Calculations Regarding Indoor Air Mixing....

Why?

What is your CO2 concentration now?

How do you know you need more outside air?

Do you work for a consulting engineering firm?

Are you a student?

Seems like an air sampler hooked to an outside door or window would do it.

Have you performed a search of air sampling equipment?

What have you done, so far?

This varies according to room use.....here is a guideline...

http://www.engineeringtoolbox.com/ventilation-air-flow-rate-d_115.html

Not a generalized number, we used (not a std)20% fresh Air-intake as recommended by Our specialist, calculation is base on geographical locality, i.e humidity , near to the sea, etc... Any number below this need not be conditioned, as it will not upset the Air-con system capacity, minimal impact. Just 1 sub-arrow of the fishbone chart.

Like Lyn asked, how do you what your CO2 concentration is? What type of equipment are you using to sample the air? And is the said equipment calibrated ? And depending on the size of your air handler and cubic footage of room/ building, it can take up to8 hours or more for the room air to stabilize.

Please refer ASHRAE guide if you want to maintain proper Indoor air quality ( IAQ).

I suggest using a "demand" ventilation program, that way you only have to bring in enough outaide air as well as condition that outside air to dilute and lower the CO2 ppm. See: https://www.energycodes.gov/sites/default/files/documents/cn_demand_control_ventilation.pdf See: http://dms.hvacpartners.com/docs/1001/Public/0B/811-10088.pdf

Determine and control required OA flow rates:

http://www.airtest.ca/support/reference/paper1.pdf

Hope this helps,

Zvi

"We want to maintain a fresh (CO2 conc = 400ppm) indoor air..."

What is the rate of CO2 generation inside the building?

If 400 ppm is is your indoor air concentration target, you're going to need outdoor air a lot 'fresher' than exists today in most places in the civilized world.

Indoor levels considered to be acceptable are around 700ppm.

http://www.kmccontrols.com/products/CO2_Sensors_and_DCV.aspx

Zvi

When I was a medical student the standard atmosphere contained 0.03% (300 ppm) CO2. Figures for the present day vary from 250-350 ppm, which I do not entirely believe, to 400 ppm, which is rather more likely.

However, what has not changed is that an adult human being at rest emits around 250 ml/min of CO2. With exercise that goes up by a factor of 10 or more. I would go along with the idea that the room level should be kept below 700 ppm, and base your ventilation calculations on that, taking into account the total CO2 production from man and machine.

A "model" will tell you nothing. You need to know the levels inside your building (even animals need a certain level of CO2 maintained). Do you have a Building Automation System? Do you have a monitor hooked to the BAS for a trend study? Do you understand the ASHRAE guidelines? If no to any of these questions - just set the minimum to say 10 to 15% and be done with it. Unless you are in a nasty metro environment (lots of manufacturing plants, maybe 20 to 30% with very good filtering system). Maybe more information would be good, ya think?

Animals maintain an internal CO2 equilibrium but no animal is dependent on an external source of CO2.

But plants are!

True, but a total irrelevance as far as the original question is concerned. (Unless you are considering a room with bright lights and lots of plants growing, in which case the police may be around shortly.)

Thats what I think this thread refers to. No?

Walk Chalk JayHawk! (Not that I ever went there, but hear you fellows are alright).

Would rather root for you than a stinky razorback, or a rangy, tick-ridden longhorn.

Aggies bailed on us, but at least we have the only Bears in Texas, and some Red Raiders....BTW I think Red Raiders had something to do with The War of Northern Aggression, but I can't remember for the life of me what.

In spite of all this crap about removing the Battle Flag of Northern Virginia from any public display, I still revere the old colors as a sad, but beautiful part of history of this glorious country, where brother was willing even to fight brother for the sake of a belief in what they held as right. Surely it could have been avoided, but that is what happens when the last seven level headed people in the room lose it.

Sorry about the all the off-topic rant, but I think I just increased the CO2 level in a few rooms. By the way, the CO2 level in Washington, D.E. (District of Ebony) is way over 2000 ppm when Congress is in session. That and the level of pure bull crap is also up to the level requiring rubber boots when crossing the street.

Cheers! Happy Birthday America! God Save the Queen!

It may depend also on the classification of the facility. If it is a medical facility you need a certain amount of air exchanges per hour to maintain new air input. If you have Medicare of other programs they may have other standards to abide by.

"If you have Medicare ......" Medicare doesn't care about shit, let alone your well being. It's cheaper for them to bury you, than provide fresh air to you!! And if you doubt that statement, just ask them (if you can find a live voice!)

All you need to do, Mildred, is a mass balance on the CO2. You know? Input = output + accumulation?

Off you go!

According to the ASHRAE STD. 62-81, outside air ventilation requirements usually run from 5cfm to 25cfm per occupancy person per 1000 FtSq. of floor area. This figures vary in accordance with space use or occupancy activity (utilization) within the occupied area. Since you do not mention if the fresh air content of your mix is for a new design or an existing facility there are some considerations you have to take into account before taking matters in your hands. If the fresh air inclusion is for a new design, then you establish your ventilation amount in accordance with the facilities utilization and use, by using the ASHRAE Stds. and proceed with the heat load & humidity calculations and specify the proper A/C unit. If your fresh air concern is for changing (augmenting) the fresh air content of an existing A/C system, then the problem is quite complex since you will have to make an assessment of the ability for the existing unit to cope with the added heat/humidity loads and decide if the existing unit will have to be changed or modified. If your problem deals with an existing unit and this unit was properly designed within the actual utilization activity and population number, then there is no need to add or subtract any fresh air amount. If the the population number and utilization activity has changed towards requiring fresh air addition, then you will have to re-calculate your equipment heat load bearing capacity and almost always this calls for a new larger A/C unit. Unless the utilization activity within the enclosed space (providing the occupation population is unchanged) is not a CO2 intensive activity, then there is no need to worry about the PPM CO2 mix. If the building occupancy and utilization has changed, then you will have to redesign the whole system accordingly to include the increase in fresh air requirements.

It does appear that the OP has enquired the one time, received back several dissertations, and has left the building for fresh air!

My suggestion: Shutter all the windows, and block the doors until you find out there is too much CO2, or too little if you are growing something under grow lights.

"It does appear that the OP has enquired the one time, received back several dissertations, and has left the building for fresh air!"

See that is why I almost never post! It would be nice to have the OP join the discussion, but, you never know.

Like so many things in life, it starts out be one thing, but ends up just being a thing.