Posted By engineer on 16 Jun 2013 09:01 AM
All our geo installs get the desuperheater option with its own buffer tank. We tend to go with an 80 gallon buffer tank, but anything down to 50 will probably work.
The desuperheater buffer becomes the source for pretty much any finishing heater you prefer, tank or tankless, electric, gas or propane. I have heard that some gas tankless water heaters don't do well with warm water at the cold water inlet, so be sure to check on that. A simple way to check on that is to pose the question whether the model is solar-compatible.
Desuperheater + a "hybrid" heat pump water heater can be a Cadillac system, particularly in warmer climates, but the ROI on the hybrid will be reduced by the desuperheater / buffer tank. By the book, hybrid reduces water heating cost by $100 per person at national average costs and conditions, but that savings might be $50 or less with the desuper.
Key factor is household size...4-6 would probably justify both, 1-3, not so much.
The issue is with all gas-fire tankless units, since they all have a minimum modulated fire. When the desuperheater's buffer tank is over 100F bumping that to 110-115F with a tankless at 1gpm is a BTU-rate below the min-fire of any gas fired tankless, and the thing simply can't regulate the temperature. When using a gas/propane fired tankless in this type of application (and solar thermal too), look carefully at the minimum firing rates- smaller min-fire is always better, but none are foolproof.
The high standby losses of standard gas-fired tank hot water heaters make them pretty inefficient in this type of application too, but a condensing gas fired tank would work. The economics won't always make sense for low-volume users though, just as with the heat pump water heaters.