I'm opening a separate discussion under a new thread as this one: http://homeenergypros.lbl.gov/forum/topics/locating-the-neutral-pre...  was getting overly congested and this topic deserves its own space.

Homes need to be tight to be energy efficient, but then comes the question of how to provide the necessary fresh air that we and our houses need.  Robert Riversong (above thread and several others) has posted some great information on passive vents and the economics of simple exhaust venting that I think offers a good alternative to expensive H/ERV installations.  However, I feel there needs to be a better understanding of just how static venting works.

Most of us in the energy business have read about, used, or advised on using some form of passive venting for replacement air that involves a form of air trap.  Robert posted his version and mentioned the "Saskatoon Loop" as methods of restricting the unwanted air flow while still providing a path for the desired air flow.  I have looked at the "duct ending in a bucket" and the "loop up at the bottom" cold air traps in the past and concluded they are not exactly what they appear to be.  Essentially they modify the height and resistance of the flow path, but otherwise do not act as an air block.


Since the explanation of the above can be long, I have put together a simple statement that I feel conveys the guidance we need when designing and installing passive vents, at least some of the guidance.

"For any fresh air vent duct passing from inside a home to the outside (under natural pressures), the effective pressure from end to end of that duct is the stack effect pressure (wrto) at the height of:

1.  the outside opening when the duct is filled with inside temperature air.

2.  the inside opening when the duct is filled with outside temperature air.

3.  the penetration through the envelope when outside is filled with outside air and the inside is filled with inside air."

I haven't reviewed this for summer conditions, but I believe the statement will hold.

When any kind of winding path is filled with the same air as is around it, it might as well be a straight shot, if the structure allows.  Alternatively, if a straight shot is not possible, a winding path will not alter the effective air flow, other than adding a bit more resistance.

The bottom line is, passive venting should follow and use the internal pressures within a home, positive, negative, and that somewhat elusive NPP.

John is very good at challenging or explaining many of my statement and he creates great artwork, so I'll post this and see what we get for input from all.


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I think we're talking past each other.

Stack effect pressure differentials occur only when there is a delta-T and a column height.

But when we talk about a vertical pipe or chimney in a house, we're talking about two interacting "chimneys" - the chimney and the house. This also occurs with a two-flue chimney and the relative pressures at each can cause one flue to back-siphon the exhaust from the other (if the first has its bottom opening in a negative-to-atmospheric pressure zone).

This also explains back-drafting of a wood-stove or any atmospherically vented combustion appliance, if the appliance chimney has less draft than the house-as-chimney. This is why an old trick for lighting a wood-stove in a leaky house is to open a window near the wood-stove to lower the NPP and reduce the negative pressure at the stove until it's hot enough to draft well.

Hey, I'm not trying to fry Bud here.  I love the thread because it is scratching at something I have yet to fully grasp.  Grasping it may lead to a big jump in our abilities to deliver solutions.  

I didn't disagree with anything, no do I think any of what you've written is "patently absurd" so don't ascribe that to me.  I just mentioned that where our natural tendency to to assign linearity to a thing, or view outcomes as typically being represented by bell curves, this orientation can some times trip us up.  I suspect that stack effect is very much NOT linear.  I suspect ACH does not have any type of uniform correlation to delta t.  

And until now I didn't even have THAT thought/question clearly defined.  So I love this thread not because it's providing answers, but because it's developing questions.  I thought that's where you still were.  

And in the HVAC side this airflow/static issue tends to be ignored/undervalued until things go significantly wrong.  So I was bringing that nugget for you to chew on.  That's why I mentioned Fan Laws.  

You appear to be scratching at the same itch, and I thought bringing that up might elicit some light-bulb moment for you that would in turn result in your further clarifying these issues for me and the rest.  

I'm not trying to be right, I'm trying to understand.  Does any of this makes sense?  

Ted, getting my sensitivities behind me, you say,

< I suspect that stack effect is very much NOT linear.>  The math is linear, but I agree that the resulting air flow would not be.  Where it is handy to use these numbers for before and after comparisons, trying to assign a number to the associated leakage area fails to be exact.  But, "exact" is a word that fits poorly into our world of energy performance.

<airflow/static issue tends to be ignored/undervalued until things go significantly wrong.>  Amen!  My crude but increased understanding of air flow has allowed me to gain substantial insight into issues I was barely introduced to in my brief energy training.  Now that "tight homes" are becoming the standard, and that is Robert's tight as well as super tight, we need to be very careful with when and how we make improvements.

