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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There are two different issues at play here.

One is using passive air inlets coupled with a mechanical exhaust fan as the lowest-cost whole-house ventilation system that meets ASHRAE 62.2. Those are generally sufficiently restrictive, with inline dust filters, to prevent air flow at less than about 5 Pa delta-P. They work best in a tight house with low natural stack effect flow, but still allow some minimal passive venting even with the fan not running or in a power outage.

The other is creating a cold-air trap to reduce natural infiltration in exhaust fan or make-up air ducting when there is no fan depressurizing the house.These have been used often enough in enough places that it's unlikely there isn't a benefit.

I don't think the duct stack pressure is as simple as you're making it out to be. First, the pressure depends on total height from inlet to outlet and average air temperature in the stack relative to inside and outside air temperatures. Second there can be significant static duct resistance, particularly with elbows (which add several feet of effective duct length) and long horizontal runs, which limits stack effect flow rates.

We know from woodstove installations that, even with high stack temperatures, long horizontal stove pipe runs and excessive elbows slows the exhaust flow by adding static resistance. The rule of thumb is no more than 10' of total stove pipe and no more than 2 elbows, with horizontal runs pitched upwards at 1/4" per foot and as short as possible.

I believe my inverted U-traps do reduce cold air thermosiphoning when there is no active depressurization of the house:

Robert, thanks for the reply.

"Those are generally sufficiently restrictive, with inline dust filters, to prevent air flow at less than about 5 Pa delta-P"

I would expect the airflow to be linear with pressure.  Maybe the amount that moves at 5 pa and below is insignificant.  I do like the heavily filtered passive vents and with some careful placement, which we will discuss next, they can provide some free fresh air.  When people object to running a fan, the more free the better.

As for minimal passive venting when the power is out, that would depend upon their effective location.  If that ended up being the NPP, I would expect almost no air flow.

"The other is creating a cold-air trap to reduce natural infiltration in exhaust fan or make-up air ducting when there is no fan depressurizing the house.These have been used often enough in enough places that it's unlikely there isn't a benefit."  I agree that these have been used and that people have indeed perceived a benefit from such, but I question whether the change occurred as a result of any significant trapping of cold air.  Yes, a downward loop filled with cold air could be viewed as a slight resistance to airflow, but once air does flow, it becomes the entire length of ducting that has to be considered.  A hose siphoning the water out of a swimming pool, once established, the middle of the hose can be lifted or lowered without affecting the flow, as long as the ends remain in place.  It might add some resistance, but any loop formed would not be a trap. 

So the Saskatoon Loop or the duct in a bucket are simply added resistance and a change in elevation for the air flow.

"First, the pressure depends on total height from inlet to outlet and average air temperature in the stack relative to inside and outside air temperatures."  A straight horizontal duct would have no height difference, but could have an airflow depending upon where it penetrated the envelope.  When an effective opening is located somewhere between the input and output, the pressure will be determined by the setup of the stack pressure in the house.  It could be zero, or in an extreme, the full house stack pressure.

Since I posted my "stacking concept" (here is my first worksheet: http://myenergyworkshop.homestead.com/hot-air.html ) John and I have been enjoying the increased understanding this alternate view of our house air pressures has provided.  I know I had many questions left unanswered when I took my first auditing class as nothing approaching a technical explanation was included.  Knowing where our baseline pressure originates and why the air in a negative basement heads to the attic through a chimney chase are now all easy to explain, and it is not because hot air rises.

Loops are less important than the location of their effective penetration of the house envelope and the pressures at that point.

Bud

Bud, I agree that the term "cold-air trap" can be misleading and some have ascribed in incorrect explanation for its function, but I use it because it replicates upside-down what we know as a plumbing trap. In fact, it operates somewhat like a water trap in that it counteracts normal "gravity" or stack pressure until there is sufficient delta-P across the pipe to initiate flow.

But I have to say that your insistence that "hot air rises" is incorrect is patently absurd. No one means by that phrase that hot air rises "all by itself". It is not "hot" unless it's warmer than its surroundings, and a warm air mass within a colder environment of air will most certainly rise (and be displaced by the cooler air). 

Everyone has had the (typically uncomfortable) experience of air temperature stratification within a home - and that's because the warmer air rises to the ceiling because it's less dense and thereby more buoyant. We all know that when you heat the air in a balloon, it rises and you can get a wonderful ride if you're in the basket connected to the balloon. It would be far more absurd to say that when you fire up the burner on a hot air balloon the rest of the atmosphere drops and displaces the balloon. 

