Water-Cooled VRF Not Meeting Heating Load

asked 2019-02-07 12:40:37 -0600

avril14
235 ●1 ●6

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 -0600

rraustad

13804 ●79 ●12 http://www.fsec.ucf.edu/

updated 2019-05-03 11:29:57 -0600

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 (or 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 system 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?

ZoneHVAC:EquipmentList,
  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 ( 2019-02-25 15:28:59 -0600 )edit

I removed the DOAS system and the cooling and heating energy jumped way up. Which makes it seem like the VRF does take into account the heating/cooling provided by the DOAS, but maybe something else is going on? I'd be happy to send you my model if you'd like to take look.

avril14 ( 2019-05-03 12:37:13 -0600 )edit

In your description you say "where the condenser loop gets too cold and the VRF shuts off". Is the boiler large enough to maintain the water heating temperature set point?

rraustad ( 2019-05-03 13:01:45 -0600 )edit

That was referring to another Unmet Hours post I had seen, I wasn't having that issue. There is no boiler in my model, the VRF is connected to a ground loop only. The project actually ended up going another way so I'm not trying to get this system working anymore, but I'm happy to send you the model if you're interested.

avril14 ( 2019-05-03 13:49:28 -0600 )edit

@rraustad Has this issue (VRF models do not recognize additional loads to zones added by DOAS systems) already been solved in the recent EnergyPlus (v22.1.0 or v9.6.0)?