# Thread: Calculating the required CFM

1. New Guest
Join Date
Mar 2012
Posts
4
Post Likes

## Calculating the required CFM

Could some one please tell me how to calculate the required CFM for a room.

Is it Calculated like - Required CFM = (Room Volume in cubic feet)
divided by
(Minutes per air change in the room)

2. It is not as simple as you suggest.
It is a combination of factors. Among them are exterior wall dimensions, ceiling dimensions, insulation of those surfaces, size of windows and skylights, compass heading of the windows, # of people in the room and the typical activities they will be doing (exercising, sleeping, cooking), heat generating appliances in the room, lights (incandescent, florescent, other), floor size and insulation, forced ventilation, etc.

It is not something which can be learned in a couple of minutes.

3. New Guest
Join Date
Mar 2012
Posts
4
Post Likes
Thank you very much for the reply kdean. I'm new to the field of HVAC and my purpose of calculating the CFM is to define it as a boundary condition to my CFD simulation.

My CFD domain is a simple room with three people whose breathing ( Inhalation of O2 and exhalation of CO2 ) is modeled for the simulation. Now the objective of the simulation is to see the effectiveness of ventilation to the room in reducing the concentration of CO2.

4. Originally Posted by saisanthoshm88
Thank you very much for the reply kdean. I'm new to the field of HVAC and my purpose of calculating the CFM is to define it as a boundary condition to my CFD simulation.

My CFD domain is a simple room with three people whose breathing ( Inhalation of O2 and exhalation of CO2 ) is modeled for the simulation. Now the objective of the simulation is to see the effectiveness of ventilation to the room in reducing the concentration of CO2.

just saying...for a simulation use only and no other real life situation use, 1 cfm per sqft.

5. New Guest
Join Date
Mar 2012
Posts
4
Post Likes
Thanks for the suggestion dlove but the simulation should of course adher to a real life context so could you please suggest if some guide line is availble to calculate the required ventilation rate in CFM to a room.

Best Regards,
Santhosh

6. The National Fuel Gas Codes requires .35 ACH(air changes per hour) for homes today. That equates to approximately 47cfm per 1000 sq.ft. This does not have anything to do with the amount of cfm for heating and cooling loads. This is the amount of air introduced to the duct systems from outside.

7. Regular Guest
Join Date
Feb 2012
Location
Upstate SC
Posts
84
Post Likes
For commercial space, 1.5 CFM planned infiltration per occupant is the guideline here. Not sure if that is ASHRAE driven or not.

8. By your question it seems you are looking for not a CFM for heating and cooling load, but only a rate for ventilation. Is that correct?

9. Perhaps this site will help.
http://www.windmaster.co.za/vbcalculations2.htm
Though it is for turbine ventilators it still has calculators for ACH.

10. New Guest
Join Date
Mar 2012
Posts
4
Post Likes
Kdean and dlove thanks for your suggestions.

Kdean as you mentioned it Im looking for a ventilation rate and the main focus in the simulation is to provide the room with such ventilation rate that can reduce the CO2 concentration in the room.

The floor area of the room is 24*16 = 384 sq.ft, it's a small office room accomodating 3 people. The ASHRAE guideline says that the acceptable CO2 concentration in a room can be 1000 ppm. I have right now a CO2 concentration of 2564 ppm.

My objective is to bring this down by providing adequate ventilation to the room.

The room has no natural ventilation and it should absolutely relay on mechanical ventilation. Since it is an office room, Im considering 8 air exchanges per hr.

Could you please comment on this

Best Regards,
Santhosh

11. Regular Guest
Join Date
Jan 2011
Posts
6
Post Likes
[QUOTE=saisanthoshm88;12837721]Kdean and dlove thanks for your suggestions.

Kdean as you mentioned it Im looking for a ventilation rate and the main focus in the simulation is to provide the room with such ventilation rate that can reduce the CO2 concentration in the room.

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.

12. Regular Guest
Join Date
Nov 2007
Location
Central Ga.
Posts
48
Post Likes
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.

Page 1 of 2 12 Last

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

## Related Forums

The place where Electrical professionals meet.