Abstract: A closed loop geothermal system has a fluid circulation loop that includes in series: a heat pump, a ground loop heat exchanger, a tank flow center and a circulation pump. The tank flow center includes a tank, a first valve, a second valve and a tank bypass line all mounted in a common housing. The tank may include a refill inlet that opens through a top surface of the housing. A cap may be attached to close the refill inlet. The tank flow center has the regular geothermal operation configuration, a system flush configuration, and a refill configuration. The tank flow center may be elevated above the circulation pump a distance to produce a water column that is greater than a minimum cavitation avoidance head pressure for the circulation pump.