Cooling load oscillations during unoccupied hours

Hi,

I was wondering if anyone could assist me with the following question in EnergyPlus. In particular, I modeled a single office room (by modifying IECC_OfficeSmall_STD2018_Denver prototype). Dimensions are 20-15-10 (ft). As far as the HVAC system, an AirToAir Heat Pump is modled to provide cooling and heating. The IDF file could be found here.

However, the issue is that the model cooling loads for early hours of the day (in summer season) is oscillating. In specific, and for instance for June 27, cooling loads falctuates betwwn midnight and 7 am between zero and 1952 W, as showed below(also could be found here).

image description

By taking a more detailed analysis, it seems for those timesteps, MAT exceeds very slightly that the cooling setpoint temperature and this is why cooling loads are set to zero and heat pump is forced to shut down: image description

It might worth mentioning that Curve:Biquadratic is applied towards Coil:Cooling:DX:SingleSpeed curve.

Any help would be greatly appreciated.

Thanks, Mohammad

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Mohammad

asked 2022-02-20 12:50:06 -0500

4.4k

__AmirRoth__

updated 2022-02-23 14:10:43 -0500

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You have a night cycle availability manager that will cause this type of oscillation every 30 minutes. You can try removing that manager to see if it really is the cause.

AvailabilityManager:NightCycle,
  PSZ-AC:1 Availability Manager,  !- Name
  Always_On,               !- Applicability Schedule Name
  HVACOperationSchd,       !- Fan Schedule Name
  CycleOnAny,              !- Control Type
  1.0,                     !- Thermostat Tolerance {deltaC}
  FixedRunTime,            !- Cycling Run Time Control Type
  1800;                    !- Cycling Run Time {s}

13.8k

rraustad

answered 2022-02-20 22:11:38 -0500, updated 2022-02-20 22:14:52 -0500

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@rraustad I truly appreciate your quick response. I changed the Always_On schedule to Always_Off and it really helped by making cooling loads to zero between hours midnight to 5 am. Thank you so much!

Mohammad (2022-02-20 22:27:41 -0500) edit

@rraustad I was wondering if you could please help me with another related question. Actually, I have to keep the AvailabilityManager:NightCycle active, as I'm testing my model against pre-cooling. This is why I selected Thermostat for Cycling Run Time Control Type. However, I'm not sure why my DX Cooling rate is non-zero for the early hours of the day (between mid-night- to 6 am), while ALL internal gains are set to absolute zero (for Golden, CO climate zone). However, for after 6 pm, all the cooling loads drop to zero. Any explanation of the system performance would be greatly appreciated.

Mohammad (2022-03-16 00:49:36 -0500) edit

Please find the IDF file here: https://drive.google.com/file/d/1ADl6... Also, I have saved the output filke for July 16, here: https://docs.google.com/spreadsheets/... Many thanks!

Mohammad (2022-03-16 00:51:47 -0500) edit

There are report variables you can use to see the predicted zone load that will help identify what is happening.

Output:Variable,*,Zone Predicted Sensible Load to Setpoint Heat Transfer Rate, timestep;
Output:Variable,*,Zone Predicted Sensible Load to Heating Setpoint Heat Transfer Rate, timestep;
Output:Variable,*,Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate, timestep;