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# Creating a ventilation system with specified overall ach and outdoor ach

I have been trying to make an air loop in OpenStudio with several zones and a standard of 10 air changes, where the recirculation rate is max 50%.

So basically I would like a minimum of 5 fresh air changes and a total supply air flow of 10 air changes. I can not figure out how to do this, I have tried setting the outdoor air flow air changes per hour to 5 and the design supply air flow rate to the double of that amount, but almost no air is recirculated when I run the simulation.

Does anybody have recommendations?

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Here are some things to explore...

• For the 5 OA (outdoor air) ACH, set your DesignSpecification:OutdoorAir object to 5 ACH. Make sure your ceiling height is properly defined in your ThermalZone. Set 'Control For Outdoor Air' to 'YES' in your AirTerminal:SingleDuct:VAV:Reheat.
• For the 10 Total ACH, in your AirTerminal:SingleDuct:VAV:Reheat set your 'Zone Minimum Air Flow Input Method' to 'Fixed Flow Rate' and then convert your 10 ACH to CFM and enter in 'Fixed Minimum Air Flow Rate' field. Additionally, in the Sizing:Zone object, set the 'Cooling Design Air Flow Method' to 'DesignDayWithLimit' and then specify the 10 ACH (converted to CFM) in the 'Cooling Minimum Air Flow' field.
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Thank you, works like a charm! Could I also put 10 ACH as the maximum flow rate and then make the fresh air ACH vary between 50% and 100% of the maximum flow rate?

( 2017-02-06 05:22:24 -0500 )edit

Yes, you could take that approach -- set the 10 ACH max flow rate to the zone and then set the minimum fraction of OA at the air handling unit to 50%. Be mindful of other zones on the system if you are setting a minimum OA fraction at the AHU.

( 2017-02-06 13:02:44 -0500 )edit