I understand a two tank system is best for heating water with the geothermal system using desuperheater. However, I don't understand how the one tank system could have been working for me these past 25 years. Aren't I circulating cooler water from the desuperheater through my hot water tank that is "usually" at 120 degrees? I suspect the tank water is more often already hot, rather than trying to get hot. So, most the time the pump is running, I'm just cooling the water.

You are circulating water at the bottom quarter of the tank so a lot depends on what the lower element is set to. There is always some stratification within the tank.

That helps. Thanks

Your correct about not much benefit with one tank. 25 yrs is a long time, same water heater?

I just purchased an inexpensive electric tank(scratch & dent) to be the storage tank(non-powered)

does it pay to hook up a heat pump water heater to a geo thermal with a desuperheater?

RAM73,

I think it's worth it if you use a non-powered storage tank connected to the DSH. Then the heat pump water heater will be the finish tank bringing the water up to final temp.

That is the set up I am using.