# Are "Stack Effect" and "Chimney Draft" the same?

A year plus ago J Brooks asked a question, "I am curious if you have found any references for the origin of the term "Stack Effect"?  Is it named after Chimney Stack or is Chimney Stack named after Stack Effect?"  post 60 http://www.greenbuildingadvisor.com/community/forum/general-questio...

The actual origin is only a curiosity, but there is a difference between house stack effect (HSE) and chimney stack effect (CSE) and understanding that difference may help in other applications.  I'll be brief.

Consider a tall house with only two 6" holes, one top and one bottom.  House is warm and it is cold outside.  The HSE pressure will be divided approximately half high and half low resulting is an inflow at the bottom opening matched by an outflow at the upper opening, each the result of half of the HSE pressure across a 6" opening.

Now, shrink that house down to 6" in diameter, in other words, a chimney.  Keep the temperatures and height the same and you will have the same total HSE pressure now equal to the CSE.  But the total pressure no longer divides equally between the upper opening and the lower opening.  The difference between a chimney and a house is that the air remains in motion where in a house, once it enters it loses its momentum and communicates with the top opening via an increased pressure.  The air flow through the chimney is subject to the full SE pressure where the air flow through the house it is only half the pressure.  Other resistances play into the actual air flow, but a chimney chase may experience a greater air flow because the momentum of the air is maintained.

So, where did the name come from?  Despite their similarities, chimney draft and HSE are actually different.  Neither one should be named after the other. (IMO)

Bud

Tags: effect, stack

Views: 1356

### Replies to This Discussion

We've had some views, but no comments, so I'll summarize.  House stack effect and chimney draft are different.  A chimney has only one pressure boundary pushing air into it resulting in a continuous flow of air which exits without the need for another "delta-p" at the other end.  House stack effect creates two pressure boundaries, one pushing air in and the other pushing a matching volume of air out.  But the momentum of the incoming flow of air is not one in the same as the outgoing flow of air.

So, where did the term stack effect come from?  My guess would be the stacking of the air which accumulates to be the 14.7 psi at sea level that we are all familiar with.  That atmospheric pressure is the result of the air stacked above that point.  At the top of a 10,000 ft mountain the atmospheric pressure is about 10.1 psi, the result of less air stacked above that point.

Bud

Hey Bud

Seems like your description:

"A chimney has only one pressure boundary pushing air into it resulting in a continuous flow of air which exits without the need for another "delta-p" at the other end"

doesn't jibe with the view that hot air does not rise, cold air falls.  How do you reconcile the two situations?

Ed Minch

Bud,

How are you wanting us to look at this?  In my mind you're creating two images.  One is a uniformly narrow river with a consistent flow rate, and the other is the same river with a large lake plopped into the middle of it.

But I'm struggling to think about conclusions, and ultimately how this leads back to improving my diagnostics and my design thinking.

Ed, the hot air is being pushed up by the pressure below created by the surrounding colder air.  I don't see the confusion.  The distinction here is that a chimney, unlike a house, has only has one delta p.  A complete model would be more complex, including the resistance of the flue, but my point is, the incoming air has momentum derived from that initial push which contributes to the flow all the way to the top.

Tedd, the river and lake are similar except the in coming water would be much slower at communicating across the lake to affect the outflow.  In a house, opening a door or window, or turning on a fan, will communicate a change in pressure at the speed of sound throughout the enclosure.  In a house we see the stack effect pressure divided in some proportion between intake and exhaust.  In a chimney or a bypass where the flow is continuous, all the calculated pressure is directed at the incoming opening.

There are real world examples but recognize I'm no expert on this.

Air ducts are often terminated with a dead end to stop the air flow and increase the duct pressure, thus forcing air out all branches with a similar pressure.

A pair of transfer grills installed one across from the other will benefit from the flow of air, from one to the other.  In a high/low install most of the momentum is lost with the 90° turns and thus the total NFA for the grills should be increased.

Accepting that warm air does not rise by itself, why does warm air flow into a chimney chase and up to the attic when the pressure in the basement is negative wrto?  It is because the delta p is positive in the basement with respect to the column of air in the chase.  And, since the entire pressure difference (not half high and half low) is driving the air flow, that flow will be greater than the house stack air flow even though they are the same height.

As for diagnostics and design considerations I hadn't planned to go beyond the "just for fun" discussion on the origin of the term "stack effect".  When I get up my courage I will try to address the "hot air" topic more thoroughly.

Bud

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