We will be installing a Geothermal closed loop + 2 stage Nordic HP with desuperheater for radiant floor heating (in northern NH). One recommended DHW installation shows a water-water heat exchanger (pre-heat tank) - using the same water as going to the radiant floor- followed by an electric hot water tank. The house is primarily a vacation house so I am wondering if it would not be much more efficient to use an "on demand" electric heater following the pre-heat DHW tank. Anybody have experience with on-demand electric water heater with geo thermal + heat pump + desuperheater?

If you don't mind the requirement for ~.5+ gal/min to turn the heater on, on demand heaters work well with pre-warmed water. I'm curious why you chose geothermal for a vacation house.

Well, radiant floor heating (very desirable) in a relatively cold environment seems to be en excellent match for geothermal (in a well insulated house) and thus it seemed we can keep the house during the un-occupied time at a reasonable temperature at minimum cost. Of course getting a 30% tax refund at this time did help with the cost plus due to new construction there is the added savings in digging for a water well. Did I miss anything?

Normally, geothermal is a high initial cost system that needs more use to get a payback. But with open loop, it may be comparable to a conventional boiler. You have ruled out designing the house so that it can safely freeze?

The current design is with SIPs, high R windows, a closed loop geothermal system and a back-up generator. I am not counting on freezing. What am I missing? (I am not an expert and am just a novice with basic engineering/science background). Thanks for your inputs and advice!

Mechanical failures is what I would worry about. It wouldn't take much to be able to winterize the house so that the heat could be completely off. This would really save energy. Maybe with a call-in system to turn the heat on before you get there (radiant takes awhile).

Pipes can be emptied with compressed air. Drains and toilet bowls need antifreeze.

The trouble with "on-demand" or tankless electric water heaters is that they use a heckuva lot of power and still may not meet the sumultaneous demands of a house, especially in a northern climate where ground water is quite chilly. During non-heating or cooling seasons the preheat tank will do nothing, so the main water heater will have to carry the whole load.

Heating a fairly modest 5 GPM from 50 to 120 would require over 50 kw, consuming more than the entire capacity of a 200 amp service.

A standard storage electric water heater turned on upon arrival would provide hot water within an hour or so of being turned on upon arrival at the vacation house. Feeding it from the preheat tank recovering heat from the desuperheater will reduce its runtime and operating cost.

> pre-heat tank - using the same water as going to the radiant floor

If the hot water heat is coming from a buffer tank that is heated by the heat pump's main heat exchanger, then there is no need for a desuperheater and it will work all year round. And when you have this pre-warmed water (perhaps 100F), the tankless works well.

Here is an example of geo heat and hot water with no desuperheater:

http://www.ourcoolhouse.com/monitor/monitor.htm

We have used 50 gal buffer tank for desuperheaters followed by on demand electric heaters (stiebel eltrons). No issues there. You could do the same.
But when you have a heatpump putting out 125 degree water, you can come up with a creative design including a high efficiency tank with a large built in heatexchanger. You can then run this tank as a designated zone which the heatpump prioritizes on and conditions at 125 F degrees.
Then you take another tank as a buffer tank for your hydronic load the heatpump conditions at the rest of the time.
Most of the systems I know in Europe are designed that way.

It makes far more sense than the "geo hot water only in summer and winter" designs.

pf,
It seems that closed loop was stated, and then a well mentioned. Which is it.
Either way you need to have bidders prepare operating cost calculations, to help you decide if geo will be a good fit for you and your goals.
Good luck,
Joe

The plan is to use a closed loop geo thermal system, (same company will also drill the well for drinking water).
I did a back of the envelope calculation and it seems with radiant floor heating the geo thermal with a heat pump is an ideal fit (with the government 30% break).
I was just wondering about how to get a good DHW system given that we will be using it only occasionally (week-ends mostly)
There are some very good internet based systems that allow us to start the radiant floor heating (via iPhone etc.) before we make the trip up north to take care of the long time constant for a radiant floor heating system.

WTW geo can be used as previously described to make your DHW, but if you are closed loop, you will need a finish tank or on demand if you would ever hope for a hot shower in Jan.
j