[Greenbuilding] ACH50 or ACHn

elitalking elitalking at rockbridge.net
Sat Jan 29 08:21:59 CST 2011


John Straube writes:

PassivHaus requiring 0.6 ACH at 50 as a voluntary program.  No problem.  But 
when they claim this number is scientifically based, it distorts, well, 
science.



My comment:

As a test protocol to demand a certain level of performance from a built 
thermal envelope, testing at 50 pascals pressure difference makes sense. 
However, this exagerated pressure difference is not a relavent in real world 
performance under natural conditions.  Where as the 50 pascal measurement is 
precise, assessing whether your site is well shielded, normal or exposed, 
expecting some variations within a zone, and different height stories 
resulting stack affect differences, the use of the n factor (ACHn=ACH50/n) 
is back to an estimate.  Though less precise for measuring, in my modeling I 
use natural conditions because I can estimate the volume of air that must be 
conditioned during normal operation.  With historical data, then ACHn can be 
used to estimate the energy cost accounting for various energy delivery 
approaches.  The environmental reaction (estimated or real) would follow 
(condensation et).



JF Straube writes:

ASHRAE 62.2-2007 is the north american ventilation standard.  It recognizes 
that both people and houses produce pollutants and hence recommends minimum 
ventilation of 7.5 cfm/person + 0.01 cfm per square foot of house area.



My comment:

Do you know the science behind this recommendation?  It does not recognize 
the variations of occupancy.  Also, it uses area instead of volume for 
recomending a volume of ventilation.  It strikes me that we need to move 
towards a qualitative assessment of conditions of air (humidity, co2, co, 
particulate matter) such that the ventilation can be adjusted according to 
the environmental conditions.  Assembly use spaces do include co2 sensors to 
adjust ventilation according to sense the occupancy needs for O2.  This is 
beyond me to invent this technology.  Therefore, I rely on "the experts" 
like ASHRAE 62.2-2007 to give me safe bases for my design goals.



The dilemma for codes and best practice recommendations is that knowledge is 
an evolving revelation.



Eli






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