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
Views: 2008
Replies to This Discussion
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Permalink Reply by Robert Riversong on -
The coefficient is based on standard pressure, not temperature. And that formula is for delta-P inside to out, not absolute pressure.
The atmospheric pressure is the weight of the column of air on the earth's surface. That column remains the same mass whether it expands from heat or cools and contracts.
But the HVAC industry, uses standard pressure and temperature to determine air flow volume.
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Permalink Reply by John Brooks on -
I think I found what I was looking for
Standard Temperature is zero degrees Celsius
Standard Pressure is pressure at sea level
Attached is a snip-it from Straube's textbook.
the formula does return Delta P if Delta H and Temperatures are known
it can also be used to solve for Delta H if Delta P and Temps are known
I used the formula to estimate the Delta Vertical Isobar Spacing for Delta T
{Isobar = a line connecting points of equal atmospheric pressure}
Delta Isobar (Pa) Spacing ( Inches) = Delta T (Farenheit) x 0.0066
So...at sea level and 32 degrees F
Isobar Spacing would be 3.14 inches
If the air in an example house is 68 F.. then Delta T = 36 F
Delta Isobar spacing = 36 x 0.0066 = 0.24 inches
Isobar spacing inside the house = 3.14 + 0.24 = 3.38 inches
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Permalink Reply by Robert Riversong on
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The US Standard Atmosphere is based on a standard temperature of 59°F (the average air temperature at the earth's surface). http://en.wikipedia.org/wiki/U.S._Standard_Atmosphere
There are many other "standard" temperatures and pressures used by various organizations:
http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_an...
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Permalink Reply by John Brooks on
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Robert, I noticed that there are several "standards"
which standard (temperature) do you think Straube is referring to?
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Permalink Reply by Robert Riversong on
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The full SI formula for stack effect delta-P is:
ΔP =0.0342 x H x A x (1/TO - 1/TI), with A = atmospheric pressure
To get Straube's conversion factor of 3465, he's multiplying 0.0342 times 101,325, so he's using that as the standard atmospheric pressure.
NIST seems to use 68°F as the standard for gas flow. But you'll have to ask Straube which is the "new normal" for him.
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