# Thread: Calculating the required CFM

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Originally Posted by tnhikr44
If the task is to simply lower the CO2 to below 1000 ppm and maintain that concentration, with no regards to temperature or humidity, I think it would be rather simple. A 100% outside air variable speed fan, say 500 cfm maximum flow, and a barometric relief damper. Full fan flow for purge, once set point is achieved control the fan speed to maintain required CO2 level. There would not be a hard fast ventilation rate number ... just the minimum to achieve set point.
Thanks for the concise statements.

2. There are actually three ways to solve your problem. One has to deal with the unknown variable of the outside CO2 level which could be over 1000 ppm itself depending on where the room is located. The other uses gas phase filtration to reduce the levels without the introduction of outside air. The third way is a mix of the two using outside and inside CO2 meters and a control system to modulate the outside air damper only when it is the best and to control the fan drawing room air through the scrubber. There is no single answer. The conditions inside the room will vary depending on the change in activity level and the occupacy level (when people come and go). CO2 meters need to be re-calibrated every few years. Your current condition is high for an office. Your meter may be out of calibration or your room is very, very tight. Do you need to be concerned with introducing harmful or corrosive gases that are outside the space when you ventilate?

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The aim of ventilation should be to dilute or replace indoor odors or objectionable off-gassing. in order to do this with unconditioned outside air the outside air must be "cleaner" then the indoor air.
Next - how to determine when the indoors needs "cleaning". CO2 is not an objectionable gas. Man has decided that the measurement of CO2 can be used as a surrogate to determine the concentration of "objectionable" gasses and thus control more or less ventilation.
Building codes or governing authorities may determine the amount of outside air to be introduced depending on the type and quantity of occupancy.

4. Your equation for an office space is:

5 x 3 (persons) + 0.06 x zone sqft.

This is per 2009 International Mechanical Code, 2009 edition. Note that variables change with different spaces; I.E. Reception areas, conference rooms etc.

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unknown variable of the outside CO2 level which could be over 1000 ppm itself depending on where the room is located.?
in order to do this with unconditioned outside air the outside air must be "cleaner" then the indoor air.
The industry standard that we use is 400-450 ppm CO2 for fresh air. Technically it is 392 ppm. And although I have tested 'fresh' air at higher levels than that, never have I ever seen it above 650 ppm. I would like to know where you guys have seen fresh air over 1000 ppm ... I want to avoid that place. As Chief Fred said, CO2 monitoring is more of a 'barometer' of other potential gases, and if you need scrubbers to clean the fresh air, well, I want no part of this office space.

6. Originally Posted by Chief Fred
The aim of ventilation should be to dilute or replace indoor odors or objectionable off-gassing. in order to do this with unconditioned outside air the outside air must be "cleaner" then the indoor air.
Next - .
Totally agree !!

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Gib's right: That works out to 38 CFM required.

8. Originally Posted by tnhikr44
The industry standard that we use is 400-450 ppm CO2 for fresh air. Technically it is 392 ppm. And although I have tested 'fresh' air at higher levels than that, never have I ever seen it above 650 ppm. I would like to know where you guys have seen fresh air over 1000 ppm ... I want to avoid that place. As Chief Fred said, CO2 monitoring is more of a 'barometer' of other potential gases, and if you need scrubbers to clean the fresh air, well, I want no part of this office space.
The highest I have measured is around an airport in California 775ppm of CO2 and 6ppm of CO outdoors in the outdoor parking lot. The highest CO2 I have read indoors is a first grade school classroom at 1900ppm. The kids wanted to fall a sleep every afternoon in class.

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Originally Posted by genesis
The highest I have measured is around an airport in California 775ppm of CO2
The typical CO2 set point I see is 900-1000 ppm. You have measured a high of 775, and my high was around 650. In both instances we would be able to maintain a level between 900-1000 with additional fresh air, because it is actually "fresher". My original question is still unanswered ... where is the O/A CO2 level over 1000 ppm, where one would have to scrub fresh air? Just curious.

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