Outside Airflow Monitoring Device

Not for VAV systems. Due to the changing airflow and pressure under normal operation it wouldn't be sufficient to install a current switch. Monitoring the fan could inform the owner about total airflow, but with the dynamic flow and pressure interactions in a VAV system it wouldn't ensure a particular OA flow at the intake, that's why an airflow monitor is specifically required.

For constant volume systems the approach for a current sensor is allowed when the system will be balanced and verified to supply the proper OA, then verifying the system is operational would be enough assuming no changes to the system. If the owner was concerned about the OA quantity they may wish to re-assess the OA flow again after several years to ensure the system continues to supply the desired OA quantity.

https://leeduser.buildinggreen.com/credit/NC-v4/EQp1#tab-credit-language

Mechanically ventilated spaces

For mechanically ventilated spaces (and for mixed-mode systems when the mechanical ventilation is activated), monitor outdoor air intake flow as follows:

• For variable air volume systems, provide a direct outdoor airflow measurement device capable of measuring the minimum outdoor air intake flow. This device must measure the minimum outdoor air intake flow with an accuracy of +/–10% of the design minimum outdoor airflow rate, as defined by the ventilation requirements above. An alarm must indicate when the outdoor airflow value varies by 15% or more from the outdoor airflow setpoint.
• For constant-volume systems, balance outdoor airflow to the design minimum outdoor airflow rate defined by ASHRAE Standard 62.1–2010 (with errata), or higher. Install a current transducer on the supply fan, an airflow switch, or similar monitoring device.

Hi Jennifer,

My understanding is that current transducers are acceptable if they are installed on constant volume systems.  The trick is that what the ME may call constant volume may not be as far as this prerequisite is concerned.  We were told by a reviewer that to be constant volume the outdoor airflow must be maintained at a constant level and at minimum outdoor air rates (with the exception of economizers).

Hi,

we are in the case of a 100%OA AHU with no mixing section therefore the airflow throught the fan is always 100% OA. The AHU is serving office spaces, which are about 80% of the floor area, and meeting rooms, which are about 20% of the floor area.

The AHU is sized according to Ashrae 62.1 AO value, based on sqm and max occupancy for the office and meeting room spaces, however in the meeting room the OA can change based on people absence (in this case the Ashrae 62.1 value based on room sqm will be provided) or presence (in this case the Ashrae 62.1 value based on room sqm + max occupancy will be provided).

Considering the above we believe that in terms of EQp1 requirement we can consider this case as a constant volume system and therefore a direct outdoor airflow measurement is not required, what do you think?

Hello! Were you able to submit the prerequisite with 100% outside air and without providing a direct outdoor airflow measurement device? We are working on a project with a similar situation in several buildings and wondering if this might be accepted. 

Thank you!