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Core zones: design day results and system oversizing

asked 2017-03-15 18:15:45 -0500

kpp gravatar image

updated 2017-03-27 18:03:35 -0500

I am running the PNNL prototype models for medium size office buildings. For detailed simulation results, I exported hourly outputs/mass flow rates at each node.In the csv file the airflow rates for the design day are higher than the simulation day results. I understand there can be differences due to the difference in weather data on design day and any other simulation day. When I compare the maximum airflow rates for design days and rest 365 simulation days for each zone the max airflow for core zones are as much as 80% higher than max airflow for the simulation days. For example, for one of the core zones the max airflow for design days is 2.8 m3/s (~6000 cfm) and max airflow for the rest of the year is 1.56 m3/s (~3300 cfm).

As I understand the design days are used to calculate the autosize variables. If the design days are predicting higher mass flow rates the equipment size will also be higher for rest of the year's simulations.

Due to this the system is getting oversized and the 30% min airflow for the VAV at that zone is almost 55% of the max airflow (yearly max). This will lead to calculating excessive amount of reheat energy.

For perimeter zones, the max design day airflow are 0.9 to 1.1 times of the max yearly airflow. However, for core zones, the design day max airflows are between 40 to 80% higher than max airflow throughout the year for San Francisco file. Are these expected results for core zones? Is there any variable that might cause this issue? Is this due to the thermal inertia?

EnergyPlus version: 8.6

Update: In bottom core zone the oversizing is higher than the top core zone. One of the reason can be, the way the ground heat transfer is calculated during sizing days compared to rest of the year. However, I am still not able to explain the overall oversizing in core zones (w.r.t. perimeter zones).

Following plots are hourly airflow rates for the bottom and top core zones.

image descriptionimage description

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Have you determined whether the airflow sizing is being driven by ventilation or heating/cooling? Calculate the min OA requirements on the design day and compare to the VAV box sizing. Also, might want to read section Addendum 90.1-07h: VAV Reheat Control of this doc

aparker gravatar imageaparker ( 2017-03-27 18:17:57 -0500 )edit

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answered 2017-03-28 07:47:56 -0500

You can force the simulation to average sizing results by using the Sizing:Parameters object. In the PNNL medium office model the averaging window is 2 hours. At times when the Tstat temperature is setup, the zone airflow will spike to meet the increased load. Since real systems cannot meet this load immediately, you do not want the simulated system to be sized to meet that load. A workaround to this method is to use constant temperature setpoints during the design days so zone airflow calculations only include the loads and not the change in Tstat set point temperature. If you would like to test this theory, change the Timesteps in Averaging Window to 1 and reevaluate the component sizes.


 1.0,                     !- Heating Sizing Factor
 1.0,                     !- Cooling Sizing Factor
 8;                       !- Timesteps in Averaging Window

!    HTGSETP_SCH_PACU_VAV_bot,             
 Temperature,             !- Schedule Type Limits Name
 Through: 12/31,          !- Field 1
 For: WinterDesignDay,    !- Field 2
 Until: 05:00,15.6,       !- Field 3
 Until: 06:00,17.6,       !- Field 5
 Until: 07:00,19.6,       !- Field 7
 Until: 22:00,21.0,       !- Field 9
 Until: 24:00,15.6,       !- Field 11
 For SummerDesignDay,     !- Field 24
 Until: 24:00,15.6,       !- Field 25

!    CLGSETP_SCH_NO_SETBACK,             
  Temperature,             !- Schedule Type Limits Name
  Through: 12/31,          !- Field 1
  For: SummerDesignDay,    !- Field 2
  Until: 05:00,26.7,       !- Field 3
  Until: 06:00,25.7,       !- Field 5
  Until: 07:00,25.0,       !- Field 7
  Until: 22:00,24.0,       !- Field 9
  Until: 24:00,26.7,       !- Field 11
  For WinterDesignDay,     !- Field 35
  Until: 24:00,24.0,       !- Field 36
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Asked: 2017-03-15 18:15:45 -0500

Seen: 90 times

Last updated: Mar 28