I have a geothermal system with Water Furnace heat pump setup with the Desuperheater, where the heat pump provides warm water to a non-powered storage or buffer tank, which then feeds another electric water heater that is connected to the power. The setup is connected per Bergy's diagram that has been posted many times in these forums. The system was setup about 8 years ago and I guess at the time there was no requirement for a thermal expansion tank, so I don't have one. I'm looking to replace the water heater that is connected to a power source with the Rheem hybrid/heat-pump water heater, and I would like to add at the same time the thermal expansion tank. My question is, should the expansion tank be connected to the cold water line feeding the storage/buffer tank, or the cold water line feeding the Rheem that is connected to the power source?