So like many on this thread, i have a LEED CI project that is 1/3 of 1 floor and is served by the base building AHU that is outside the scope and LEED boundary of the project. The base building AHU is a single massive unit that serves the entire building (15 floors) and does not have any airflow measurement device on the outside air intake. At this scale, installing an OA measurement device for the non-dense space criteria is not financially feasible for our small CI project. I would like to propose an alternative and see if y'all have any feedback.

We know that the design outside airflow for the base building AHU is 15%; this is primarily based on the interviews with the facility operator since there are no as-built drawings. We based our design and subsequent ASHRAE 62 calculations on this premise. Essentially, as long as the outside air ratio is at least 15% for the supply air, our project's indoor air quality is sufficient.

So what i propose is the use of 3 CO2 sensors to measure the %OA for the supply air. If it is below the 15% value then an alarm kicks in for the occupants of the space. The CO2 sensors would need to be installed such that one is located in the collective return, one is located anywhere outside or at the OA intake, and one is located in any supply air location. The %OA would then be calculated similar to how we were all taught to determine mixed air temperatures.

I believe this follows the intent of the credit that we are measuring actual conditions to identify if we are meeting the design assumptions used within the ASHRAE 62 calculation that shows we have sufficient ventilation.

So here are the questions to you all: Does this make sense? Has anyone had success with this strategy in the past? Am i missing a simpler way to achieve the same thing (i do have a tendency to complicate things)?

Thanks for you time...