Is there were a way to build a one line diagram for the plant loop and then export it to EnergyPlus?

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Have you tried the free open source software OpenStudioApplication?

It is a graphical user interface built upon the OpenStudio SDK and which uses EnergyPlus as a simulation engine (you can grab the IDF if you want and keep modifying it, though I would recommend sticking with using an OpenStudio Model (OSM) generally speaking). One of the abilities it has is to provide a clean usable way to create AirLoopHVAC and PlantLoop objects without having to manage nodes and branches manually.

See the tutorial on Air, Plant and Zone HVAC Systems for some screenshots.

Another possibility is DesignBuilder, which builds air and water loops in a graphical environment.

HVACDiagram that comes with EnergyPlus will help you visualize the IDF model that you have already created. It is not exactly what you want but will help verify what you have done. It runs as part of the EP-Launch that comes with EnergyPlus. Just open the SVG file. If you are not on WIndows, you need to run HVACDiagram by hand. You can learn more about it here. It is important to have a good SVG viewer, many internet browsers work but you might want to consider Inkscape or other programs that are dedicated to viewing drawings.