A partial benefit of a geothermal heating/cooling system is that it can be used to assist in water heating.  At the same time now, tankless water heaters are getting a lot of hype.  Tankless water heaters are more efficient than traditional tank heaters but there are many disadvantages (check the link below).  It seems a great way to take advantage of geothermal and the energy efficiency of tankless hot water heating is a hybrid system like shown in the link below.  In a hybrid system, you have both a tankless water heater a very small hot water tank.  This eliminates the problems with tankless systems and allows the geothermal system to help by providing heat to the water in the small tank.    

http://carbonbigfoot.blogspot.com/2007/04/hybrid-tankless-waterheater.html

i have a similar hybrid system currently under construcktion, however my tank is placed up stream of my tankless hot water heater, i have a 85gallon super insulated plastic hot water heater tank, i say tank because it is not ever going to be hooked up electrically, this tanks has 2 sources of heat, 1 being the desuperheater on my 3-ton forced air, i believe this is set to heat the water to 145 degrees, the second source of heat is my 2 -ton water to water geopump, it has a third heating zone which is a heat exchanger and small pump system, this will cut in when the tank temps fall below 90 degrees or so, the second source is for the parts of the year when my heating or cooling loads are not enough to allow my desuper heater to heat the tank right up. the hot water output of my tank goes to a 29KW electric tankless water heater. i chose the biggest tankless unit my electrical service would allow to make sure i had maximum capacity, and because i am a typical man and like to have the biggest and the best. this unit is not immune from all the drawbacks of tankless water heaters, because it is the biggest one in this company's line up, it also has the highest gpm cut in, my shower barley turns it on. i have got the usual complaints from my wife, i am hoping once the geothermal component of my hot water system is complete most of the issues will be addressed and my tankless will hardley ever have to turn on. this tankless water heater does sense incoming water temps and adjust the heat accordingly. another note to would be tankless water heater purchasers, although this water heater is the second largest from this company, (the 36KW they offer requires a 300amp service) it does NOT have the capacity to keep up with the roman faucet on my jacuzzi, if i throttle the faucet back to about half and do not turn the cold on i can fill the tub with hot water. i do believe natural gas models do have more capacity, i chose the electric because i do not intend to have natural gas hooked to my house. if you have any questions, fire away.

Most heat pump desuperheaters will only heat water to 120°. This only happens when the heat pump is running to provide space heating or cooling. During the spring and fall when the is no demand for heating or cooling there is no hot water assist coming from the heat pump.

It doesn't make any sense to put the storage tank after the tankless heater. The water would already be heated and the desuperheater can not add any heat.

If I was going to do this, I would have two tanks. One before the tankless that the desuperheater can pre heat and then one after the tankless for reasons described in the link in the opening post.

'Most heat pump desuperheaters will only heat water to 120°. This only happens when the heat pump is running to provide space heating or cooling. During the spring and fall when the is no demand for heating or cooling there is no hot water assist coming from the heat pump.'

this is why i added the second system to heat the water, my water to water unit will cut in just to heat my tank, only to 90 degrees or so however, this should help minimize my power consumption from my tankless and add capacity to my tankless for things such as my roman faucet. i think the second, downstream tank would be unnesasary, preheating the water goin into the tankless will eliminate the cold water 'sandwiching' described in the first post, as for speeding up hot water delivery time i have considered one of those convection flow circulating systems to each of my faucets, especially if it turns out my geo component of my hotwater system has more than enough capacity for my DHW needs. as for temps, i am not trying to argue, you are obvious the geo guru here, but on my previous 3.5 ton system i frequently saw temps well above 120 degrees from my desuper heater, i would suspect that as the storage tank water increased in tempurature, the amount of BTU's the desuper heater provides would decrease.

Thanks for your input. I always want to hear what other people are doing and how they have have solved problems. We can all learn from exchanging ideas and especially hearing about what is actually working

It sounds like your system is working fine. You obviously have put a lot of thought and planning into it.

I was mostly responding to the original poster where he wanted to put the tank after the tankless.

