How to simulate hot water pipes heating system?

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EnergyPlus does have a Pipe:Indoor object that transfers heat to a zone. You would define a Construction for the Material layers of the pipe wall from outside (touching the zone) to inside (touching the hot water), similar to how you would define a wall assembly. The best example file to review on how to use this object is PipeHeatTransfer_Zone.idf, where the indoor pipe exchanges heat with a plenum zone, and is located downstream from a cooling tower (the two components are on the same Branch object) on the supply side of a condenser water loop.

I should note that EnergyPlus will allow a pipe to be the only component on a branch, but a pipe (of any object type) cannot be referenced by the ZoneHVAC:EquipmentConnections object that sets what components are connected to a zone and provide heating or cooling to that zone. This means that you will need some other heating component to be part of the equipment connections (like your baseboard heater). Then, your baseboard heater and indoor pipe should be part of the same Branch in your hot water loop. This way, when the zone needs heating, the baseboard heater will request hot water from the boiler, and that hot water will flow through the indoor pipe as well as the baseboard heater. To make sure the indoor pipe is the only component providing heat, set the baseboard heater capacity to zero.