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Modeling Self Contained Water-Cooled AC Units in OS or E+

asked 2014-11-03 15:00:12 -0500

Has anyone modeled SCUs (self contained water cooled AC units) in OpenStudio or EnergyPlus?

They are basically a VAV air handler that has a large water cooled heat pump in it in place of coils. The heat pump is connected to a condenser loop, and then also there is a separate water side economizer in the unit. Downstream of the AHU are VAV boxes with reheat. An example unit is the Trane PKG-DS-6.

I can't figure out how to model this because E+ won't allow water to air heat pump coils on an air loop. Has anyone modeled one of these in the past? If not, does anyone have any idea how to? I will be modeling it in OS, but if someone knows how to do it in E+ I can write a script to do it in OS most likely.


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E+ has an AirLoopHVAC:UnitaryHeatPump:WaterToAir. Have you checked that out?

Julien Marrec's avatar Julien Marrec  ( 2014-11-03 15:24:43 -0500 )edit

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answered 2014-11-03 20:56:13 -0500

updated 2014-11-04 15:56:30 -0500

I solved this problem by placing the heat pump coils as a single component inside the AirLoopHVAC:UnitarySystem. Essentially using the AirLoopHVAC:UnitarySystem like you would the DX coil wrapper model. I encountered one issue getting the unitary to size the inner coils appropriately and that has been resolved here.

It is possible to place the coils inside the unitary using OpenStudio, but you will need to use the API to accomplish it. I can provide an example in Ruby if you want to go down that path.

(update) I should have pointed out that AirLoopHVAC:UnitarySystem is unique from most (all?) other unitary systems because it allows you to use setpoint managers for control instead of using internal (single zone) control logic. This is the key feature that allows you to use it in a multi zone situation like you have in mind.

(Update 2) here is an example in Ruby.

  m =
  air_loop = m
  node = air_loop.supplyOutletNode
  coil  = m
  unitary = m
  unitary.setHeatingCoil coil
  unitary.addToNode node

There is just one little problem. You need access to the "Control Type" field in the AirLoopHVACUnitarySystem object so that you can set the value to "SetPoint". That field is private in OpenStudio Model. In C++ you can get to the private method through a sneaky back door. No such trick in Ruby. So I'm going to show you an even more sneaky trick and please use it this one time and pretend you never saw it after that.

unitary.setString 2, 'SetPoint'

That setString method lets you access the private data fields directly. This is almost always a bad idea, but in this case it is ok. The two indicates that you are setting the third field (they are indexed from 0) to the value "SetPoint". The outlet node of the unitary system will now require a setpoint manager for things to run. Disclaimer we could close this loop hole at any moment. More likely we will just give proper public access to the control field, but that requires some thought because typically you will need set point managers on all of the internal nodes between components in the unitary.

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A ruby example would be great so I can start writing a measure.

Thanks for all of the help.

TaylorRoberts's avatar TaylorRoberts  ( 2014-11-04 12:02:31 -0500 )edit

Kyle, I just wanted to add that your response was tremendously helpful. A whole world of DOAS with condenser loops has opened up. I'll try to use the setString() method only when it is necessary and I look forward to having the proper public access of the control field.

chriswmackey's avatar chriswmackey  ( 2017-07-14 21:22:40 -0500 )edit

Kyle, I am fairly new to OS. It appears that sense this post the setpoint has become available. What type of setpoint manager would you use? I want to use once unitary system to control the leaving conditions via a cooling coil down to 42 deg F and another unitary system to bring the leaving temperature back up to 68 deg F. Any help would be greatly appreciated as I am right in the middle of a 1400 ton geothermal project and I want to model the impact of the DOAS units on the well field size.

jmoody's avatar jmoody  ( 2018-02-12 08:27:03 -0500 )edit

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Asked: 2014-11-03 15:00:12 -0500

Seen: 554 times

Last updated: Nov 04 '14