[Greenbuilding] JStraub response: Sensors to evaluate ventilation needs

[elitalking]

JStraub's response to earlier message off list.  He approved posting message.  I think it is of interest.  

RH is not a measure of IAQ.  It is a measure of human comfort, and too high levels can damage buildings. 
ASHRAE 55 covers the comfort side and suggests RH levels of 20 to 60.
ASHRAE 62.2 is a standard.  Of course the number of people is not equal to bedrooms + 1.  But what guess would you make?  The ventilation rates have been calibrated against this assumption. There is no good science to tell you what cfm/sf or cu ft will provide good IAQ.  All we do is set a value that by consensus solves most problems most of the time.
You cant make a sensor for IAQ, because we dont know what to measure: which of the thousands of chemical compounds in the, say, cleaning and building products should you focus on?  Does that mean you should ignore the smell of burnt meat? Foul body odors? Wintertime RH?
The answer has been we ventilate a certain amount, one rate for people (it varies from 3 to 20 cfm per person depending on which standard and when) and one rate to deal with building emissions (varies from zero to 0.06 cfm/sf or 0.2 ACH).  
We have experience with hundreds of thousands of homes with mechanical ventilation systems and thousands of commercial buildings (offices use 5 cfm/person +0.06 cfm/sf for example). If ventilate at about half this rate, problems begin to crop up at an increasing rate.  If you are good about avoiding polluting materials, and have some tolerance for odors, you can decreases the rate further with no problems.  Rates of twice this much dont seem to have any noticeable effect on peoples perception or even most pollution levels measured.
CO2 is a reasonably good measure of human occupancy and activity as well as background ventilation.  Hence it is explicity allowed in ASHRAE 62.1 (commercial code) and widely used.  Even when you use CO2 measurement, background ventilation to remove buildng generate odors and emissions is recommended. CO2 only measures the human side of the equation.




On 2011-08-13, at 2:35 PM, elitalking wrote:


  John Straube wrote:

  The problem with RH is that it varies wildly with with outdoor conditions.  You can have all the windows open in the summer and the RH will be high, and have too little ventilation and the RH be very low in cold weather in Denver.  Hence RH is a horrible measure of ventilation need unless you add a lot of other non measured info like outdoor conditions, floor area, etc.
  CO2 is a good measure of occupancy and intensity, which correlate fairly well with the need for ventilation.

  On 2011-07-26, at 9:29 AM, Corwyn wrote:

  > On 7/24/2011 10:50 AM, Peter Kidd wrote:
  >> dioxide for ventilation control
  >> 
  >>> On Sun, 24 Jul 2011 09:21:36 -0400, elitalking
  >>> <elitalking at rockbridge.net> wrote:
  >>> 
  >>>> 
  >>>> Does anybody have a good inexpensive CO2 monitor recomendation? What
  >>>> levels of CO2 are tolerable for good indoor air quality?
  > 
  > Greetings,
  > 
  > I find it hard to believe that the first warning sign of drastically low ventilation levels is going to be elevated CO2 levels.
  > 
  > Why not use humidity?  That can be done easily, cheaply, and many systems are already configured for it.  Unless you have a CO2 generating source which is NOT putting out vastly more H20 than CO2

  My Comment:


  John Straub and Corwyn correctly surmised my interest in CO2 monitoring for evaluating the air quality and the need for ventilation.  It is interesting that these two smart and generous contributors of their knowledge to this list disagree on the implications of relative humidity in evaluating air quality. 
   
  I am wondering if their might be developments in a combination of sensors to evaluate objectively the need for ventilation air.  It strikes me that the ASHRAE guidelines for Required ventilation/min = (.01cfm/sf x Area(sf)) + (7.5cfm/occupant x #occupants) is incomplete.  Using bedrooms as an indicator , Occupants =#Bedrooms +1, makes an assumption that the master bedroom is double occupied with a couple.  This strikes me as a rule of thumb.  It seems odd to give a volume ventilation rate based on an area.  Also, the # of bedrooms does not recognize the great variations of occupancy.  If the bodies are away at work, they are not consuming O2.  If we have a party, there are a lot more bodies breathing and exhaling moisture. 
   
  Hence, my interest is in objective sensors for regulating ventilation to reduce energy cost for conditioning air when it is not needed. 
   
  I know that assembly functions such as a courtroom have CO2 sensors to evaluate when ventilation is needed.  I suppose CO2 is an indication of O2 consumption.  Is it a true indication of O2 available?  Would an O2 reading be more direct to determine if there is enough air?  Does high CO2 mean low O2?
   
  Can humidity be used to evaluate when you need air?  It can be used to evaluate when drying is needed.  However, as John points out, ventilating because of indoor high relative humidity is counterproductive if the exterior air has higher humidity such as a rainy day. 
   
  Can there be sensors for other pollutants in indoor air that are problematic (Dust, CO, VOC)? 


  Perhaps this is too complex and a genral flushing of the air specified by ASRAE formula is the appropriate method. 
   
  Eli


John Straube
www.BuildingScience.com





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