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VRF Control in EnergyPlus

asked 2016-02-15 15:44:46 -0500


I have a model which used Sefaira to build/create the geometry and HVAC systems. I have downloaded the EnergyPlus file and imported it to E+ v8.4. I have gotten the model to run, but I have a ton of unmet hours which I cannot figure out. The system is set up as a DOAS with VRF; all of which are currently set to autosize. I have figured out the autosizing parameters and increased those, as well as modified the schedules to have the setpoints ramp back up rather than an abrupt change - but neither of these appear to have been the issue. The autosizing appears to be sizing the coils...but not operating the coils in numerous zones. Is there an additional thermostat control / control setpoint that needs to be assigned or verified? Thank you in advance for your assistance. DA

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Did you check Zone Cooling/Heating schedule setpoint schedule and setpoint /Availability Manager for DOAS airloop?

ngkhanh's avatar ngkhanh  ( 2016-02-15 16:38:23 -0500 )edit

Ok. It appears I do have a DOAS Availability Manager & Assigned DOAS schedule, and the values there make sense. I also have a Zone Heating & Cooling Schedules setup. Also, for ZoneControl:Thermostat I have a control schedule of 'HVACTemplate-Always 4 assigned. I am assuming this means that the zone system is always available, even if the DOAS is 'off'?

Additionally, my Zones have a ThermostateSEtpoint:DualSetpoint assigned, with this object referencing the correct [only] Heating & Cooling schedule objects....

dradair's avatar dradair  ( 2016-02-15 19:26:31 -0500 )edit

'HVACTemplate-Always 4 means you have dual setpoint with deadband for thermo stats. did you add Output:timebin; and see summary of temperature in each hours and each month? i

ngkhanh's avatar ngkhanh  ( 2016-02-16 02:07:56 -0500 )edit

I'd bet it's a sizing problem and that your question is exactly the same as this one.

Julien Marrec's avatar Julien Marrec  ( 2016-02-16 05:38:25 -0500 )edit

Julien, Thanks for that link, but I'd found that thread. I had assigned those controls to the VRF units, but it appear to have no impact on the unmet hours situation. Though, at this point, I'm going to assign it to all of them to see. Im quite confused... I will also try the reporting method from ngkhanh above. It may be worth noting, I attempted to assign an AvailabilityManager to the VRF units, but the input boxes remain pink in the idf editor...I assume this means its an error. But running it both with them assigned and removing them also seemed to have no impact. I could post model..?

dradair's avatar dradair  ( 2016-02-16 07:57:21 -0500 )edit

3 Answers

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answered 2016-02-27 13:58:20 -0500

I would suggest putting the following output variables into your model SystemNodeTemperature and SystemMassFlowRate, run your model and open the ESO file. Then follow your air node by node,

firstly through your DOAS system and make sure

  1. the fresh air is coming into the building when it should be,
  2. the temperature of supply air into each zone is what you have set in your setpoints.

Then take a look at your VRF system and make sure

  1. there is flow through your VRF system when required to meet zone temperature
  2. the temperature of the air is what you expect it to be entering the spaces.

Once you have done these diagnostics you can tell which component specifically is not responding to your controls and more clearly understand your problem. From my experience (and i have modelled a lot of these systems) generally when it comes to controls it is incredibly important to understand and program your ZoneSizing, SystemSizing and PlantSizing correctly as well as making sure your SetpointManagers are set correctly.

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Annie; Question on following these variables through the nodes. Selecting those two variables alone generates a vast file nearly 13million entries. How have you gone about processing/organizing/viewing this? Have you/firm built a tool? Or one available online? Or is it a 'reality' of modeling to dig through these?

dradair's avatar dradair  ( 2016-02-29 14:30:53 -0500 )edit

Yes it does! luckily EnergyPlus has already given us something to use, if you download the ESO viewer, it will let you look at these data points in graphical formatting. It is great we use it ALL the time. We are also developing a spreadsheet tool which we are naming simulation diagnostics so that you can see these patterns more easily and it will include a library of some common patterns, this is under development at the moment.

Annie Marston's avatar Annie Marston  ( 2016-02-29 15:10:54 -0500 )edit

answered 2016-03-01 17:27:12 -0500

A mistake I once made with this system type is that I left the outside air items on the VRF systems to "autosize", thinking that it would just autosize it to come from the DOAS. This inadvertently doubled the outside air to the zones, so that I got double the required OA through the VRF fan coils AND the DOAS. If by chance you did the same, you will need to set this to zero.

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answered 2016-02-26 21:32:56 -0500

DA, forget about all those issues and verify if the VRF system can maintain the zone loads mid- to late-day. Look directly to the control zone. What is the terminal unit and condenser PLR on the design days? What is the VRF sizing results compared to the zsz results? Setpoints don't typically ramp up, they set back or set up, it's the system that responds to this step function gradually. VRF systems can't respond to load in a zone when other zones are in the opposite mode (e.g., cooling vs heating), unless heat recovery is used. Even then there are limitations in performance. Focus on the design days to see what the limitations are in each season.

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Luckily in this case I did have heat recovery on the VRF systems, so trading heat/cool around was an option. Otherwise I will still have had significant unmet hours. Though, the biggest drop/change came after make the system's MasterThermostatPriority:ControlType 'LoadPriority', rather than some arbitrary selected zone. Is there a report that combines these zone temperatures or is it just through examination of the individual zones in the html output file?

dradair's avatar dradair  ( 2016-02-29 14:34:51 -0500 )edit

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Asked: 2016-02-15 15:44:46 -0500

Seen: 1,397 times

Last updated: Mar 01 '16