401.2 Ventilation required. Every occupied space shall be ventilated by natural means in accordance with Section 402 or by mechanical means in accordance with Section 403.
401.3 When required. Ventilation shall be provided during the periods that the room or space is occupied.
403.3 Ventilation rate. Ventilation systems shall be designed to have the capacity to supply the minimum outdoor airflow rate determined in accordance with Table 403.3 based on the occupancy of the space and the occupant load or other parameter as stated therein. The occupant load utilized for design of the ventilation system shall not be less than the number determined from the estimated maximum occupant load rate indicated in Table 403.3 . Ventilation rates for occupancies not represented in Table 403.3 shall be determined by an approved engineering analysis. The ventilation system shall be designed to supply the required rate of ventilation air continuously during the period the building is occupied, except as otherwise stated in other provisions of the code.
Exception: The occupant load is not required to be determined, based on the estimated maximum occupant load rate indicated in Table 403.3 where approved statistical data document the accuracy of an alternate anticipated occupant density.
That is the code. The exception to 403.3 is where the design professional would use judgement to determine how much outside air to bring into the space.
In practice, as you don't really want to be bringing unconditioned air into your datacenter, you can tap off a convenient building comfort system with sufficient flow to get the required OA.
... when incorporated in the design, provisions for ventilation air was functional during start-up only. O&M disabled outdoor make-up to reduce the hassle of controlling dust particles infiltrating into the data center through outdoor air.
If outdoor air is not taken, then you risk the chance of infiltration of outdoor air through door gaps, window gaps, etc., or at times of door openings. It's better to take some outdoor air purely from the point of view of positively pressurising the room and avoid inflitration of outdoor air. Ventilation requirements as per local code also needs to be taken care.
I agree with all. O.A. is code required plus a pressurized building is a good thing.
The most recent project I had utilized downflow CRAC units with plenum return above the ceiling. I calculated the required O.A. and used a roof mounted filtered supply propeller fan dumping the O.A. directly into the return plenum. The required O.A. was minuscule compared to the total of the CRAC supply air. O.A.Fan operates 24/7.
I am in the electrical field, but we have many Mech guys here I've worked with for quite a few years. Seems their consensus is that even though it is "mostly unoccupied", if the space is occupied on a regular basis, such as a data center with a systems supervisor or operations tech, then it must be treated as occupied. Wether he is there one hour or eight hours a day. If in fact the space is strictly a mainframe room with the operators outside the space-and not requiring regular maintenance, then we have used self contained units such as the Liebert systems with outdoor condensers supplying the space via the raised floor plenum, with zero outside air. This may be affected by local IAQ or Energy Code requirements. As always it would be beneficial to check with your AHJ.
Just buttin' in while I was surfing through. Good day.
The purpose of outside air is not only for IAQ, but also for pressurisation. Even if the space is wholly unoccupied, you need some outside air to maintain positive pressurisation.
Ventilation also deals with off-gassing and enviormental factors beyond human occupancy. VOC's bake off of floor and wall finishes and code minimum ventilation is designed to deal with them.
A typical number for an unoccupied space would be 0.06cfm/sqft.
As others have mentioned, positive pressurization is a good idea. Helps with infiltration, VOCs and generally helps preventing dust/dirt accumulating in the space, which is bad for datacenter equipment.
A data center needs closed control precision type of air conditioning units, which concentrates more on humidity than temp. A data center should never be provided with untreated outside air. There should be outside air but treated. It can be generally 4 l/s/m2, but check the local codes and regulations.
Myself, I would bring in fresh but conditioned air, to keep the data centre under a slight positive pressure, to try and keep out dust from the rest of the building.
Perhaps even the current 2007 62.1 'area rates' would be practical for a lightly occupied space and pressurization purposes. You could always add a telaire CO2 sensor to step up the fresh air perhaps if it got occupied to the point that CO2 levels rose.
Take the "V" out of HVAC and you are left with a HAC(k) job.