Similarly, a helium balloon rises when you let it go because helium is more buoyant than air, and a submarine rises in the ocean when it's made more buoyant than the sea water. When it's made less buoyant, it sinks - we don't say that the rest of the ocean has risen around it.

What is essential to dispel from our vocabulary is the false notion that "heat rises". Since heat is merely average molecular kinetic energy, it moves or transfers (according to the Second Law of Thermodynamics) isotropically (equally in all directions). This is true of heat transfer by conduction as well as heat transfer by radiation. But heat also moves by convection, either forced or natural, due to the movement of warm fluids (liquids and gasses). It moves naturally because warmed fluids expand, become less dense, and hence more buoyant in relation to the cooler surrounding fluid. Warm fluids in a cooler environment move only upwards unless forced by non-buoyancy pressure differentials (wind or current or mechanical force).

So, sorry Bud, but hot air most definitely rises. That terminology is the most straightforward for common understanding and the most accurate to describe what we experience. It is also scientifically accurate as long as context is considered.

Robert, I'm glad you started out by agreeing with something :). 

I think the biggest flaw in your effort to "fry Bud" is here:  "No one means by that phrase that hot air rises "all by itself"."  In fact, many do and I can dig up lots of extreme quotes if needed.  I agree that the phrase is the simplest way to describe what we see happening, but it is technically and scientifically not correct.  The moving buoyancy forces do not come from within the hot air volume, but from the heavier colder air around it. 

"It would be far more absurd to say that when you fire up the burner on a hot air balloon the rest of the atmosphere drops and displaces the balloon. "  Although not as convenient an explanation, that is exactly what happens. 

Left to less than a clear explanation, and yours is farther off than mine, people, including energy auditors, have come to believe the frequently found statement, "the warm air rises and pulls its replacement air in behind it".  This is a harmless misstatement until someone concludes that the pulling creates a negative pressure behind it, which it doesn't.  An example of how this extension has found its way into our energy field is the reasoning behind removing those gable vents once a ridge and soffit path has been established.  They will describe the so called short circuit problem where the warm air exiting the ridge will pull its replacement air from those left behind gables rather than the soffits where we want it to.  The warm air exiting the ridge is being pushed up and out by the cold air pushing in the lower soffits.  Just another example of stack effect, but NO pulling involved.

The challenge I have for you and other academics, et al, that are calling me names for trying to clarify an important aspect of our energy business, is to contribute something to the effort and not just throw mud at me.  The above example of how this convenient slip of science has grown into a true misunderstanding is just one of many.  No, we should not try to push this onto our customers, unless they ask, but yes, we should be teaching our energy professionals that hot air only moves up when something else is pushing it.  And we should include an explanation as to how those pressure come about.

Bud

First, Bud, there is nothing in my comment which would even suggest that I'm either "frying" you or "throwing mud" at you, so I'm not sure why you are turning this into a personal battle rather than a discussion of the science. You have only yourself to blame for that.

Second, your description is NOT scientifically accurate, but is just as incorrect as those you castigate.

Newton's Third Law of motion has not been repealed. There is no physical action without an equal and opposite reaction. There is no push without an equal pull, there is no rising without an equal falling.

It is just as accurate to state that the air rising out of the ridge vent pulls in air from the gable vents (and gable vents are a short-circuit problem with ridge and soffit venting) as that the weight of the atmosphere is pushing replacement air into the gable vents. But the former statement is far more consistent with our experience and hence a much better description if our intent is to aid understanding.

Awesome thread Bud.  

This makes so much sense:  

I don't think the duct stack pressure is as simple as you're making it out to be. First, the pressure depends on total height from inlet to outlet and average air temperature in the stack relative to inside and outside air temperatures. Second there can be significant static duct resistance, particularly with elbows (which add several feet of effective duct length) and long horizontal runs, which limits stack effect flow rates.

 

I love the image of air sitting in a trap block flow nearly as effectively as water, and the idea breaking inertia takes more energy than you might suspect.  

 

If you've ever read Malcom Gladwell's stuff he illustrates that human beings tend to have a bias toward linear thinking:  

I would expect the airflow to be linear with pressure.  Maybe the amount that moves at 5 pa and below is insignificant.  I do like the heavily filtered passive vents and with some careful placement, which we will discuss next, they can provide some free fresh air.  When people object to running a fan, the more free the better.

