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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My secret urinal revealed! I didn't remember you taking that picture when you were (not) here last. It's a good thing that the electrical outlet below it is electrically downstream from a GFI.

Good question. I'll have to check on the next cold night with an incense stick. With the woodstove running, the NPP could be down low, even though the triple track aluminum storm windows are all high on  the 7' walls and the cathedral ceiling terminates at an exposed ridge board at 7'-8" which undoubtedly leaks air through the fiberglass insulation.

Given that the NPP may not be the governing element in these down-and-out devices, but rather the pressure difference between the two ends of the "duct", I'm ready to concede that without actual air-flow testing there may be no theoretical way to prove an advantage, other than the advantage of excellent condensate draining (or urine draining in this case).

In your illustration, there would definitely be air flowage in low and out high (depending somewhat on the rate of respiration of hot air of the two people).  ;-)

Robert, I remembered your urinal photo from your "How's the Weather" thread at GBA

This example shows a "3 ft cold air column" located just slightly below the Neutral Pressure Plane (not Robert's house)...just an example

assuming  the "cold air column" is Equal to the outdoor temperature...

the vertical pascal spacing should be very close to the outdoor pascal spacing.

Looking at the illustration....I would think there is a very slight tendency  for the outdoor air molecules to "spill into" the indoors...So I do not see an advantage for a downturned air column.

More likely....if the column is inside the wall...

the air in the "column" would be slightly warmer than outdoor air....

and the pascal spacing inside the tube would be greater than outdoor spacing...

and (in this example) the air would be even more likely to "spill into" the indoors.

I'll go with the tortuosity theory and the benefit of an effective condensate drain.

Also, I don't like placing a kitchen or bath exhaust termination under a roof soffit, as the humidity will be drawn into the soffit vents (when one of the goals of roof venting is to eliminate moisture).

And the most foolish way to duct an exhaust fan is straight up through the roof. Not only does this put an additional penetration in the roof, but it creates a reverse condensate drain, a significant thermal bridge through the insulation (with the potential for humidity to leak into the insulation) and a nice passive ventilation hole in the point of maximum positive pressure.

Where did you find John Straube's isobar spacing formula and what is it?

How about a downturn for drainage and tortuosity  and nix the vestigial 3 ft extension?

and speaking of tortuosity ....

Doesn't tortuosity decrease the effective area of the woodstove fresh air intake duct ?

and perhaps cause a 4" duct to perform like a lesser diameter duct?

oops, I think you were editing your post....

the 3 ft extension moves the exhaust futher from the soffit ...OK

that makes sense.


Concerning Isobar spacing....and the John Straube formula....

I adapted  formula [3] from BSD-014


I was trying to visualize the incremental difference in Vertical Pascal spacing for some common ranges of temperatures.

I plugged his formula into a spread sheet and noticed that the incremental diifference for Isobar spacing  in inches was approx 0.0066 times Delta T (Farenheit)

In my example ...68 degrees inside minus 24 degrees outside = 44 degrees F

incremental difference = 0.0066 times 44 = 0.29 inches

In my example the choice of indoor Pascal spacing of 3.4 inches is arbitrary (borrowed from one of Bud Poll's examples).

I calculated the outdoor Isobar spacing by subtracting 0.29 inches from 3.4 inches = 3.11 inches

John Straube states in that report that "Stack effect pressures are generated by differences in air density with temperature, i.e. hot air rises and cold air sinks."

Don't tell Bud ;-)

At elevation of 1000 ft above sea level, the change in atmospheric pressure is 3.67 Pa/Ft or 0.306 Pa/inch. At sea level, it's 3.81 Pa/Ft or 0.318 Pa/inch.

RE: "Don't tell Bud"

yeah I know ... I have mentioned this to Bud

that even John Straube says "Hot Air Rises".... and he continues in his latest FHB article about Stack Effect.....

Bud and Julius Sumner Miller and I are certainly in the minority.


LOL Robert, I saw that.


I wouldn't bother with anything that Julius Sumner Miller said (he also pimped for Cadbury chocolate in TV commercials, telling kids how nutritious it was). He was a student of Einstein's at Princeton and probably believes that there is no force of gravity, but that space is curved - how silly is that?

In the 100s of attics I've been in the only moisture problems I've seen we're due to baths vented into the attic or to vented soffits (then rolling into soffet vents).

Well, there was one where a thermal bypass was delivering moisture to a gable end, homeowners were over humidifying.

Even venting moist air to UNvented soffits would make me uncomfortable, although I have nothing but intuition to base this on. I suspect rollback and condensation could be a result.

I've seen both attics and cathedral ceilings with black mold on the underside of roof sheathing, even with no obvious moisture source such as in a newer addition on an old farmhouse that contained no plumbing (but was over an occasionally damp basement), and only ceiling lights or wiring holes in plates or a masonry chimney to channel warm, moist air upwards.

In fact, the chimney had Kraft-faced fiberglass batts around it and with the air flow and the heat of the chimney, the paper facing had several different colors of mold.



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