[Greenbuilding] Natural ventilation.

[John Straube]

Hi Frank and Michael
Actually, 60 cfm through a 4" vent is easy if you have a 4" opening in a window.  It is not so easy if we are moving air through, say, 100 feet of equivalent smooth pipe (each 90 degree bend being equal to around 15 ft).
Below are the pressure drops in Pascals for flows of 10 to 90 cfm through 100 ft of 4" pipe 
10	2.1
20	7.9
30	17.1
40	29.5
50	45.0
70	85.4
90	137.6
As you can see, even 30 cfm has a pressure drop of 17 Pa, which would require either a serious wind at the top of the stack or a significant temperature rise between the stack and house (eg 100 F or more)
Now 20 cfm might be more reasonable as 8 Pa should be feasible for a solar chimney or a cold winter night or normal light winds.

What will this do for cooling?
Well if the earth tube cools the air to 60 F (plausible but optimistic in 100 ft) with a 75 F indoor temperature, then the cooling provided would be
(75-60)*20 = 300 Btu/hr.
This is equal to 1/20 of a ton of cooling or about the heat output of a person doing housework.
I am certain this level of cooling will be completely unnoticed and will not cool the house by even 0.1 degrees.
Adding a 20 W fan to move 70 cfm would increase the cooling by a factor of 3.5, and still be essentially useless BUT it would easily bring in the 50 to 60 cfm Micheal rightly pointed out is about what is needed for ventilation.

OTOH.  A fan, using say 20 Watts blowing air at 200 feet per minute over a persons body, would make a house at 80 F feel like it is 72F to a person lightly dressed.



On 2011-08-30, at 9:31 PM, Michael O'Brien wrote:

> Hi, Frank--
> 
> If you accept the ASHRAE whole-house ventilation rate, you can easily provide it with a 4" vent in a small house. For example a 3-bedroom 2000 SF house is recommended to have 60 cfm ventilation capacity in Standard 62.2.
> 
> I believe that this rate is fine for houses which have been built using safe non-toxic materials and have low to normal moisture loads.
> 
> Best,
> 
> Mike O'Brien
> 
> 
> On Aug 30, 2011, at 6:19 PM, Frank Tettemer wrote:
> 
>> Frank King, writing in "Ventilation", is correct:
>> Air once breathed has reduced food value.  And ingesting it reduces ones' sustaining power.
>> So fresh air is truly what we need.
>> Thanks for that info, Rob.  Priceless, really.
>> 
>> And thanks, Mike, for the link to Duro Dyne.  I think I can make use of their spring loaded 4" damper, for the Summer months.
>> 
>> Admittedly, Alan, the volume of air exchange won't be huge.  But in the compact, well-sealed home we're building for the Cool Hemp folks, a steady night-time/day-time trickle of "pure, free air of field and forest" through a 4" opening, both in and out, should make some sort of difference.  Of course an opening skylight of 2 foot by 3 foot would be much better at air exchange.  But as Rob Tom has said,  I'm cringing and I am tempted to say "Gggggnnnaaaah!"
>> 
>> A four inch pipe is pretty insignificant, in the big scheme of things.   But it's better than nothing at all, and it takes double advantage of the required 4" combustion air supply, during the few non-wood-burning months we get in this climate.
>> 
>> (That's a pretty funny appellation, Alan.
>> "Tar Sand Nation" also makes me cringe.)
>> 
>> Frank
>> 
>> Frank Tettemer
>> Living Sol ~ Building and Design
>> www.livingsol.com
>> 613 756 3884
>> 
>> I suspect that fellow MooseHugger Frank, like I, cringed and uttered "Gggggnnnaaaah!" at warm-clime AA-man's suggestion to install a skylight.
>> 
>> Oh! The horror!
>> ...in a well insulated house, where there ought to be little stratification in winter time, why would a skylight be any less energy efficient than a window of the same size and glazing quality?  Further, if it were strategically located and obtained some winter daytime gain, and or if the homeowner from Tar Sand Nation installed a two inch block of styrofoam in the opening from November to March, it might be a net asset, not to mention to provide the summer stack effect bonus.
>> 
>> AA
>> 
>> -- 
>> 
>> 
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Dr John Straube, P.Eng.
Associate Professor
University of Waterloo
Dept of Civil Eng. & School of Architecture
www.buildingscience.com