For the case in the link I posted, I think it makes sense to have the tank after the tankless and it seems to me to be more efficient that way. The key to the link is that the tank has a low power heater and the tankless by itself provides delayed hot water and provides hot water only if there is minimal water flow (not a trickle). The tank provides some efficiency by storing just enough hot water to provide immediate hot water while the tankless gets going, and would also provide hot water even if there is just a trickle demand. This would not work as well with the tank upstream of the tankless, the tank would then require a much stronger heater, would defeat the purpose of the tankless.

I'm thinking about a geothermal system, and I already have a Bosch AquaStar 125FX. Since the desuperheater supplies (say) 120° water and the Bosch will raise the temperature 45-90° depending on water flow, doesn't this mean that I might have 200° water? What's to keep that from happening?

what is that Bosch?? is it gas?? I didn't think Bosch made a gas/propane heater they would recommend for that application. If it is electric it should have temp sensors on it to maintian the output temp.
It has been a while since I looked at bosch..but if it is gas.. I cant stress enough not to put it into a desup application...it isn't meant for it.
If they haven't change the specks I dont even think they want you running a grundfos comfort system of them or a Leang ATC303BTW even.

Electric no prob...look at a Steibel Eltron Tempra 29

I thought I'd toss in "what I have". A) I have a ground source heatpump (although the loop is in a river) w/desuperwater heater. B) I have a standard electric water heater that receives any available heat from the heat pump. This tank is powered and set for 100F. The 240V feed to the water tank is also regulated by a timer which restricts it to low time-of-use electrical rate. C) There is also a recirculating pump at the far end of the house that draws water from the hot water line and returns it to the hot water tank. It is controlled by an aquastat & a timer. The aquastat is set to 100F and the timer is set to be on during "house occupied & active" times. D) After the hot water tank is a propane (rural property) demand/tankless water heater set at 125. The draw from the recirculator is insufficient to cause the demand/tankless water heater to kick on. Therefore: - The static water temp in the lines (during occupied & active times) is 100F & takes very little time to reach the 125F provided by the demand/tankless water heater. - When the heat pump is supplying hot water up to 120F the demand/tankless water heater adds only 5 degrees, but - When the heat pump is not providing sufficient hot water then the electric elements in the hot water heater kick in to keep it at 100F, except during peak electical load/cost. Done P.S. I don't know why all the line breaks disappear when I post this - it makes it hard to read.

Posted By jambsi on 06 Jul 2012 07:13 PM
I thought I'd toss in "what I have". A) I have a ground source heatpump (although the loop is in a river) w/desuperwater heater. B) I have a standard electric water heater that receives any available heat from the heat pump. This tank is powered and set for 100F. The 240V feed to the water tank is also regulated by a timer which restricts it to low time-of-use electrical rate. C) There is also a recirculating pump at the far end of the house that draws water from the hot water line and returns it to the hot water tank. It is controlled by an aquastat & a timer. The aquastat is set to 100F and the timer is set to be on during "house occupied & active" times. D) After the hot water tank is a propane (rural property) demand/tankless water heater set at 125. The draw from the recirculator is insufficient to cause the demand/tankless water heater to kick on. Therefore: - The static water temp in the lines (during occupied & active times) is 100F & takes very little time to reach the 125F provided by the demand/tankless water heater. - When the heat pump is supplying hot water up to 120F the demand/tankless water heater adds only 5 degrees, but - When the heat pump is not providing sufficient hot water then the electric elements in the hot water heater kick in to keep it at 100F, except during peak electical load/cost. Done P.S. I don't know why all the line breaks disappear when I post this - it makes it hard to read.

Just curious how much propane do you use each year? I use about 200 gallons per year as I do not have my desuperheater connected to my tankless.

I've used an average of 152 USGallons the past 3 years. That's servicing the hot water, a fireplace and a BBQ. I'd guess 75 - 80% of that is for hot water of that is for hot water. It was 1320 USGallons the last 2 years before geothermal space heating. Note I live in a "heat-centric" climate. Apples-to-apples is tough in the energy use comparison game.

