My Geo installer plans to add a 50 gallon buffer tank (for the DSH) to use with my existing 80 gallon natural gas water heater.  The existing water heater is about 14 years old.  Due, in part, to the age of the water heater, I was thinking about having a new one put in when my geo goes in.  Part of my thought process is that I would rather have all of the plumbing done to a new water heater than one that is close to the end of its life expectancy. 

Seemed like simple idea till I started to see all of the options out there....

Does it make any sense to buy one of the water heaters that has supplemental heat capacity?  My question is whether it is possible to hook up the water heater so that it supplements the heat produced by the Geo for Stage 3 needs - rather than use the more expensive resistance heat.  Then again, I am not sure I understand how these types of water heaters work, so maybe this isn't even possible.

Thanks!

A buffer tank does not heat the water, only holds it. To me it would make more sense to convert your current 80gal heater into your buffer tank. With the pre-heating of the water from the DSH & holding in the buffer tank, your powered water heater shouldn't have to work nearly as hard - you may be able to get away with a 50gal instead of 80gal. Just a thought.

On the other hand, the tank is 14 years old - you may want to scrap that one too. That's the age where it wouldn't be unexpected that it could start to leak. I'm guessing that the general consensus is that if you have 2 tanks - a heating tank and a buffer tank - that you'll probably want the larger of the two as the buffer tank.

Do you use lots of hot water? You may wan to read the thread Justifying Two Electric Water Heaters.

I don't believe excess capacity in a water heater or buffer tank would be able to store a sufficient amount of heat due to the relatively low size. Others can explain this better than I can.

If you can, go with new tanks. Marathons are great, but expensive, and have a lifetime warranty on the tank.

Here is an example of the product that prompted my question:

These hybrid products offer the dual performance of both water heating and space heating simultaneously. They are an economical alternative to installing both a heating system and a water heater. Combi1™ models utilize a single wall heat exchanger that provides approximately 40% more space heating output when compared to the Combi2™. Combi2™ models utilize a double wall heat exchanger that provides additional protection between the heating fluid and surrounding water (required in certain municipalities).

http://www.bradfordwhite.com/products_list.asp?id=13

I can't find much information about how it discharges the "heat". I was curious if the thing could be configured to turn on the space heating function when the geo would otherwise call for stage 3 heat and that heat could be connected into the ducts. If so, could I reduce or avoid the cost/need for stage 3 resistance heat?

I did see the Marathon WH's, but they appear to only sell electric and I would prefer gas.

Thanks for the link. I follow what you're talking about now. I believe I've read that it is possible. Is it worth it?

How many hours per year is your aux estimated to run? Compare the costs of using gas and electric and figure out your estimated savings per year.

How much would it cost to do this part of the project? You would need a water coil installed in the duct work, a pump, triggering devise, piping, labor, anything else?  You should also include the incremental cost of a water heater capable of doing this to one that you would otherwise consider.

Then figure out your estimated years to pay back this investment (divide total cost for this part of the project by the estimated saving per year.)

How long will your installer warranty this part of the setup?  What if there is a leak in the coil and water gets into the geothermal equipment and circuit boards?

Would the water coil reduce airflow from the geothermal unit when you don't need it the majority of the time?

I would be concerned with the stagnant water in the coil that may sit there for months at a time. This may not be an issue if this water won't mix with the water in the water heater.

Have you considered gas logs instead? They can be used instead of aux electric heat. It is also possible to set them up without electricity to provide heat during a power failure. Of course, if you have a generator, this may not be an issue.

Marathons make great buffer tanks too. You will need a buffer tank (not necessarily a Marathon) if you want to have a gas water heater.

I'm in the same boat.  My electric water heater is 12 years old.  When I get my geo system, the installer I'm working with has suggested a buffer tank to hold the warmed water from the DSH.  I could spend $500 installed for a buffer tank (his quote), or I can buy a nice electric water heater in the 80 gallon range and use my current water heater as an unpowered storage tank.  It seems to be well insulated since it is never warm on the exterior and we never seem to run out of hot water unless we're both taking 30 minute steam showers at the same time. 

