From the SDK documentation, if use addDemandBranchForComponent method, it will create a new ControllerWaterCoil if hvacComponent is a CoilCoolingWater or a CoilHeatingWater.

So from OpenStudio, should be like this:

however, when I add heating water coils to loop by this ruby script:

coils = model.getCoilHeatingWaters
plant = model.getPlantLoopByName('Hot Water Loop')
coils.each do |coil|
   plant.addDemandBranchForComponent(coil)
end

there is no controller in OpenStudio:

So any suggestions?

From Form the SDK documentation, if use addDemandBranchForComponent method, it will create a new ControllerWaterCoil ControllerWaterCoil if hvacComponent hvacComponent is a CoilCoolingWater CoilCoolingWater or a CoilHeatingWater.CoilHeatingWater.

So so from OpenStudio, openstudio, should be like this:

coils = model.getCoilHeatingWaters
plant = model.getPlantLoopByName('Hot Water Loop')
coils.each do |coil|
   plant.addDemandBranchForComponent(coil)
end