Posted By jambsi on 06 Jul 2012 10:14 PM
I've used an average of 152 USGallons the past 3 years. That's servicing the hot water, a fireplace and a BBQ. I'd guess 75 - 80% of that is for hot water of that is for hot water. It was 1320 USGallons the last 2 years before geothermal space heating. Note I live in a "heat-centric" climate. Apples-to-apples is tough in the energy use comparison game.

You have an interesting setup but as you are on a smart meter already and the cost of propane is similar to peak electricity, why not just have a 300-400L electric tank on a time of use (which you already have). The added cost for maintenance and propane doesn't make any sense to me.

I have an air source heat pump in a clients house and this is what we do, although there is also a solar thermal system preheating the tanks as well. It is all going to floor heat.  

Posted By MikeSolar on 07 Jul 2012 07:42 AM

Posted By jambsi on 06 Jul 2012 10:14 PM
I've used an average of 152 USGallons the past 3 years. That's servicing the hot water, a fireplace and a BBQ. I'd guess 75 - 80% of that is for hot water of that is for hot water. It was 1320 USGallons the last 2 years before geothermal space heating. Note I live in a "heat-centric" climate. Apples-to-apples is tough in the energy use comparison game.

You have an interesting setup but as you are on a smart meter already and the cost of propane is similar to peak electricity, why not just have a 300-400L electric tank on a time of use (which you already have). The added cost for maintenance and propane doesn't make any sense to me.

I have an air source heat pump in a clients house and this is what we do, although there is also a solar thermal system preheating the tanks as well. It is all going to floor heat.  

My setup was an evolution not a "design from scratch". House was built all propane: furnace, storage DHW tank, etc. First to be replaced was storage DHW tank - replaced by propane tankless. So when I tossed the propane furnace for Geo, ripping out the propane tankless didn't make sense. Like I said; apples-to-apples is tough.

http://welserver.com/WEL0288/

Stiebel eltron after buffer tank. Works great. WEL has lost internet connection, it is a vacation home.

So much of this depends on return on investment for me. Our desuperheater is plumbed to a separate tank, which, in turn, feeds a fairly new 60 gal. standard electric water heater, tied to a timer, allowing us to make use of cheaper night electric rates.

According to the energy analysis from our installer, the geo should be supplying 58% of our annual how water at a cost of $46. The water heater does the other 42% for a cost of $164. This is based on $0.11 / kwh electricity (we're currently paying about $0.125 for wind power).

I live in the northeast (central NY) and the geo contribution in the summer time is probably negligible, though during the heating season, it's doing almost all of our water heating. I often though it would be neat to do a simple, unglazed (like the pool heaters) solar water heater for the summer months. This could be a drain back that would be unneccessary once the air temps made an unglazed collector inefficient around here. However, the effort and cost even for a DIY setup would never justify the $164 I might save on an annual basis. It would have to be a labor of love or something.

Posted By Krisps on 23 Feb 2008 07:01 PM
For the case in the link I posted, I think it makes sense to have the tank after the tankless and it seems to me to be more efficient that way. The key to the link is that the tank has a low power heater and the tankless by itself provides delayed hot water and provides hot water only if there is minimal water flow (not a trickle). The tank provides some efficiency by storing just enough hot water to provide immediate hot water while the tankless gets going, and would also provide hot water even if there is just a trickle demand. This would not work as well with the tank upstream of the tankless, the tank would then require a much stronger heater, would defeat the purpose of the tankless.

You are correct that the upstream tank won't solve your complaints, but there is lots of data shared here that running a DSH to a powered tank often has the consequence of the heat pump actually taking heat from the tank raising operating costs. So one could do as you suggest with no DSH or add an additional buffer tank where DSH is employed.

Posted By joe.ami on 12 Nov 2012 01:58 PM

... there is lots of data shared here that running a DSH to a powered tank often has the consequence of the heat pump actually taking heat from the tank raising operating costs. ...

Particularly true if the tank is a gas-fired one.

Best regards,

Bill