I have a couple questions for the experts.  I am replacing my forced air oil furnace with an open well geothermal system.  I live in Northern Michigan where we get lots of cold and snow.  I have a couple quotes from contractors, but here is what I need to know. 
 
One said if I have a 52 gal or larger hot water tank, I don't need a seperate tank for the desuperheater.   

I am putting in a dedicated well to supply the water to the geo only.  Do I need a pressure tank, or can I just wire a start relay to turn on the 220v pump when the sytem turns on?  I am only 10' from excellent ground water in sand.

Thanks  

It is true - you don't need a separate preheat tank for a desuperheater...as long as you are content to have it merely sit there and look pretty.

If, however, you want the desuperheater to actually recover and store heat and thereby actually reduce your hot water costs, offsetting its initial cost and ultimately saving you money...in that case you'll need a separate preheat tank, regardless of whether your primary heater is 0 gallons (tankless) or 120 gallons, or anything in between. Some of we regulars here prefer to defer to the expertise of local installing contractors, but folks selling desupers often fail to understand the dynamics of superheat recovery and domestic hot water use. Almost no one submeters or otherwise measures hot water costs before and after a desuper install, so the no-preheat-tank myth lives long and prospers.

I've beat this horse hard, if not to death, elsewhere here - search my posts for specifics.

I would second a good sized pre heater tank. Often you will find them sized for a day’s load of hot water, but the larger you go the less the main water heater will run and the more $ it will save you in the long run. We only have a 40 gallon pre heat tank and we regularly empty it between heap pump runs and often when the heat pump is running it gets up to temp and cycles on / off. So at that point we are losing heat.

If you have the space add a larger dedicated tank.

As far as the pumps being turned on add off that should all be controlled by the system.

I prefer a buffer tank.
I also think a pressure tank and a reliable entering water pressure makes the most sense. Open loop systems are recommended by manufacturers to be maintained at a positive pressure to help avoid build-up on the heat exchanger. Reliable water flow is also required. Use of the well for anything else or even an occasional hic-up in the flow will not impede performance if a pressure tank is employed.
Joe