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.
I'll try to keep this short.
Robert, I apologize if I was too defensive and too sensitive.
Ted, I apologize to you as well.
To both of you, I welcome your participation, so let's continue. If my work on "hot air rises" is indeed incorrect and patently absurd, then my arguments will fall apart with further discussion. I would first like to return to the topic of this thread, "Controlling Passive Air Flow" with some analysis of Robert's inverted loop. Later, in this thread or another, we can resume the discussion on how air stacks up, the hot air issue.
The "Saskatoon Loop" or "Saskatchewan loop" as Don posted, just doesn't seem to be workable. With the cold air starting well above the loop, in the lower portion of a home where stack pressure and the weight of the air pushing in, there would always be air flow in cold temperatures. Robert, your inverted loop does offer some resistance, but there are some issues.
1. You specify the determining factors as, "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. Equally important will be the location of the inlet and outlet in reference to the pressure balance inside the home, AKA the NPP.
b. Average temperature inside the stack (vent) does not reflect the height of the incoming cold air plug, which is what must be pushed out of the way for air to flow.
2. Once an exhaust fan has been cycled and the entire length of the vent duct is filled with cold air, it doesn't seem that it will stop until the stack pressure goes away.
3. Even when the pressure builds slowly and cold air begins to be pushed into the duct and the block is working, that cold air will warm. As it warms it will gradually be pushed up and over the top of the loop. Once a mix of air begins to flow down the other side the pressure will increase and more air will be forced up and over. At what point this will flip into a full flow of cold air I have not pondered, but it seem like a possible event.
4. If the top of the loop is too far below the NPP, then the temperature (weight) of the air inside the incoming upward portion of the loop will be insufficient to block the flow. In other words, the resulting weight of 3' of cold air needs to equal or exceed the net vent pressure once the air has been pushed to that height. A first order approximation would be, the length of the loop needs to be as long as (or greater than) half the distance from the inlet to the NPP. At exactly half the distance, when the inward leg of the loop is filled with cold air, the reduced pressure across the vent will equal the weight of the cold air blocking it. Given the uncertainty of the NPP, a considerable margin might be needed.
This is just work in progress to try to understand the inverted loop and identify applications or improvements related to static venting. Although the NPP is an elusive parameter, with intentional openings it may be possible to design or adjust its final location.
I agree that the Saskatoon Loop won't do much more than add static pressure drop and if it's going down into a basement it will just be a siphon.
My woodstove combustion air inverted U-trap mostly just adds tortuosity and duct length to increase static pressure drop. If it's direct-coupled to an air-tight wood stove then it should resist movement when there is no fire and the outside temperature at both ends of the "chimney" is the same.
But the primary use of my 3' down and out system is on all exhaust and inlet ducts, both to allow condensate to drain outward and to resist cold air back-siphoning.
It takes 12.4 Pa (0.0018 psi) to lift a 3' column of cold air. There will never be that much negative pressure inside a 2-storey house without mechanical forcing.
The three foot head of cold air would weigh approximately 12.4 pa, however, it is the difference in weight between warm and cold air that produces the stack effect pressure or moving force. For discussion I like to use a ▲35° which yields approximately 0.25 pa per foot. Thus 3' would yield 0.75 pa. Am I missing something?
Connected to the wood stove would close the path from outside to outside and that should stop the air flow. Is the loop necessary?
Any thoughts of preplanning the location of the NPP and using it or the known pressure balance above and below to regulate passive ventilation?
For a force to move an object requires that the force be greater than the normal vector component of the weight of the object plus the resistance of the pathway. While a 0.75 Pa delta-P would move air through a horizontal opening (where there is no gravity to overcome), it won't move a mass of air against gravity if it's only 0.06 times the weight of the mass.
The loop is helpful in a closed system because the wood stove, stove pipe and chimney is warmer than outside air, thus creating a chimney effect unless there is sufficient resistance in the pathway.
Now we are getting somewhere. If I'm interpreting this correctly, and forgive me if my approximations are not as accurate as yours, but the 12.4 pa must be the weight of the cold air (approx 3.4 pa per ft.) plus some ▲t stack pressure added in.
So what would be the force to move air through that loop if there were no ▲t? Say a bit of wind pressure wanted to move some air in.
Pascals are a unit of pressure, not weight. It gets confusing in the English system, since both force and weight is measured in pounds.
I just calculated cold air density times the volume of a 3' high duct, the psi required to lift that mass, and converted to Pascals.
The pressure operates at the surface area of the duct, while the weight of air column is dependent on total duct volume. Since volume is proportionate to length for any duct diameter, the same pressure differential would be required for any duct size.
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 (assuming the outer termination flap is stuck open).
I think the direction of flow thru a duct/tube depends on the absolute pressure at each end.
If the absolute pressure at the exterior end of the tube is greater than the absolute pressure at the interior end of the tube ... then the flow would be from outside to inside.
By estimating the height of the NPP ... we should be able estimate the absolute pressure at each end of the duct/tube and predict the direction of air flow.
The rate of flow would depend on Delta T, the size of the openings, vertical distance between openings, and to use Robert's term (the tortuosity) of the passage.
Good morning John,
<The rate of flow would depend on Delta T, the size of the openings, vertical distance between openings, and to use Robert's term (the tortuosity) of the passage.>
AND the starting pressures at each end!
Stack effect pressure differentials are dependent only on delta-T and height.
Stack (or chimney) effect flow volume and rate is dependent only on delta-T, height, orifice size and discharge coefficient.
<Stack (or chimney) effect flow volume and rate is dependent only on delta-T, height, orifice size and discharge coefficient.>
AND the stack effect pressure inside the home at the source of the air flow.
Granted, a hot chimney can almost ignore the small stack effect pressures, but with a cold chimney, as in an outside chimney, those stack pressures become very important.