Drain water heat recovery as heat source to a Heat Pump?

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There isn't an object to model drain water heat recovery directly in EnergyPlus, but you can still model it with some custom objects and code.

First, add the "Water Use Equipment Drain Water Temperature" output variable and calculate the drain water temperature and flow rate from the water use schedule.

After that, I can think of two options for modeling the drain water heat recovery:

The first option is to use the HeatPump:WaterToWater object coupled with a PlantComponent:TemperatureSource object with some EMS code to adjust the source temperature based on the load and drain water flow/temperature. Here is example EMS code for modeling ground source heat pumps. You will need some custom operation scheme to control the load management on the loop.

The second option is to use the PlantComponent:UserDefined object to model the drain water heatpump directly on the loop. Here is example EMS code to model an air source heat pump. You'll need to adjust it for the drain water flow/temperature.

Good luck! Please reply back if you manage to get it implemented so other people can borrow code, or post it as a measure to the UnmetHours Github.