 

To a surprising extent this orientation is completely wrong and leads to some humorous and not so humorous errors in design.  If you look at fans laws the curve is geometric.  Some curves can be oriented like hockey sticks (CO2, global warming?)  Intuitively you would think there is little resistance to airflow, turns out there can be quite a bit.  

 

 

 

Bud, thanks for starting the new thread...tell me if I go off course

Robert,

Thanks for all the great feedback....

I am attaching my perception of the Intentional Openings in your Warren VT House.

I would think during the Heating Season ...

the house should be "negative" or mostly negative WRTO

whether exhaust fans are on or OFF

(or during power outage)

In other words the passive Neutral Pressure Plane should be "Higher"

It seems to me that it would be better if Intake Portals CONNECT to the outdoors "low" on the prevailing windward side.

And Exhaust Portals CONNECT to the outdoors "high" on the prevailing leeward side.

And the "total area" of the Exhaust portals should be significantly greater than the "total area" of the intake portals.

John,

I'm not sure why you think the NPP would be above center with no fans operating. And your illustration is misleading since the mechanical exhaust ducts all have exterior dampers (some have an interior damper at the fan unit as well), while the always "open" orifices are the passive air inlets divided approximately equally between each storey (and the dryer make-up air inlet is in a partially pneumatically isolated mudroom).

Using the stack effect pressure formula, at those interior and exterior conditions, the total delta-P within the house would be 5.22 Pa and the NPP would be at 8'.

Robert,  OOps... I think I see how you misunderstood my comment

poor wording on my part

 

I am not predicting the NPP of the Warren house to be above center

I am saying/asking wouldn't it be better IF the passive NPP were higher?

And the way to raise the NPP would be to have larger  openings up rather high

And to have not-so big passive intake openings down low

Better why?

When there is a significant enough pressure differential to drive exfiltration (and create condensation in the envelope), it is because a fan is operating, but when an exhaust fan is operating the interior pressure is mostly (or completely) negative in relation to outdoors.

I'm not worried about the minimal natural ventilation rate, since the 12" of cellulose in the walls acts as an effective moisture buffer and the walls are 5 times as moisture permeable on the outside as on the inside, while still allowing some diffusion to the interior as well.

Where I put the "holes" in the envelope has far more to do with other design decisions, such as where the kitchen, bathrooms and laundry are best located and where fresh air is needed for the occupants.

Hi Ted, this is brutal, but I will try.  First, did you see Don's post on the other thread?  Here is the top of it:

"Reply by Don Fugler 4 hours ago

Bud, CMHC tested the flow vs. pressure performance of a Saskatchewan loop back in 1987. The report is likely still around but essentially the looping just reduced the flow rate compared to a straight inlet, by introducing a 180 degree turn. The test results did not bear out the theoretical advantages of a column of cold air sitting in the vertical tube."

A water trap is effective because the water is significantly heavier than the air it is blocking.  A trap filled with cold air has very little weight and is easily moved by a very small pressure.  But here is the catch.  Even a water trap poses no resistance to flow, once a flow is established, other than a little more length.  A length of duct bringing cold air in from a rim joist to a basement floor will establish a flow, because that is the direction of the stack pressure.  Once it is flowing, it will continue to flow, all-be-it slowed somewhat by the 180° bend at the bottom.  It is the added resistance to air flow from the change in direction and net reduction in height resulting from the upward length of the bend.  The reduction may be fine, it is just that there is no air trap formed by the cold air in the bend.

As for the linear relationship between the pressure and airflow through an opening, at these low pressures and low volumes of air, I'll stick with my assumption.

I answered this above, but the pressure across the duct is related to its effective penetration height through the envelope and the stack pressure at that point.  Once you establish the pressure from end to end, you can then assign a ▲p at the input opening, a ▲p for resistance over the length of the duct, and a ▲p at the exit opening.  Since the air entering must equal the air exiting, both ends must have the same ▲p.  Discounting any temperature change along the way.

Ted, you love the thread, but you disagree with everything I said.  I'm confused.  Some of it must be getting through.

Bud

Bud, you're confusing pressure and flow. The delta-P of an orifice is the difference between pressures at each end - there is no delta-P except in relation to two distinct pressure regimes.

Air flow through an orifice has to be the same at both ends in terms of volume but not necessarily mass as the density will change. But air flow through an orifice occurs only in relation to a difference in pressure at each end. There is no delta-P at each end, only P.

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