A building provides outside air for ventilation based on CO2 concentrations in the return air. When CO2 concentrations exceed ambient + 590 ppm, the OA damper is opened to allow more OA to mix with return and reduce CO2 levels in the space. Fans speeds are also increased for OA AHU, floor AHU and return/relief. This essentially allows the building to operate with a lower amount of OA than the ASHRAE 62.1-2007 calculation requires. This “lower minimum” would essentially be the OA rate required based on the area OA rate (i.e., 0.06 cfm/sf) and distribution effectiveness, the people OA rate would be zero. In operation, as the building increases in occupancy, the CO2 concentrations increase, and OA rates are then increased to meet “demand” as measured by CO2 concentration. OA rate is increased until the CO2 concentration reaches acceptable levels (ambient + 590 ppm). As CO2 concentration drops, the OA damper is closed back down and fan speeds reduced to conserve energy.
The reference guide (O&M) even states that costs can be mitigated with CO2 monitors and demand control ventilation. However, there is no “Case 3” that lays out the requirements for how to operate and document the system as such.
Has anyone been able to use DCV to achieve this Prereq?
I'm thinking about writing up a CIR and submitting…
ASIDE: Wouldn’t it be nice if there was a place we could share “Project CIRs”? I wonder if LEEDuser would consider adding a feature for that…share the knowledge!
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Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11477 thumbs up
June 20, 2010 - 11:34 pm
Ralph, feel free to use this forum to share "Project CIRs"! (And encourage others.) If it starts to become popular we might be able to set something up.I am looking for USGBC to soon announce a key change to its CIR process, FYI.
Roger Chang
Principal, Energy and Engineering LeaderDLR Group | Westlake Reed Leskosky
LEEDuser Expert
398 thumbs up
June 22, 2010 - 11:09 am
It isn't completely clear what you're trying to achieve. All designs need to be ASHRAE 62.1-2007 compliant. The breathing zone airflow rates have been carefully developed through research and application and should be used to set your design airflow rates. Demand controlled ventilation can be used to reduce the amount of ventilation air brought in, for energy savings, but I'm not sure how that would help you meet the prerequisite. Also, CO2 monitoring should occur in the breathing zone, not in the return air ductwork, to be effective.
Scott Armstrong
Manager, Building Science & SustainabilityMMM Group Limited
5 thumbs up
November 1, 2010 - 11:26 am
If I understand the question correctly, it sounds like you're having trouble demonstrating through testing that the ASHRAE 62 minimum OA is being delivered by the demand-control ventilation (DCV) system. I had a similar question come up with a client and haven't resolved the matter.
To illustrate, the DCV approach introduces OA to maintain CO2 at a prescribed level. Therefore, the OA is not based on zone-based occupancy numbers but, rather, on CO2 feedback (in the breathing zone). Thus, any testing done to demonstrate OA deliver will yeild a lower number since the space likely is not fully occupied (and less OA is being delivered).
While troublesome, would it be possible to correlate actual occupancy at the time of testing (showing adequate OA is being delivered) and demonstrate that the AHU has the capacity to deliver the required OA at full occupancy (through calculations)? A possible augmentation to this would be tracking of CO2 levels in the breathing zone.
AshLee Eustace
Sales and Marketing Executive AssistantSustainable Solutions Corporation
46 thumbs up
April 26, 2012 - 9:19 am
Has anyone documented this prerequisite with demand control ventilation? I'm going through this with a project right now, and it's a concern because the minimum OA setpoint is below the ASHRAE 62.1 requirement. If we take our measurements at the current minimum, it will surely not meet the prerequisite. However, if we increase the minimum to the amount of OA required to meet 62.1, then the DCV is not saving us any energy. Does anyone have any insight on this?
Thanks.