As for forced air, I haven't ventured into that as yet.  Another topic that I received almost no training in.




Well, you will definitely enjoy that. There is only so much air you can blow through a straw before additional units take huge amounts of additional energy, and understanding how that plays into design, comfort, and EE will fire a lot of lightbulbs.

Do you have any reference to external static pressure? Good hydronic guys tend to get flow concepts better than most air guys, do you know any?

I'd encourage you to go to hvac-talk and troll around. Take with you the orientation that ideal ESP is usually between .1 and .4.

<Do you have any reference to external static pressure?>

Explain.  Are you talking the inside to outside pressures?


Static pressure is the resistance of any duct or piping system, due to length, restrictions (such as filters or valves) and "tortuosity" (shape, texture and angular changes which create flow turbulence).

External Static Pressure is a term used by air-handler designers/installers to designate the total return and supply duct and associated equipment (filters, coils) pressure losses in an air distribution system that a fan must overcome to deliver the required CFM.

I wish I'd coined that word.  

So Bud, ESP is tortuosity (copyright RR) of the whole distribution system.  Ideally you insert your probe into the fan cabinet and above the heat exchanger, and at high fan speed your at less than 1/2 inch.

You can then test tortuosity of every component (coil, filter, supply system, return system) to understand how restrictive each is.  

Go to hvac talk and search static, esp, airflow, I'm sure you'll have some fun.  

I wish I'd coined the word "tortuosity" but it's used in the description of compound curves, rivers, hydrogeology, blood flow, fluid diffusion in porous materials, acoustics and other fields.

A tortuous river.

It comes from the same root as does "contortion".

Further up ...Robert Said : "I could be off base, but this is what makes sense to me. It's somewhat similar to an atmospheric inversion with warm air over a cold air mass preventing convective uplift. Cold air will not rise into a warmer air mass unless forced.

If I'm correct that it would take a 12.4 Pa delta-P to move a 3' cold air column upwards, then anything more than a 10 mph wind would have sufficient stagnation pressure to do the trick"

Robert are you saying that unless the wind is blowing.....a "3 ft cold air column" would prevent air from flowing into the bottom opening in my attached illustration ????


John, I'll let Robert answer your question.

One more for Robert,

If you calculate the atmospheric pressure at both elevations of your 3' drop you will find the missing pressure required to move that air column.  The equation/s we use simplify the process of calculating and subtracting the inside and outside atmospheric pressure and gives us just the difference.  Once air starts to move, then we can subtract friction and turbulence, but the movement and direction of movement has been established.


First, let me correct my math. The effective weight of the cold air column would be the difference in density between outside and inside air, which at 68°/24° temperatures would require about 1 Pa to move.

But the two locations for exhaust ducts for kitchen or bath fans would be 1) at the ceiling of the 1st storey near the NPP where there should be no infiltration pressure or 2) at the ceiling of the second floor where there is positive outward stack effect pressure.

Your illustration doesn't locate those properly, ignores the dampers in the termination caps, and makes it appear that the only openings are the duct and an equal or larger "hole" at the point of maximum exfiltration. In reality, the house has an ELA of 1 sq. in. of openings @4 Pa due to the upstairs air inlets at ceiling level (2 sq in total for all inlets).

For a very unscientific example, since I have no indoor plumbing in my cabin (a compost bucket outhouse 30' away), I have a "urinal" in my mudroom so I don't have to go out barefoot in the snow on winter nights, that consists of a garden hose through the floor which extends a little down the slope behind the cabin, and topped with a funnel. There's probably a 5' total drop to the outlet.

The cabin is leaky and heated by a woodstove (which is operating now as it was in the 40's last night). I can feel a strong draft being drawn in at the base of the door next to the "urinal" but no air coming up the hose.

Hi Robert,

My last illustration was not an attempt to simulate the openings at your Warren Vermont House.

(By the way your projects {and methods}are among the best I have ever seen)

I was only trying to isolate the "3 ft cold air column" and consider if there is any benefit for the strategy.

You said : "But the two locations for exhaust ducts for kitchen or bath fans would be 1) at the ceiling of the 1st storey near the NPP where there should be no infiltration pressure or 2) at the ceiling of the second floor where there is positive outward stack effect pressure."

I agree... and wonder if the "3 ft air column" is making things better, or worse or virtually makes no difference.

As to your mud room "urinal funnel" ... I wonder where the NPP is in your mud room?


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