CFM Quantity for a 24' x 16' Office Area

[mpeck1982]

Greetings!

Does anyone know how to calculate a CFM quantity for a 24' x 16' office area? There isn't any servers or extra heat load in the area, it is just people. Currently I have 1200-CFM serving this room and the users are still complaining about heat/cool airflow issues. Any guidance?

 

[willard3]

This is not a simple rule of thumb problem. If you arent familiar with heat gain/loss, find somebody who is.

 

[mpeck1982]

There isn't any exterior heat loads from glass. The room doesn't have any glass on the perimeter. Isn't there a standard CFM quantity per square foot for people office use?

 

[317069]

- what is your cooling/heating load?

 

[CarolinaPE]

That is over 3 cfm/sf which is pretty high if it is a standard office area as you have described. Are you certain they are getting the 1200 CFM? You will need to do a load calc as others have suggested.

 

[IRstuff]

And what temperature is your 1200 cfm? 1200 cfm of hot air isn't going make anyone comfortable.

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[mpeck1982]

discharge from the air handler is 55 deg F.

 

[MintJulep]

How many people are in this room?

What exactly is the nature of their complaint?

Back in the day the rule of thumb was 15 cfm fresh air per person. If we guess that you have 20% fresh air in your 1200 cfm then that's enough for 16 people.

16 people in a 24 x 16 foot space is pretty dense.

 

[ChasBean1]

Yes. 1 cfm/ft2 for office space, but that is old school – you can probably get by with less. Esp. for air conditioning: too much capacity or cooling airflow means cyclical control and overcooling and excess airflow. There are intermittent periods of nipple-hardening cooling followed by off cycles of humid air intake.

You have over 3 cfm/ft2. Cut it down to 0.7 cfm/ft2. You might need to change your diffusers…

 

[ProcessHVAC]

I think you might get a not so bad answer if you use rules of thumb, but chances are you might not get the best answer just based on how you have asked the question. Hire an engineer if you want the best answer to your client. If you can't answer the what are your loads question, you are taking a big risk by doing rules of thumb.

 

[chicopee]

To add to the above responders, where in relation to the office floor are the supply air and return air grills; and where in relation to the supply air grill is the return air grill? 16 people in a 24'X 16' space sounds like a sweat shop and not an office.

 

[BronYrAur]

Good point, Chicopee. A sweatshop that probably contains 16 computers and monitors as well. The "rules of thumb" don't sound like they apply here. If it's truly an interior space with no walls or roof, surrounded by conditioned spaces, then you load is primarily people, lighting, and computers (and other equipment).

If you threw in 2.5kW per sqft for lighting, 500 BTUH per person, and 500 BTUH for each computer, you are still well below 3 CFM/sqft. The ventilation load is covered in the AHU. Bottom line, I don't think you really have 1,200 cfm of 55 deg air entering the space. You are either not getting that much air, or you have a reheat valve stuck open or something.

Now that I think about it, I have seen offices full of stock traders that are crammed together in small spaces. Each one has anywhere from 6 to 10 computer monitors on his desk. Is there some big heat load like that?

 

[urgross]

How many people in the room are going through change of life? My experience in that area is it will never be warm or cool enough.

 

[ChasBean1]

2 W/ft2 is old school; 2.5 KW/ft2 is unheard of (wear sunglasses with that light level), and about 1 - 1.5 W/ft2 is current. But you really do have about 3 times the cooling capacity that you need. Grossly oversized DX systems cause problems big time...

 

[RossABQ]

Other questions:
-- why do you think there is 1200 cfm going to the room; actual measurement, or numbers on plans?
-- constant or variable flow? if variable, where is the thermostat for the VAV?
-- have you personally visited the room to see what conditions are?