I'm probably going to go that route.  I can add the new water heater myself, plumb the old water heater as a buffer, and set everything up so I can bypass the buffer if I wanted and to shut off the water through DSH if I wanted as well.  I figure that if the buffer tank idea doesn't do any good or I don't like it, it will be easy to remove from the system and I could simply have the DSH plumbed directly to the new water heater with just the turn of a couple valves. 

Trust me, a buffer will work better then plumbing DSH into main powered water heater.

I suggest sizing the powered tank for your worst projected hour of hot water use and sizing the buffer tank for a day's hot water use.

An additional benefit of a double tank is, electrical capacity permitting, the ability to temporarily power the buffer tank during times of low DSH production but high hot water use, such as if guests are in town and staying with you.

I like that idea!

Since you chimed in, I'll ask you a question, engineer.  Would you suggest using the old, but working, water heater as the buffer or would you suggest installing a new water heater as the buffer?

My two cents...take the old one, drain it and flush thoroughly then set it up as your buffer tank. Buy a new one for your main hot water needs.

Off the top of my head I think I'd go with the old tank as the buffer and add the ability to valve it out completely

Reasoning:

1) When older tank does start leaking you aren't stuck w/o hot water

2) Newer tank may be better insulated, and much of the time main tank is warmer than buffer, so it is the better place for better insulation.

Aside from that, I might also place the older tank wherever it may most easily be accessed for changeout. You could fairly easily arrange it so either tank could become primary tank.

I have read several threads regarding the use and size of tanks for taking advantage of desuperheater.
I am contemplating a GSHP install and find another factor is the layout of the space available.
Though you don't have to place the storage tank close to the GSHP I would think the closer it is
the less heat loss you have.  Does anyone have a rule of thumb or formula that would give you
the distance between the desuperheater and the storge tank after which you can little or nothing
from the hook up?

I like the idea of the 2 tanks.  For 30 years  have lived with a heat recovery system on my air source
heat pump using a single tank.  If I was in the shower when the unit cut on I could feel the water
temperature drop (not uncomfortable, but you could tell).   I would think a buffer tank would eliminate
that possibility.   When I discussed this with the proposed installer he said he was recommending and'
installing an 80 gallong AO Smith Sun 80.   These units have a mid tank connection for the circulating
pump and a single element.   I talked to some owners that had this set up, and those with a couple
of teenagers found they needed the element on to achieve the duration of hot water they wanted.
Marathon makes a similar tank but recent price increases have pushed even large Marathon tanks
beyond my budget.  The dollar quote on the AO Smith tank was $650.   Would you still recommend
two 40 or two 50 gal regular electric water heaters?  (My current tank is 50 gal, serving 2 baths,
kitchen and laundry, while a second 30 gal tank serves a pool bath, outdoor kitchen and possible
2nd laundry ("mother-in-law" apartment that is currently not used).  Would 2 tank performance
be better than Smith Sun 80 specifically designed for use with alternative heat source?  How about
cost?

The Waterfurnace Envision units that are proposed have a 130 degree temperature limit on the
desuperheater.  The installer also indicated that he can set up these units to run when there is
either a demand from the living area or from domestic hot water.

IMHO:  If I am spending a lot of money on GSPH, I would (will) replace aging hot water tank.
            I am even tempted despite cost increases, to go with a Marathon unit, with titanium
            elements, given hard water in my area (Tampa, FL).

I'm not sure if you can buy a Marathon with the titanium elements already installed as a factory packaged unit. May need to buy the titanium elements separately. If this is the case, you could try using the original elements (suggested to me by Marathon) to see how long they last if you are unsure about your need for titanium. If you are sure you need the titanium and the elements you are replacing are the same size as mine, PM me if you want to sell them. :-)

You could run 1 (or more for that matter) DSH's into one buffer tank. I believe the buffer tank could then supply more than one water heater. We discussed this setup on another thread, possibly in another forum. Where am I again?