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DOAS modeling with varying/scheduled OA cfm

asked 2019-03-01 10:11:54 -0500

manatee gravatar image

updated 2019-03-02 12:51:41 -0500

Currently I am modeling a high-rise residential building and specifically have issues on modeling DOAS.

  • MAU supplying OA at 72F to residential units: Design supply air flow rate (Max) and hourly actual cfm needed are known (smaller than max).

Firstly, I've tried to mimic the DOAS system using AHU (vav) with min/max fraction of OA schedules.

  • OA flow rate is working as intended (checked system OA flow rates), but realized this is not anymore 100% OA, but a fraction of it.

So here I am wondering if there is any way to model this AHU (actually DOAS) with varying OA (=SA) CFM following the schedule value I made based on a real-life OA flow rate calculated each hour.

In summary, wondering the way to model DOAS system in EnergyPlus,

1) 100% Outdoor Air

2) conditioned to 72F using H/C coils

3) Max OA (=SA) design air flow rate known

4) different/varying hourly CFM during the day known

5) so hourly OA (=SA) flow rates following the schedule, not the count of people or CO2 or else.

Thank you so much for any kind of helps/insights/comments in advance!

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answered 2019-03-05 16:52:03 -0500

The current best practice for this is:

  1. Use AirTerminal:SingleDuct:ConstantVolume:NoReheat which can reference a DesignSpecification:OutdoorAir object to set the flow rate. See example file ZoneWSHP_wDOAS.idf (which still uses the obsolete AirTerminal:SingleDuct:Uncontrolled. Just substitute AirTerminal:SingleDuct:ConstantVolume:NoReheat for that.)

  2. Be sure to use a full DesignSpecification:OutdoorAir object which includes a schedule field which can be used to control the varying flow. The OA flow fields will be used to calculate the flow rate and the result is multiplied by the current schedule value. Also note that the full terminal unit object has a field for "Per Person Ventilation Rate Mode" (CurrentOccupancy or DesignOccupancy).

  3. For the DOAS air loop, define a half loop with no return path and no outdoor air system. The "Supply Side Inlet Node" is an outdoor air node, and the "Demand Side Outlet Node Name" is left blank. See example file DOASDXCOIL_wADPBFMethod_NoReturnPath.idf.

In this case, since you already have an airloop defined, you can just use the Controller:OutdoorAir fields for Minimum Fraction of Outdoor Air Schedule Name (and Maximum . . .) to force the OA fraction to always be 1.0 and the OA flow will track the supply flow requested by the terminal unit(s).

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Are you saying to apply Controller:OutdoorAir to the DOAS half loop AirLoopHVAC? How can you use Controller:OutdoorAir without an AirLoopHVAC:OutdoorAirSystem? What node would you list for the return node on the Controller:OutdoorAir object if it's 100% OA?

ericmartinpe gravatar imageericmartinpe ( 2019-03-24 01:43:59 -0500 )edit

No. If you configure the DOAS as a half-loop with no outdoor air system, then you do not need a Controller:OutdoorAir object for that loop. The OA flow will equal the DOAS supply flow which will be governed by the terminal unit flow request(s).

MJWitte gravatar imageMJWitte ( 2019-04-19 10:45:49 -0500 )edit

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Asked: 2019-03-01 10:11:54 -0500

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Last updated: Mar 05