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DesignSpecification:OutdoorAir for fixed ventilation rates

asked 2015-05-13 15:51:41 -0600

updated 2017-05-03 19:15:04 -0600

The DesignSpecification:OutdoorAir object in EnergyPlus applies the occupancy schedule fractions to the design Outdoor Air Flow per Person (cfm/person), so the required hourly minimum outdoor air rate during the simulation is less than the design specification. This turns the DesignSpecification:OutdoorAir into a variation of a demand control ventilation. How can one specify the Outdoor Air Flow per Person inputs in the DesignSpecification:OutdoorAir while maintaining fixed outdoor air flow rates during occupied periods of the hourly simulation?

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answered 2015-05-14 14:16:47 -0600

updated 2015-05-14 14:18:09 -0600

Depending on your HVAC system you may need to reference the DesignSpecification:OutdoorAir object elsewhere (in addition to the Sizing:Zone object and the Controller:MechanicalVentilation object). For the AirTerminal:SingleDuct:VAV:Reheat object you need to select the corresponding OutdoorAir object under 'Design Specification Outdoor Air Object Name' otherwise the terminal unit will not be controlled for outdoor air flow. If this input is left blank, minimum OA may not be provided to the zone at every hour and the minimum OA volume may vary.

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This is what was happening in my model. I'll have to make sure to carefully check other air terminal units.

jmcneill gravatar image jmcneill  ( 2015-05-15 11:10:37 -0600 )edit

answered 2015-05-13 23:55:45 -0600

When you specify OA flow/person in the DesignSpecification:OutdoorAir object in EnergyPlus, during the sizing run, the min OA flow rate for the system is determined at each hour during of the design days like this: (people * people fraction * oa/person) + (area * oa/area) + (oa fixed rate). The min OA flow for the system is then set to the highest value that occurs during the sizing days. During hourly simulation, if DCV is not enabled, the min OA flow will be set to the value from the design day no matter how many people are in the zone. If DCV is enabled, the min OA will be calculated based on the number of people, and the system will ramp down OA accordingly.

In OS, you turn DCV on/off using the toggle on the AirLoop > Controls tab. By default, it is turned off. image description

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@aparker the comment below from @LyleK was what was occuring. OpenStudio was not setting the Design Specification Outdoor Air Object Name in the AirTerminal:SingleDuct:VAV:Reheat. I'm using the API and didn't find a method to set this input. Am I missing a method somewhere else?

jmcneill gravatar image jmcneill  ( 2015-05-15 11:09:36 -0600 )edit

@aparker, assuming the system is designed to go all the way down to 0 OA, should the "constant minimum airflow fraction" and/or the "fixed minimum air flow rate" for the air terminal object(s) be set to zero?

willyJohan gravatar image willyJohan  ( 2016-10-19 19:30:57 -0600 )edit

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Asked: 2015-05-13 15:51:41 -0600

Seen: 765 times

Last updated: May 14 '15