[Greenbuilding] Stale Air

[jfstraube]

Ventilation rates are not set based on the physiology of O2 requirements as there is a lot of O2 available in the air as John states  The rise of CO2 is more of an issue, but submariners regularily spend months at around 5000 ppm, whereas we usually ventilate to 1000-1200 ppm (2 to 3 times outdoor levels).  Of course, most IAQ issues are not related to either O2 or CO2, but CO2 is a good indicator of general ventilation quality as a function of human occupancy.
ASHRAE standards have historically been set based on early research around odour: it is human smells that set the ventilation rates. Depending on culture and time, these rates have, under ASHRAE's guidelines, varied from about 3 cfm to 30 cfm of fresh air per person.
If a building is ventilated to 1000 ppm, this is about equal to 15 cfm per person.  800 ppm is about 20 cfm.
Current residential ventilation rates by ASHRAE are 7.5 cfm/person + 0.01 cfm/square foot.  This has worked rather well, as IAQ problems never crop up when this ventilation is actually delivered, and the building is not too polluting. Increasingly the problems of ventilation are 1.  the ventilation is not delivered and 2. the building or outdoor air supply is the source of the pollutant.
In the old days, 1. was a problem in commercial buildings because a ventilation system was not installed or fresh air was not delivered to the building.  Today I find the problem is that the ventilation system does not actually deliver fresh air to the space where people are reliably. VAV systems in commercial buildings are particularly bad.  The response to these problems has been to increase base level ventilation rates rather than fix the problem with ventilation systems.  For example, in a hydronic heating system with an exhaust fan acting as the ventilation system, bedrooms often dont get ventilation air if they have their door closed (not uncommon in master bedrooms).  Hence people think they are ventilating when in fact the fresh air being brought in (and heated) is not being delivered to that room.   A fresh air supply to each room (via a duct or via an air handling system with a FanCycler controller) provides assurance that every room gets ventilation.
The ventilation rate of 0.01 cfm/sf explicitly recognizes that buildings, not just people, generate pollutants.  Lots of off-gasing in new buildings, but once past the first year or so, things like pets, cleaning chemicals, or moldy buildings are more likely.  You can control this ventilation rate requirement by how you build.  Many houses might require a higher rate because of the way they are built.  Some might require less per sf.  Of course, in commercial spaces we often find a problem that the ventilation intake is from the loading dock, or next to the dumpster, or 8" above the black tar roof exposed to sun:)

My experience is that if you ventilate reliably to every space, during all hours, there are no problems.  the rate is not the issue, although of course it must reasonable.  the problems come from not actually delivering ventilation air to each space under all weather and operating conditions.

John


On 2010-12-12, at 10:31 PM, JOHN SALMEN wrote:

> Sorry Stephen – I know you know all that stuff, I liked the question and was thinking out loud and still thinking out loud. One of the interesting things for me is that when someone is cyanotic or hypoxic in a medical situation it is pretty apparent. Basically co2 drives our breathing as we eliminate it. Chronic high(er) levels of co2 in our built environment may be an issue. It would never be the case that there was not sufficient oxygen – more that the body becomes less able to utilize the oxygen as it becomes less able to eliminate co2 or other mixtures of gases. So we end up with chronic problems.
>  
> That is one thought and then a companion thought is that co2 is used as an indicator – if it is present at higher levels then other gases are so is a broad measuring tool for IAQ and these gases interact. I think we are allowed approximately double the exterior co2 level for indoor environments. Perhaps that is too high for an aging or health compromised population.
>  
> Increasingly I think ventilation rates are too low (and people spend too much time indoors but ironically not that much time in their houses). Are we heating and ventilating houses far too much when are unoccupied and then ventilating sufficiently for when they are? How well is demand controlled ventilation actually working?
>  
> Thanks for the question – has me scratching my head as well.
>  
> John
>  
>  
> JOHN SALMEN ENVIRONMENTAL DESIGN
> 4465 UPHILL RD,. DUNCAN, B.C.  CANADA, V9L 6M7
> PH 250 748 7672 FAX 250 748 7612 CELL 250 246 8541
> terrain at shaw.ca
>  
> From: Stephen Collette [mailto:stephen at yourhealthyhouse.ca] 
> Sent: December 12, 2010 4:44 PM
> To: JOHN SALMEN
> Cc: Greenbuilding at bioenergylists.org
> Subject: Stale Air
>  
> Thanks for your thoughts John. I am wondering how useful the measurement of CO2 in homes is. I have a CO meter and pull it out on occasion and to my memory have not found anything with it in a home. Typically with a couple of people or small family in a single family dwelling, I wonder whether they alone could ever get the levels up to something dangerous? Now blended families or extended families all living under one roof, I think that may be possible. 
>  
> I'm not disagreeing with you at all, what you are saying is what I talk about too, but again, is it actually valid? I don't know, and hence the head scratching. 
>  
> Grateful for your time thinking about this.
>  
> Stephen
>  
> Stephen Collette BBEC, LEED AP, BSSO
> Your Healthy House - Indoor Environmental Testing & Building Consulting
> http://www.yourhealthyhouse.ca
> stephen at yourhealthyhouse.ca
> 705.652.5159
>  
>  
>> Hi,
>>  
>> There are a lot of adjectives for air – stale, stagnant, dead, fetid…
>> Air basically means for us oxygen as a requirement – so stale air could simply be oxygen poor air.
>> .
>> With ashrae I guess there is adequate ventilation and inadequate ventilation and stale would be an excess of unventilated air (stale??). Ashrae uses co2 concentrations as the indicator for adequate ventilation so there definition is ppm for various uses.
>>  
>> In emergency first aid I measure ventilation rates,  blood oxygen levels as well of level of consciousness – all of which could reveal an emergency condition in students in a classroom at the end of a day subject to averaged ventilation and subsequent ‘stale’ air. Tidal volume (breath) is about 500ml with about 14% oxygen and 4.4% co2 exhaled – outside air is about 21% oxygen and .04% co2.
>>  
>> So a significant amount of c02 is released on each breath in comparison to the intake – so we could say that ‘stale air’ is any air exhaled.
>>  
>> Fun question.
>> John
>>  
>>  
>>  
>>  
>> 
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> 
>  
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John Straube
www.BuildingScience.com



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