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How is simple ventilation controlled when using ideal loads in OpenStudio?

asked 2014-10-29 13:20:24 -0500

nfonner gravatar image

updated 2017-08-06 07:21:12 -0500

I usually rely on ideal loads when modeling during the earliest stages of any design for evaluating architectural configurations and envelope considerations. I recently discovered that providing a schedule in the OS:DesignSpecifcation:OutdoorAir object does not apparently impact the energy associated with ventilation for a project. This is admittedly only a quick test, but I could not verify any changes at all in the ABUPS report when swapping out schedules.

Is there another controlling factor such as population or an "Hours of Operation" schedule that controls this say during unoccupied periods? For example, Trane Trace 700 controls system operation around a threshold of 5% population.

I see that the "Minimum Outdoor Air During Occupied Hours" reads 0.011 ACH for Simple Venitlation and the "Average Outdoor Air During Occupied Hours" reads 0.695 ACH for Simple Ventilation. Clearly something is controlling the OA rate. I just do not know what that variable is.

Lastly, where can I look to see the actual simple ventilation during a given timestep? Or, should I just give up on ideal loads if I am looking for this amount of feedback from the model?

Thanks for any help.

Nic

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answered 2014-10-29 14:52:37 -0500

updated 2017-06-20 14:23:25 -0500

Your discovery is correct. The schedule in DesignSpecificationOutdoorAir does not influence the ventilation provided by ideal air loads. At least this is true when the idf file was created with OpenStudio. Instead if your design oa spec has ventilation specified per person, then the ventilation schedule will follow the occupancy schedule defined by your people instance for that space. If you define oa per floor area or in terms of total cfm, then OpenStudio will use the always on schedule for ventilation.

Now for a little implementation detail. When you turn on ideal air loads in the OpenStudio interface the OpenStudio Model just turns on a boolean flag in the ThermalZone object, and removes any existing zone hvac or attached AirLoopHVAC systems. When OpenStudio translates the model to EnergyPlus idf format, it does two things for zones with ideal air loads turned on. First it creates a ZoneVentilation:DesignFlowRate object and populates the input fields with information that you provided in the DesignSpecificationOutdoorAir input. It uses schedules according to the rules that I described in the first paragraph. The second thing it does is creates a HVACTemplate:Zone:IdealLoadsAirSystem object that is configured with no ventilation since that is handled by the ZoneVentilation:DesignFlowRate. That hvac template is later expanded by the EnergyPlus ExpandObjects tool to a ZoneHVAC:IdealLoadsAirSystem object and connected to the zone. At some point in the future we might bypass ExpandObjects and just insert the ZoneHVAC object directly. If you are feeling adventurous or curious you can follow this code in the OpenStudio source here.

(Update) I forgot to answer the last part of your question. To see timestep level data of your ventilation you will be interested in the following report variables.

Output:Variable,*,Zone Ventilation Current Density Volume Flow Rate,hourly; !- HVAC Average [m3/s] 
Output:Variable,*,Zone Ventilation Standard Density Volume Flow Rate,hourly; !- HVAC Average [m3/s]
Output:Variable,*,Zone Ventilation Current Density Volume,hourly; !- HVAC Sum [m3]
Output:Variable,*,Zone Ventilation Standard Density Volume,hourly; !- HVAC Sum [m3]
Output:Variable,*,Zone Ventilation Mass,hourly; !- HVAC Sum [kg]
Output:Variable,*,Zone Ventilation Air Change Rate,hourly; !- HVAC Average [ach]

You can make these requests by inserting these objects into the idf directly or by using an OpenStudio Measure. You probably want to change the "hourly" frequency to "detailed" or you will be subject to averaging if your schedules move within the hours. There is already a Measure on the BCL to make report variable requests so you don't need to start from scratch.

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After a bit of poking around, looks to me like there is no outdoor air ventilation being included when ideal loads is selected in OpenStudio. Looks like there is no point in OpenStudio's writing a ZoneVentilation:DesignFlowRate object because the ideal loads system won't use it. As I understand it, there must be an entry in the ideal loads system for DesignSpecificationOutdoorAir Object Name. Seems like Openstudio should put in that entry for zones with ideal loads systems.

-Erik

Erik Kolderup gravatar image Erik Kolderup  ( 2016-03-28 14:49:42 -0500 )edit
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answered 2014-10-29 16:32:09 -0500

nfonner gravatar image

Good to know Kyle. I had been using cfm/sf to hold the OA rate constant through the day instead of allowing it to vary based on occupancy. Correct me if I'm wrong any and all takers, but this would be equivalent to demand control ventilation where my OA rate matches the demand of the occupancy rate.

I should be able to just keep the occupancy rate fairly constant, but lower than the peaks of the default rate by a good deal to act as a normalized rate and then set the OA as cfm/person only.

Thanks for the output variables. I was only looking in the .rdd file and must have missed those.

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Asked: 2014-10-29 13:20:24 -0500

Seen: 1,492 times

Last updated: Jun 20 '17