GBlist: Humidity/CO2/IAQ

Richard Kadulski (
Sun, 9 Mar 1997 20:32:44 -0800 (PST)

The question posed:
> There are a variety of parameters to control ventilation: CFM
per person,
> Air Changes, CO2, and RH. If you had to pick one, which one
would you pick
> and why?
> Greg Thomas

David Brook already raised the question is it residential or commercial
applications that we are looking at.

>CO2 level is the most accurate means of controlling the need for outside

CO2 can be considered as ONE means of controlling air needs. It should be
rememberd it may not be in all cases - in many, especially residential,
situations humidity may well be the more significant concern.

> EPA requires level to be less than 1,000 ppm and I understand that that
>may be dropped to 800 ppm very soon. If a fixed cfm/person is used to
>bring in fresh air, the maximum amount of air will be introduced
>weather the max or min amount of people are inside the building. By
>monitoring CO2 levels inside and having C02 sensors controll outdoor air
>dampers, the amount of fresh air will be introdced in proportion to the
>internal ventilation demand.

Let's not forget that ambient CO2 levels are around 350 ppm. I understand
that trying to meet 800 ppm limits on a continuous basis will be very
difficult to achieve in practice if we are also trying to maintain a
reasonably energy efficient building.

Also, while we need yardsticks, CO2 is not the most lethal of gases we
should be concerned about. The human body is quite tolerant of a range of
CO2 levels, and while it is not the best indicator, lets not forget that the
military allows levels up to about 10,000 ppm in submarines. Lest someone
get the wrong impression, I am not implying that is an appropriate level,
only to point out that even such a high level is tolerated for shorter
periods in an environment where presumably there is an expectation for
people to function at a high level of mental alertness.

In many situations, if the levels move up and down from the target, and as
long as there really is a continuous ventilation strategy to avoid high
levels, there should be no detrimental conditions if cocentrations vary -
and magic levels presumably need not to be considered with the same gravity
as for more toxic substances (e.g. ozone, formaldehyde, etc).

> This is called demand controlled ventilation DCM, and is used primarily
>with direct digital control DDC systems that are able to accurately
>control dampers based on sensor signgals.

In commercial and institutional applications, demand controlled ventilation
definitely appears to be a good approach. The French have developed a system
that has elements of demand controled ventilation that can be used in
residential applications (American Aldes sells the system).

>Energy can be saved by only bringing in the amount of outdoor air
>necessary at any particular time.
>Thanks for the question.

Richard Kadulski Architect
208 - 1280 Seymour St.
Vancouver, B.C. V6B 3N9
Tel/Fax 604-689-1841

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