Controlling Passive Air Flow

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.

Bud

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Replies to This Discussion

Permalink Reply by Robert Riversong on September 22, 2012 at 1:35pm

Here's a cathedral ceiling over one of the ceiling light fixtures. This was a "vented" roof with fiberglass batts and no vent channel.

Attachments:

IMG_5032.JPG, 111 KB

Permalink Reply by John Brooks on September 22, 2012 at 2:11pm

Julius Sumner Miller was a student of Einstein and a "fellow" student and a close friend.

.......

Robert said: "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."

I think Straube's formula is based at sea level

Robert, what is the temperature for your examples?

For the sea level example...

the reciprocal of 0.318 Pa /inch will return a Vertical Isobar spacing of 3.14 inches

and 0.306 Pa/inch (elevation 1,000 ft) translates to a "Spacing" of 3.27 inches

Permalink Reply by John Brooks on September 27, 2012 at 5:35am

Robert said: "the most foolish way to duct an exhaust fan is straight up through the roof"

It seems to me that in a heating climate (or during the heating season) ...

It would be desireable to have the "passive NPP" high

This would make the house more "negative" and would reduce (or eliminate) positive pressure near the top of the enclosure.

Therefore terminating exhaust stacks "HIGH" might not be so "foolish".

It is not-so-foolish to terminate fireplace or woodstove chimneys (stacks) high rather than low.

It is not-so-foolish to terminate a sanitary sewer vent HIGH...

even if it does require going thru the roof.

Of course it would be better to have fewer penetrations thru the ceiling and roof...and condensation should be considered and drained properly.

If it makes sense to run a sewer vent thru the roof...why not an exhaust stack?

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Permalink Reply by Sean Lintow Sr on September 27, 2012 at 6:41am

You nailed the reason - condensation

The next reason is dampers & wind

In some cases going through the roof is the only option, but for most of us we will look first for the nearest gable end

How about soffits - nope as you will be pulling it back into the house

By the way, the more negative a house, the more cold dry or warm moist air you are pulling in - while that maybe good during some parts of the year in certain parts of the country it's not the best thing to do

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Permalink Reply by John Brooks on September 27, 2012 at 7:51am

I did say "During the Heating Season"

Permalink Reply by Robert Riversong on September 27, 2012 at 9:22am

We run a chimney through the roof (and minimum 3' above, and 2' above anything within 10' horizontally) both to maximize the updraft, to get smoke away from the house (including from staining the roof) and to prevent downdrafts from wind passing over obstacles such as roof peaks).

We run a plumbing vent through the roof to also maximize stack effect draft and to get the stink away from all fenestration. Similarly with a a radon vent.

Maximizing static draft pressure and velocity in an exhaust fan that's not supposed to be moving air when it's turned off is generally not a wise option. If the exterior backdraft damper were to get stuck open, the house would have a passive chimney moving large quantities of air.

If it's connected to a bath fan that's used on low speed 24/7 as a whole-house ventilator, then it's probably fine.

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Permalink Reply by John Brooks on September 22, 2012 at 2:19pm

On a side note...we have run out of chances to "Reply to This" again

So I will just "reply" wherever I can

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Permalink Reply by Robert Riversong on September 22, 2012 at 2:40pm

I didn't see any formula for absolute air pressure in Straube's article.

The formula I'm using is local barometric pressure is sea level pressure (assuming 14.696 psi) / 2^(h/18,400), with h being the local elevation above sea level. The formula is independent of temperature.

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Permalink Reply by John Brooks on September 22, 2012 at 2:48pm

Isn't the "14.696" assumption based on a certain temperature ?

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Permalink Reply by John Brooks on September 22, 2012 at 2:52pm

At least the sidebar in John Straube's latest article is a little more "Bud Friendly"

http://www.buildingscience.com/documents/published-articles/pa-air-...

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Permalink Reply by Robert Riversong on September 22, 2012 at 3:25pm

Yah, but buildings don't grow - they get built.

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Permalink Reply by tedkidd on September 22, 2012 at 3:59pm

I'm not confident that airs tendency to mix doesn't overcome this "house natural stratification" theory.

In fact, I'm thinking that you have to have a fair delta to overcome mixing. Anybody experiencing wood stove heat in a cabin knows that big delta will stratify, but low gentile heat tends to be uniform.

I've been in homes where unintentionally conditioned attics end up sauna temps, hottest area in the home, from downstairs wood stoves. When the same home is heated with the oil furnace the attic temps stay below 60, coldest room in the home.

The implication of stratification w/o rt heating source temperature seems to miss a really big slice of the stack concept, and lead to some poor assumptions for many situations. In fact, I'd propose lumping stack and stratification together is confusing apples and oranges.

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