what's the best program for modeling and simulating a recirculating loop with various components connected such as the Domestic Hot Water System...

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A small addition to mehrdad's answer,

I strongly recommend you review the software tools comparison section in this journal paper:

A review of modelling approaches and tools for the simulation of district-scale energy systems

This detailed comparison will give you the ability to select your best tool based on the case you are simulating.

BEST? How would you define BEST? User-friendly? Development time? Simulation time? Accuracy?

I have worked with these three, which are capable of doing what you described:

• TRNSYS: User-friendly, Not free, Need much more simplifications compared to real systems, reach components library, flexible HVAC loops, has no special structure for HVAC loops
• EnergyPlus: Free, takes much time to develop, lighting simulation and control, a lot of predefined standard loops, reach documentation.
• OpenStudio: Free, based on EnergyPlus, covers a lot of HVAC systems, the best starting point for advanced energyplus HVAC simulations