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We have a VAV system where single AHUs serve multiple zones. Do we need to take outside air measurements at each VAV box?

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EBOM-2009 IEQp1:Minimum IAQ Performance

VAV minimum for Ds

Hello All: Is there any specific requirement from LEED for what the VAV minimum should be during cooling or heating? I have adjusted my minimum to above the typical 25% or 30% in various zones to be able to meet the ventilation during heating mode. Does anyone see anything wrong with this approach, trying to comply with ASHRAE 62.1. The boxes in question are cooling only and i am not reheating. Appreciate all input and advice.

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Ben Stanley
Senior Sustainability Manager
WSP - Built Ecology
LEEDuser Expert
188 thumbs up

Fri, 08/23/2013 - 14:56

Hi George, It sounds like your referring to the Ds value in the ASHRAE calculations? That value should be based on the actual setpoint for the VAV at each potentially critical zone during the worst case condition (i.e. the lowest supply air during normal operations). LEED and ASHRAE don't prescribe a minimum value. As to your approach, it's not clear to me if you're adjusting the input to the calculation to meet the prerequisite or adjusting the actual set point of the VAV boxes to meet the prerequisite? Can you explain the scenario a bit more?

Fri, 08/23/2013 - 16:06

I am adjusting the actual minimum set point of the VAV boxes. I am not revising anything on the calculation. On another point about CO2 sensors. We are not doing DCV, but the design calls for CO2 sensor in the return duct. I understand this is not acceptable for the critical zone, but it is used to set a maximum airflow. The absolute minimum during occupied mode is what the 62MZCalc gives, and in the case that the CO2 in the return goes above 1100 ppm i am setting it to a max value. Is this an acceptable approach or should it matter to LEED? I will always provide the calculated value during occupied mode. I am asking this because for reasons that i do not know the design has CO2 sensors in the return duct only, so we have a comment from LEED to explain how the outdoor air is controlled. I guess the reviewer wants to make sure that we never go below what the spreadsheet calculates, since we do not have space CO2, correct? Thanks for the response.
Profile picture for user bstanley
Ben Stanley
Senior Sustainability Manager
WSP - Built Ecology
LEEDuser Expert
188 thumbs up

Fri, 08/23/2013 - 16:42

Yes, I think that the reviewer wants to know that the actual outside air provided always meets at least the ASHRAE required airflow regardless of what the CO2 sensor is reading. Your explanation/approach above seems good to convey that idea.

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