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How to model a working (VAV) HVAC system in energyplus

asked 2014-12-17 17:37:59 -0500

RyanStochastic gravatar image

updated 2017-05-17 12:42:13 -0500

Greetings,

I am aware of the energyplus example files, the input/output reference, and engineering reference, and i have spent a while with all of the above.

I'm still not able to figure out what is required to model an HVAC system in energyplus, or how to properly model VAV controls so that the volume of airflow changes to meet the load in each zone. There are terminal HVAC objects, fans, airloop objects, supply loops, demand loops, branches, branchlists, controllers, nodes, schedules, and plenty more. How do they all fit together?

Can someone explain in a concise response, what energyplus objects are required for a VAV system, and what the role of each object is? I've read the documentation on most all of the objects in the VAV system example files, but there are just too many, and I can't keep it straight. Perhaps a comprehensive, brief summary exists somewhere that I haven't looked?

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Ryan, I'm not trying to be condescending, just an honest question: how familiar are you with VAVs in the real world in the first place? Second, have you put the .svg files created with the examples to use? (I recommend using Inkscape, open source SVG reader, instead of classic IE on Windows).

Julien Marrec gravatar imageJulien Marrec ( 2014-12-18 02:02:07 -0500 )edit
2

Hi Julien, I take no offense to your question - and no, I'm not a VAV system expert in the real world. However, even if I had expert knowledge of real-world HVAC systems, the modeling and simulation methods of energyplus are tough to sort out from the documentation, in my opinion, and I figured that i wouldn't be the only one with this question - so i asked. In Jason't answer below, there are 31 different objects required for a VAV system (and he didn't include individual Node objects). Figuring out that list is one challenge; learning how they all interact and connect is another.

RyanStochastic gravatar imageRyanStochastic ( 2014-12-18 14:17:22 -0500 )edit

Individual node objects? None required, except perhaps a nodelist or two for convenience. Yes, there are many pieces, but that's part of the price of giving flexibility rather than standard fixed system configurations. For those, that's why HVACTemplate and GUIs were developed to help eliminate the complexity of the connections.

MJWitte gravatar imageMJWitte ( 2014-12-18 16:29:39 -0500 )edit

TRNSYS if you want to simulate "how an engineer thinks," and E+ if you want a 1990's "object-oriented" approach where you can't model/simulate that object in isolation. Modelica for the 21st century.

Sam gravatar imageSam ( 2014-12-18 22:48:36 -0500 )edit

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answered 2014-12-18 06:34:08 -0500

updated 2015-01-09 14:20:25 -0500

I do not know of a brief summary to answer your question but I can suggest an approach for you to learn the answer. Run the HVACTemplate-5ZoneVAVWaterCooled.idf file in the ExampleFiles directory. It uses the HVACTemplate objects: HVACTemplate:Zone:VAV and HVACTemplate:System:VAV to describe a VAV system. When you run the HVACTemplate-5ZoneVAVWaterCooled.idf file with EP-Launch, a expidf file is created which is an input file created by converting these HVACTemplate objects into full detailed EnergyPlus objects. Take a close look at the expidf file that is created. About half way through the file will be a line that says "New objects created from ExpandObjects" below this line is all the detailed EnergyPlus objects. This will show you all the objects that need to be created as well as how they are all connected to each other using names and nodes. The file will include portions related to how the boiler and chiller are are used closer to the bottom.

The objects that are created related to the VAV system are:

ZoneControl:Thermostat 
DesignSpecification:OutdoorAir
DesignSpecification:ZoneAirDistribution
Sizing:Zone
ZoneHVAC:EquipmentConnections
ZoneHVAC:EquipmentList
ZoneHVAC:AirDistributionUnit
AirTerminal:SingleDuct:VAV:Reheat
Coil:Heating:Water
Branch
Sizing:System
AirLoopHVAC
AirLoopHVAC:ControllerList
BranchList
AirLoopHVAC:SupplyPath
AirLoopHVAC:ZoneSplitter
AirLoopHVAC:ReturnPath
AirLoopHVAC:ReturnPlenum
AvailabilityManagerAssignmentList
AvailabilityManager:NightCycle
Schedule:Compact
SetpointManager:Scheduled
Coil:Cooling:Water
Controller:WaterCoil
SetpointManager:MixedAir
NodeList
Fan:VariableVolume
AirLoopHVAC:OutdoorAirSystem
AirLoopHVAC:ControllerList
AirLoopHVAC:OutdoorAirSystem:EquipmentList
OutdoorAir:Mixer
Controller:OutdoorAir

Many of these objects are used to link objects together. The most important to understand the inputs related to VAV would probably be:

AirTerminal:SingleDuct:VAV:Reheat
Sizing:Zone
Sizing:System
Coil:Cooling:Water
Coil:Heating:Water
Fan:VariableVolume

Another approach is to start looking at the AirLoopHVAC object and trace how it is connected to everything else.

Of course if you want to avoid all this, you can just use the HVACTemplate:Zone:VAV and HVACTemplate:System:VAV objects directly.

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Hi Jason, thanks for your answer! It's definitely helpful, and your lists of objects are exactly what i need to get started. One thing I'm still not clear on though are the controls - could you comment on VAV controls in energyplus? Of the 31 objects you've listed there, which ones actually govern how much airflow goes to each zone, or through each system?

RyanStochastic gravatar imageRyanStochastic ( 2014-12-18 14:21:51 -0500 )edit

Controls are specified in ZoneControl:Thermostat (which I will add to the above list), Controller:..., SetpointManager:..., and AvailabilityManager:.... You may find the training ppt files available here to provide a useful overview http://www.gard.com/services/energypl... In particular, see presentations 5-9.

MJWitte gravatar imageMJWitte ( 2015-01-09 14:17:53 -0500 )edit

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Asked: 2014-12-17 17:37:59 -0500

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Last updated: Jan 09 '15