SB: Re: Re: air takes the easiest path

["GuyW" <guyiii@cox.net>]

That counts. Thanx


----- Original Message ----- 
From: "Derek Roff" <derek@unm.edu>
To: "GuyW" <guyiii@cox.net>; <strawbale@crest.org>
Sent: Friday, March 22, 2002 6:36 AM
Subject: Re: Re: air takes the easiest path


> Check issue # 32 of The Last Straw, page 9, for an article by Rob 
> Tom, which covers many issues from this thread, including air and 
> moisture exchange, heat recovery ventilators, infiltration and energy 
> conservation.  Rob includes a sketch copied from the 1993 ASHRAE 
> Handbook of Fundamentals, page 23.4, which diagrams the air pressure 
> differentials in a heated house.
> 
> The basic idea is that, in a convection situation, hot fluids (such 
> as air) will rise to displace colder fluids, which will sink.  In 
> popular terms, "hot air rises."  In a heated house, temperature and 
> pressure will tend to have the highest values for both temperature 
> and air pressure at the highest points in the heated envelope, and 
> lower values down low.  This is often referred to as the "stack 
> effect."  Unless you cool your house down to ambient exterior 
> temperatures, there will always be a temperature and pressure 
> differential, which will tend to drive air out of the house in the 
> upper half, and suck air into the house in the lower half.  It is, of 
> course, possible to pressurize or depressurize the entire house 
> (relative to outside).  This can lead to problems with driving 
> moisture into the walls or sucking combustion products into the 
> living space.  Again I refer you to TLS # 32.
> 
> Cracking open a window in the winter lets air pass freely.  But that 
> window may have open area of 0.15 m2.  Air doesn't pass easily 
> through the average ceiling, but the ceiling may have an area of 
> 100-200 m2.  Meaning that the ceiling may have an area that is a 
> thousand times more than that of a partially opened window.  In 
> addition, ceilings have high potential for air leakage at walls and 
> junctions.  These factors can make air leakage at the ceiling an 
> important part of the total air exchange.
> 
> I'm not sure I have made this much clearer, but I did use more words. 
> Does that count?
> 
> --On Thursday, March 21, 2002 3:46 PM -0800 GuyW <guyiii@cox.net> 
> wrote:
> 
> >> ....but it won't change the fact that the second floor
> >> ceiling will experience the highest air pressure in the house, and
> >> the first floor floor will have the lowest air pressure.
> >
> > Can you explain more fully why these air pressures are as you
> > describe?
> >
> > -Guy-
> 
> Derek Roff
> Language Learning Center, Ortega Hall Rm 129, University of New Mexico
> Albuquerque, NM 87131  505/277-7368 fax 505/277-3885
> Internet: derek@unm.edu
> 


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