[Greenbuilding] Sensors to evaluate ventilation needs

Corwyn corwyn at midcoast.com
Sat Aug 20 18:16:44 CDT 2011


On 8/20/2011 2:30 PM, elitalking wrote:

> 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 suspect we are coming at it from different perspectives.  For example, 
John complains that a humidity sensor will be useless if the windows are 
open on a humid day.  From my perspective of smallish buildings in 
Maine, if it is nice enough outside for the windows to be open, 
ventilation is never a problem.

> 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.

True, but most places with bedrooms have roughly the same ratio of 
volume to area (i.e. ceiling height).

> 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.

I am coming to the conclusion that many 'rules of thumb' such as the 
ASHRAE recommendations are on their way out.  We are now getting to the 
point where we can just model each building individually.

> Hence, my interest is in objective sensors for regulating ventilation to
> reduce energy cost for conditioning air when it is not needed.

In the case, of a party full of people, I see a room full of SUBJECTIVE 
sensors being of far more utility.  When people feel stuffy they open a 
window.

> 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?

Possibly (I don't know if such exist), but CO2 is measured in PPM and O2 
in percent.  Thus, precision is easier with CO2.  Most processes that 
consume oxygen, produce CO2 as a by product, and vice versa.  It is 
interesting to note, that human physiology measures CO2, not O2.  If you 
scrub the air of CO2 but let the O2 diminish, people will be unaware of 
any issues, including their failing brain functions (the part that 
notices such things going first perhaps?) as they die.

> 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.

Humidity is a proxy for human activity.  While it isn't 
counterproductive to ventilate when outdoor humidity is high, it will 
positively feedback you sensor.

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

I invite you to do the calculation for heat loss cost of running an HRV 
24/365 at a rate for sufficient ventilation in the worst case, and 
compare that to the cost of any sensor you are considering.  For my 
house that number is around $37 per year (plus electrical cost).


Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.greenfret.com/
topher at greenfret.com
(207) 882-7652




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