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Water-Cooled VRF Not Meeting Heating Load

asked 2019-02-07 12:40:37 -0500

avril14 gravatar image

I have a water-cooled VRF system with DOAS set up in Energy Plus. The condenser loop is a ground loop. The VRF is set to autosize, but it isn't meeting the heating loads for the building. I've seen a few other posts with similar issues, where the condenser loop gets too cold and the VRF shuts off, but that doesn't seem to be happening here. It looks like there is always flow in the VRF system, just not enough. I've tried hard sizing the flow but I'm still not getting enough heating capacity and the DOAS is overcompensating. If anyone has any insights I would really appreciate it. IDF file is here. Thanks!

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answered 2019-02-22 17:58:07 -0500

updated 2019-02-22 17:59:18 -0500

This is a little confusing. You say that the condenser loop is always active. This means that the supply side (the air into the zone) is also always active. So if the VRF is not meeting the load then there must not be sufficient capacity to meet that load (ot the VRF does not know the load). It doesn't matter how much flow is in the condenser loop, that would just impact delta T on the water side, the sysytem would still be operational.

In your model you have a VRF system with a DOAS to each zone. I just checked and the VRF model does not account for the impact of other zone equipment on the operation of the VRF system (for developers: VRF uses OutputRequiredToCoolingSP instead of SequencedOutputRequiredToCoolingSP). What this means is that the VRF model does not know that the DOAS system has added additional load to the zone and therefore can't meet that load "in addition to" the zone load. This was a mistake in programming (if I have assessed this correctly). If you remove the DOAS does the VRF system operate as expected?

  Thermal Zone: L1 C - Circulation Equipment List,  !- Name
  SequentialLoad,          !- Load Distribution Scheme
  ZoneHVAC:TerminalUnit:VariableRefrigerantFlow,  !- Zone Equipment 1 Object Type
  VRF Zone Terminal 4,     !- Zone Equipment 1 Name
  1,                       !- Zone Equipment 1 Cooling Sequence
  1,                       !- Zone Equipment 1 Heating or No-Load Sequence
  ZoneHVAC:AirDistributionUnit,  !- Zone Equipment 2 Object Type
  ADU VAV No Rht,          !- Zone Equipment 2 Name
  2,                       !- Zone Equipment 2 Cooling Sequence
  2;                       !- Zone Equipment 2 Heating or No-Load Sequence
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You're right, the DOAS was the issue. I intended for the DOAS to provide all ventilation but did not set the OA in the VRF object to 0. Thanks!

avril14 gravatar imageavril14 ( 2019-02-25 15:28:59 -0500 )edit

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Asked: 2019-02-07 12:40:37 -0500

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Last updated: Feb 22