You can develop and edit HVAC systems in OpenStudio and there are templates for setting up most of the standard ASHRAE systems and scripts for generating prototype buildings. What you can't do is read the HVAC system from an EnergyPlus model that you created outside of OpenStudio.

[Updated per Eric's comment]

You can develop and edit HVAC systems in OpenStudio and there templates for setting up most of the standard ASHRAE systems and scripts for generating prototype buildings. What you can't do is read the HVAC system from an EnergyPlus model that you created outside of OpenStudio.

You can develop and edit HVAC systems in OpenStudio and there templates for setting up most of the standard ASHRAE systems and scripts for generating prototype buildings. What you can't can do is read the HVAC system from an EnergyPlus model that you created outside of OpenStudio.