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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

Air flow measurement for non densely occupied zones

Dear Sir/ Madam, I have a project that is a hotel with 200 guest rooms that are non-densely occupied. Each guest room floor (20 rooms per floor) is supplied with 3 AHUs. Therefore 100% of the OA suppled by these AHU are served in to the non-densely occupied spaces. However if I calculate based on the total OA requirement of the building there is no single guest room that takes 20% or more OA from that total OA requirement, but add up 200 guest rooms and then altogether they take more than 20% OA. Based on that I have the following questions. IEQc1 requires direct air flow measurement devices mechanical ventilation systems where 20% or more of the design supply airflow serves non-densely occupied spaces. But I'm not sure whether I should define "space" as a single guest room unit or all guest rooms together (because one AHU supplies air to multiple guest rooms). Secondly I'm also not sure whether 20% OA requirement is based on the total OA requirement of the building or the outdoor air supplied by each AHU. I appreciate clarification on this. Thank you very much.

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Profile picture for user jkmcfarland
John McFarland
Director of Operations
WorkingBuildings, LLC
LEEDuser Expert
39 thumbs up

Wed, 03/12/2014 - 11:02

Hi Magda, The air handling units each need to have direct airflow measurement. You add up all the airflow going to the non-densely occupied spaces. If that is greater than 20% of the total then the requirement applies. Don't forget that all densely occupied spaces still need CO2 sensors in order to earn the credit. Good luck, John

Wed, 03/12/2014 - 15:43

Thanx John. So according to your explanation since the total amount of OA supplied to guest rooms is more than 20% of the total OA supplied to building, I have to have direct air flow measurement. Now next question is at which point the measurement should be taken? Is it at the AHU (one measurement per AHU) or is it at each diffuser point (one measurement per guest room)? It makes more sense to measure at each diffuser point because that is the only way to ensure that in each guest room the air flow is maintained within the required limits. But this is a costly exercise. Will LEED accept if I have air flow measurement devices at AHU level?
Profile picture for user jkmcfarland
John McFarland
Director of Operations
WorkingBuildings, LLC
LEEDuser Expert
39 thumbs up

Wed, 03/12/2014 - 16:11

Hi Magda, You measure at the outdoor air intake or better downstream of the filters but before any recirculation of return air (if applicable). I say downstream of the filters because some airflow measuring devices get clogged by particulates so it's better to filter those out. I'm guessing your system is constant volume. If that's the case once each room is balanced to receive the proper amount of outdoor air, the flow rate won't vary. That's why you don't need to measure at each room. If you have a variable flow system, then we may need to talk further. All the best, John

Thu, 03/13/2014 - 03:55

Hi John. It is a VAV system. What do you think is the appropriate strategy then?

Thu, 03/13/2014 - 14:21

Magda, to clarify what John was saying; the 20% is of the total supply, air not the OA. If you have a 5000 cfm VAV system and more than 1000 cfm serves non densely occupied spaces then an air flow measuring station would suffice in the OA duct. The OA setpoint should be determined using the 62mz calculator for vav systems.

Fri, 03/14/2014 - 03:47

Thank you very much Andrew.

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