You should find that you will have more hot water capacity with a desuperheater and a buffer tank for any given active tank size. The buffer should be 40 gallon and never be active. I would recommend 60-gallon for the active tank. I would also recommend that a desuperheater from each geothermal unit feed the buffer tank.

In your case, where the active tank would be gas heated, you also would have the option of, instead, installing an 'On-Demand' gas DHW system fed by the desuperheater(s) & buffer tank.

SR

Check out www.geoexchange.org and www.greenbuildingtalk.com to learn more and get more answers.

I have a 50 gal buffer and an 80 gal domestic hot water heater. The buffer simply recieves the extra heat from the DSH and passes the warmed water on to the heater to be raised the rest of the way. It is not on ordinarily, but it can be turned on should I ever need a lot of extra hot water, such as when we have a lot of guests during the holidays or summer weekends.