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HVAC Split System - More than one in a single zone?

asked 2015-10-27 15:35:10 -0500 gravatar image

updated 2015-10-27 15:35:30 -0500

Hello everybody!

I'm new to all this energy modeling thing and I'm having a hard time trying to model my HVAC system :/ The model is for my project in University and it is from a real building, where I'm going to study the economic benefits of implementing a green roof.

The problem is: the building is full of Split Air-Systems!

I already know that to model that in E+ I need:

  • HVACTemplate: Thermostat
  • HVACTemplate:System:Unitary
  • HVACTemplate:Zone:Unitary

But...there is more than one Split AC in each zone, and I can't figure it out how to reference that single zone in multiple systems - I keep getting errors about that.

Is there a way to have more than one AC in a single zone?


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See if this post helps. It discusses the limitation of one air loop per thermal zone in EnergyPlus.

David Goldwasser gravatar imageDavid Goldwasser ( 2015-10-27 15:49:17 -0500 )edit

Yeah, it is kind the same problem, but...the solutions won't do for what I want, right?

I can't model them as just one, as the main objective of my project is how much less energy they will consume, and modeling as only a big one would distort the results, right? (No equipment would have that power) And my zones are already separated per room, so...I don't think I can split them even more...

Is that a dead end to me?

Should I try to model them as a central AC? With the average efficiency of the split ACs? gravatar ( 2015-10-27 17:03:06 -0500 )edit can you describe in more detail what your "split air-system" actually is so we can tell you whether you are actually dealing with something that needs an air loop or a zone hvac equipment?

Julien Marrec gravatar imageJulien Marrec ( 2015-10-28 11:06:57 -0500 )edit

Yeah, sure! What I mean with this is that I have a lot of these equipment inside the rooms: (of course the outside part is outside the building). But they are not connected with each other at all, all of them work separately, besides the electrical control - they are switched on and off at the same time. gravatar ( 2015-10-28 11:14:57 -0500 )edit

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answered 2015-10-28 13:25:01 -0500

In order to model multiple split AC units within one zone, you'll have to get away from using HVACTemplate objects. These are the big-ticket objects you'll need for your model:

Note that you will need to define lots of other objects to have your model run successfully (the cooling coil, fan, outdoor air mixer, thermostat, etc.). As Julien mentioned in his answer, you can use the expidf file to see these underlying objects created from the HVACTemplate objects. The links I included are for documentation of v8.3 of EnergyPlus, but when I compare the IDD file for the objects above between v8.3 and v8.4 the only difference of interest is that there are some new input fields at the end of ZoneHVAC:EquipmentConnections in v8.4 that can modify return air flow back to a central air system.

If you do have a central air system (roof top air handler, DOAS unit, etc.), the HVACTemplate:System:Unitary object will only create a system that is constant volume with DX cooling and your choice of heating. If this doesn't match your building, you will need to also create the air loop from scratch.

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answered 2015-10-28 11:11:54 -0500

If you intend to use and AirLoopHVAC:UnitarySystem, you should be fine.

Just stop using your HVACtemplate objects, use the expidf file to see the underlying objects.

You should be able to put several AirLoopHVAC:UnitarySystem in a ZoneHVAC:EquipmentList.

In the Input/Output Reference guide, v8.4:

1.31.2 AirLoopHVAC:UnitarySystem

The AirloopHVAC:UnitarySystem object is intended to replace all other air loop equipment, although other system types are still available. This system is unique in that it can accommodate all fan and coil types whereas other system types are specific to the type of fan and coil available for simulation. Additionally, although the AirloopHVAC:UnitarySystem is intended for use in the primary airloop, this object can be modeled as zone equipment (i.e., listed in a ZoneHVAC:EquipmentList) or as an outside air system component (i.e., listed in a AirLoopHVAC:OutdoorAirSystem:EquipmentList).

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I'm not convinced this is correct. If you check the IDD file for v8.4, ZoneHVAC:EquipmentList has a predefined list of possible objects that can connect to zones. AirLoopHVAC:UnitarySystem IS NOT one of them. I tried to alter an example file to test this, and I got a hard EnergyPlus crash (you can see my GitHub issue HERE). This may be an example where the InputOutputReference and IDD are in disagreement, in which case the IDD is ALWAYS correct.

Aaron Boranian gravatar imageAaron Boranian ( 2015-10-28 13:01:34 -0500 )edit

The AirloopHVAC:UnitarySystem was intended to be used as zone equipment and has been minimally tested as such. For the configuration tested, it did work. The IDD probably missed this nuance. If there is an issue with using UnitarySystem as zone equipment, then yes an issue is warranted.

rraustad gravatar imagerraustad ( 2015-10-28 13:23:05 -0500 )edit

@rraustad are you able to share this test file? I'm trying to make this system work and want to better understand how to configure the zone connections. I'm getting an error "Severe ** AirLoopHVAC:UnitarySystem "UNITARY SYSTEM": The zone inlet node in the controlled zone (SPACE5-1) is not found."

jmcneill gravatar imagejmcneill ( 2016-01-20 11:13:32 -0500 )edit

answered 2016-01-20 12:13:01 -0500

updated 2016-01-20 12:32:50 -0500

For those wanting an example of ZoneHVAC, AirloopHVAC, or OutdoorAirSystem using AirloopHVAC:UnitarySystem see this example. I just ran it in V8.4 without errors. I changed the extension from idf to jpg to attach.


  Zone1Equipment,          !- Name
  AirLoopHVAC:UnitarySystem,  !- Zone Equipment 1 Object Type
  Zone1PTAC,               !- Zone Equipment 1 Name
  1,                       !- Zone Equipment 1 Cooling Sequence
  1;                       !- Zone Equipment 1 Heating or No-Load Sequence

  Zone1PTAC,  !- Name

  OA Sys 1 Equipment,      !- Name
  AirLoopHVAC:UnitarySystem, !- Component 1 Object Type
  OACoils,  !- Name      !- Component 1 Name
  OutdoorAir:Mixer,        !- Component 2 Object Type
  OA Mixing Box 1;         !- Component 2 Name

  OACoils,  !- Name
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Asked: 2015-10-27 15:35:10 -0500

Seen: 1,170 times

Last updated: Jan 20 '16