SB: Re: Re: air takes the easiest path

[Derek Roff <derek@unm.edu>]

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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