[Greenbuilding] Sensors to evaluate ventilation needs

elitalking elitalking at rockbridge.net
Sat Aug 20 13:30:37 CDT 2011


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






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