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If my project is using air handlers that supply 100% outside air at a constant volume, do I still need to install an airflow monitoring device?

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NC-2009 IEQc1:Outdoor Air Delivery Monitoring

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Omer Moltay
Co-founder
Mimta EcoYapi
LEEDuser Basic Member
133 thumbs up

Although there are spaces in our project that require the installation of flowmeters for this credit, our mechanical engineer is arguing that these are unnecessary for the following reasons: - Spaces are served by 100% outside air constant flow systems, flowrates of which will be fixed during balancing, - The only reason why the flowrates might deviate are because of system pressure changes, triggered by either clogged filters, broken dampers or errors with the air handling system, all of which are monitored through sensors and are reported immediately in the building automation system. - In addition, there are air pollution sensors in the AHUs, again triggering alarms in case pollutants in the outside air will reach certain limits. From an engineering point of view, all of these seem sufficient to guarantee the conditions which LEED is trying to achieve by monitoring airflow. Would this be a plausible argument to earn this credit without installing flowmeters? Have there been any instances achieving this credit with these measures?

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Tue, 12/13/2011 - 14:06

So whether you have 5 people in the room or 500 people in the room the air is still the same constant volume, say 10 air changes per hour (and I made that up)? Wouldn't you need more fresh air for 500 people than the 5? How would the system be able to respond to the change in occupancy if it is fixed? These are the questions you'll want to answer if you don't install the flow meters. Reread the credit intent and ask if the engineer's solution is meeting that intent.

Tue, 05/22/2012 - 16:51

In a similar vein a project team engineer has proposed the following for the non-densely occupied areas. Since the systems for non-densely occupied areas are provided with fresh air with a constant flow the following system is suggested to ensure sufficient quantities of outdoor air and to alert occupants as necessary. 'The fresh air intake dampner should be provided with a venturi meter, or similiar, so that flow can be measured and the dampner set to a suitable position to achieve the required air flow. A fitted alarm will sound if the AHU operates with the FA dampner below this opening setting to alert building occupants. Since an alarm will sound with a change in airflow values and that value can be set sufficient above the 10% change requirement would this be sufficient? Thanks in advance for your help. I look forward to your comments on what has become a much discussed issue within the project.
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Dylan Connelly
Mechanical Engineer
Integral Group
LEEDuser Expert
342 thumbs up

Wed, 05/23/2012 - 01:17

Omer, Sorry for the delay in answering the first question. There is a LEED CIR (ID#2099 and the LEED 2012 version will plainly state this in the requirements) that states if you are using systems that are constant volume with a constant amount of OA, then you can use a CT (current transducer) to ensure the unit is on and operating correctly. Susan, To meet code you design the OA for the normal maximum occupants. You are not required to design for or increase the OA in the event a space is packed in with more people then usual. For LEED, you are required only to have an alarm, not provide more air. This does hopefully motivate the facility manager to go investigate what is causing the problem. Francis, The statement seems adequate. The flow meter and alarm will be sufficient. Resetting the airflow values when the system alarms is optional.

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