Radiant Low Temperature Hydronic

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I am assuming you already know how to use EnergyPlus (setting thermal zones, constructions, ZoneHVAC):

• Create a new Construction:InternalSource. Refer to the linked I/O reference about what inputs are needed and what they are.
• Assign the construction to your floor surface of the zone(s) with the underfloor heating.
• Create a new ZoneHVAC:LowTemperatureRadiant object, either VariableFlow or ConstantFlow. Add it to the zone(s) equipment list(s).
• Make the appropriate connections to your heating PlantLoop.

The ZoneHVAC:LowTemperatureRadiant objects are not controlled by your zone thermostat, instead, they have an separate temperature schedule that will look at the metric that you select in the Temperature Control Type parameter.