures, as in 'inside vs outside', we do see air movement, as in infiltration and exfiltration.  I think it boils down to gravity is a force and stack effect is a pressure.  I'll check the video later.

Allison: I posed two questions, but I'm sure you are busy so I'll post the answers.

1. "what actually causes the NPP to shift when we open an upstairs window?"  Opening a window above the NPP simply lets some of the air out of the house.  As the air exits, the pressure within the entire house drops simultaneously reducing the exfiltration and increasing the infiltration, seeking a new balance for the "in" and the "out".  The reverse happens if you open a window below the NPP.

2.  "how can we use our manometer to locate the NPP?"  By knowing the inside and outside temperatures we can calculate the total stack pressure over the inside height of the house and thus determine the ▲p per foot on the inside.  In John's examples, that is running about 1/4pa per foot as you move above or below the NPP.  Then use your manometer to measure the pressure outside a high or low window.  If you measure 2pa at a high window and the house stack pressure is 0.25pa per foot, then the NPP would be 8' below the test point.  Note, the test point is where the manometer tube passes through the envelope.

Bud

1. "what actually causes the NPP to shift when we open an upstairs window?"  Opening a window above the NPP simply lets some of the air out of the house.  As the air exits, the pressure within the entire house drops simultaneously reducing the exfiltration and increasing the infiltration, seeking a new balance for the "in" and the "out".  The reverse happens if you open a window below the NPP.


Nice Bud.  But is it correct?  

This was how I was looking at it when I thought balance point would go LOWER when you open a window high.  Resistance/positive pressure at the top is reduced, and air more freely leaving would increase vacuum/negative pressure at the bottom driving balance point down.  

For me it's pure speculation.  

Others think it goes the other way, and they may know better.  

Ted, it is correct, confusing but correct :).  If you provide an additional source of exfiltration, the house needs to balance the equation.  It will both increase infiltration and decrease exfiltration.  It does this by reducing the exfil area and increasing the infil area, along with the lowering of all house pressures.

Here's a question for you.  If a house is experiencing a strong temperature difference, thus a strong stack effect, and you turn on a 50cfm exhaust fan, how much does the total exfiltration increase.  Think of those exhaust only ventilation fans that get recommended when a house is too tight.  They get very specific as to what size they should be, yet the answer to the above will suggest the result will fall somewhere between 50% and 100% of the specified requirement.

The answer to the above is, under a strong stack effect, that 50cfm fan will result in only 25cfm of extra exhaust air.  Depending upon a buildings location, it will experience some average stack effect, thus some average reduction in the results from an exhaust only fan.  NPP is fun.

Bud

This creates light bulbs:

 It will both increase infiltration and decrease exfiltration.  It does this by reducing the exfil area and increasing the infil area, along with the lowering of all house pressures.


Basically, the additional "suction" makes areas that were ex-filtration points now become infiltration points.  


That helps me "see" the balance point shift up. Thanks.

see http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-245.html

"Measurements on a two-storey house with a balanced system indicate that the total ventilation air supply rate increases as the outdoor air temperature decreases...The ventilation air supply rate for a similar house equipped with an exhaust-only system is relatively insensitive to both wind and temperature difference and remains essentially constant."

error-file:tidyout.log 

This also answers John Brooks question about how an open chimney changes the NPP.

I continue to agree with Bud ... that the atmosphere is totally relevant to stack effect.

I agree that hot air does not rise (on it's own)

I also agree that opening a window above the NPP will cause the NPP to move up.

I believe that the NPP will always move toward  any new opening.

the larger the new opening...the more the NPP will move toward the new opening.

 

I do not agree that opening a window will cause exfiltration to go down.

Opening a new window either above or below the NPP can only cause exfiltration AND infiltration to increase.

Closing openings above or below the NPP will reduce exfiltration AND infiltration.

 

Exfiltration and Infiltration will be EQUAL except for the instant (speed of sound) that the new orifice is opened

I think Bud's comment "It will both increase infiltration and decrease exfiltration" probably refers to ex-filtration excluding the opened window.  In other words, when you open the window, all the other ex-filtration leaks diminish (path of least resistance).  


Inclusive of the window ex-filtration should INCREASE.  At least that was how I interpreted that.  

Ted, you are probably right... I tend to take things very literally... I see your point

I visualize each opening as a portal to a particular altitude ...and each altitude has a particular Barometric pressure...

The size of the opening will determine how much influence the opening has on the NPP

 

If there is only One significant opening... the NPP will be exactly at the Barometric Pressure associated with the opening

If there are Two openings...the NPP will be in between the Barometric pressure of the two openings and closer to the larger of the two openings

Hi John,

Actually, although the timeline is short, when a window above the NPP is opened, there is a spike of exfiltration that begins the deflation of the house.  At that moment, the stack exfiltration and window exfiltration are greater than the stack infiltration.  As the pressure in the house drops (we are only talking a pascal or two) the total exfiltration drops and the infiltration increases.  Once the "area shift" and "pressure shift" result in a balanced airflow, the new NPP is located above where it was.

But the shift isn't always directed by above or below activity.  It relates to infiltration or exfiltration.  A chimney chase is often located below the NPP, but can be a source of exfiltration.

Bud

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