Background: I'm a homeowner in SE Michigan with a vertical closed loop system that provides forced air for all floors of my home (2200 sq ft unfinished basement, 2000 sq ft first floor, 1000 sq ft second floor) and hydronic radiant floor heating for the basement slab. My house was completed summer of 2015. The forced air component is working just fine without issue, but we've had issues with the hydronic component in the basement. Heat pump: WaterFurnace Synergy 3D (SDV064) Hydronic storage tank: 80 gallon State Industries Geothermal Storage Tank (SGV821OTS 110) We've dealt with some past issues (tank leaking) that required the installer to replace the storage tank once already. Prior to them replacing the tank, we had no issues with the heat pump properly maintaining the water temperature in the storage tank. Since the tank was replaced in October 2016, though, the heat pump is not controlling the temperature in the tank properly. Explanation of the problem: If water in the tank is at 60 degrees, and I turn on the hot water feature on the Synergy 3D (flip the switch on the unit to on), the system will start heating up the water in the storage tank, as long as there is no demand for forced air. But, the system never stops running until there is a high pressure fault. It seems that the heat pump properly detects when the temperature is low and it needs to heat the tank, but never detects when the temperature reaches the set point to shut off. It will heat the tank up to over 120 degrees, even when the tank thermostat is set to 90 degrees, and the pressure gauge out from the tank hits its maximum readout of 60 psi, with a return pressure of 50 psi. The loop from the tank to the floor and back stays at 40 psi. I have tried to get the installer to come back out to fix the issue, and he refuses. At least he responds to texts (sometimes) during my troubleshooting, though. I've had another professional come out to look at the system, and they are suggesting all kinds of additional equipment that wasn't necessary for the system to work before the storage tank was replaced, and they've been dragging their feet on getting me a quote to do the bare minimum to get the system working properly. At the installer's request, I've measured the red wires from the tank that are supposed to be the thermostat wires (per my installer) and they never read anything other than 0 ohms. I suspect that the storage tank thermostat is either wired incorrectly to the heat pump, or it is a faulty thermostat. Additional information: The manuals and documentation from State Industries are worthless crap. They don't even acknowledge the presence of the lower thermostat. Their website barely acknowledges the existence of their geothermal storage tanks. With the original storage tank, the breaker to the tank was always left in the off position so that it would only be heated by the heat pump, and never the internal element. This suggests that the lower thermostat in the tank does not require power. I assume the heat pump is measuring a resistance in the wire that is a function of water temperature. The tan jacket wire from the heat pump is run to the storage tank, and the red and white wires from the tan jacket are wired to red wires at the top of the tank. The red wires run to the lower thermostat of the tank, where they are connected to two yellow wires. See attached pictures. In the attached picture of the lower thermostat, there are two screw terminals that are unused. My installer says those don't need to be used for the heat pump connection, is that correct? Since local professionals aren't being much help (at least not in a reasonably timely manner), I turn to you all. Does my conclusion seem sound, or might there be something else at play here? The only thing that has changed between when the system worked correctly and now is that the storage tank was replaced.