I have read threads on using 1 standard hot water tank vs 2 when connecting a GSHP desuperheater to your DHW.
 
I plan on replacing my old air source heat pumps soon, and with GSHP alternatives would include a desuperheater.
(the current and replacement installation in west central FL, described in another thread, have a 3 ton unit which serves master bedroom,
bath, second bath, kitchen, laundry, dining, and living room, while the 2 ton unit, at the other end of the house, serves
3 bedrooms, pool bath, outside "kitchen", and is plumbed for a 2nd or relocated laundry room).

Both existing electric hot water heaters are near the end of their lifetimes, so I will be replacing them as well.
One is 50 gal and the other is 30 gal.   There is no hot water line between the two sections of the house.
I seldom have a problem with having enough hot water.

One GSHP installer is suggesting using a single tank water heater purposely designed for use with solar systems.
These tanks have separate connections for the solar panel, or in our case the desuperheater.  They also have a
single (optional) heating element which, from the illustrations appears to be positioned about midway up (down)
the tank.  AO Smith has an 80 gal unit, model SUN-80, while Marathon sells an 85 gal unit, MTS85200.  Both companies
have larger units but only Marathon has one smaller, 50 gal MTS50200.  Marathon suggests using a 3800 W Titanium
heating element, if you add an element, but they don't say they sell it.

The questions are:

Would one of these 80 gal purpose built units perform better than 2 standard 40 gal hot water heaters where one is
not active and serves only as a buffer tank for the desuperheater?

The Waterfurnace Envision series has a 130 degree limit on water into its desuperheater, assuming long enough run times that
would heat all 40 gal in a buffer tank to 130 while leaving the 40 gal of water in the active tank at its thermostat setting (120).
Would a single 80 gal tank reach 130 hence utilizing more of the heat available from the GSHP?

With only the pool bath and outdoor kitchen using hot water from my second unit, is it worthwhile to even install a desuperheater
on the 2 ton unit?  If yes, then what size single tank storage should I use, or if a separate buffer tank is better what should
the size of the buffer and active tanks be for this second unit? 

Are there other manufacturers of purpose built single tank water heaters like those I described?

Anyone know where you can get titanium water heater elements?

Thanks for any feedback.
Jim

Have you read the thread "Justifying two electric hot water heaters"? For BEST performance, get a buffer tank. Do you NEED BEST performance for the amount of hot water that you use? Given the additional cost vs payback, extra space requirements, additional potential for leaks, etc., we decided on a single tank system.

We have a Marathon MR85245 (it has upper and lower elements) that works great for us in a single tank setup with 2 DSH's plumbed to it. I don't know what the additional cost is, and what the additional benefits are, to a tank with extra connections. Last time I checked, Marathon sells titanium elements (upper and lower) for our water heater. No idea why they don't sell them for the model you listed. I don't see any reason why you couldn't get a Marathon without the extra connections and get the titanium elements if you need them, and still be able to connect them any (proper) way you want.

With a single tank, you can reduce the lower element setting to 100f. You may need to increase this setting during shoulder months (if FL has shoulder months) when your systems don't run as much (not a big deal to me, but may be to some). If your hot water usage is at predictable times, I have, use, and love our digital water heater timer. It works great with the low heat loss of the Marathon. With this setup, I keep both elements at 120-130, and set the timer to finish the tank (and then turn off) just before anticipated hot water use. Works wonderful for us.

If your hot water supply is currently sufficient for your needs (and you don't mind making an occasional adjustment), I would consider getting a 75 and a 50 gallon Marathon (a little more capacity than you currently have due to the new setup and settings), keep the locations the same, and plumb 1 DSH into each water heater (2 DSH's total).

If you need the best performance and don't want to ever mess with settings, you should consider 2 buffer tanks (one for each water heater), or 1 buffer tank with a "T" that supplies both water heaters, or 1 buffer tank and 1 water heater. You'll have to decide what is best for you given tank cost, water usage, plumbing cost, etc. If you like the benefits of a Marathon, and can swing the price, don't let anyone talk you into getting a less expensive steel tank.

Make sure the Marathons will fit in the space you have. They are bigger than conventional water heaters due to the additional insulation. Let us know how you make out.

Geome,

I've just read the "Justifying two electric hot water heaters" thread.

Having somewhere to store the heat pulled from the desuperheater would seem to be the overriding
parameter in the efficiency equation.   But then if you don't use it (the hot water) all the costs of
capturing the energy are for naught.  Seems like one has to find some correlation between a home's
hot water use and how much capacity you build to store the energy collected from the desuperheater.

In my home the hot water usage pattern is fairly random.  In addition electric rates do not vary by
time of day (Progress Energy rates are .11 / kwh for the first 1,000 kwh and .13 / kwh over 1,000 kwh.
I.e., timers don't help much.  I really don't know precisely how much hot water we use.

2 40 gal tanks - my thoughts were:
   a) good - little or no mixing of cooler water with heated due to desuperheater circulation pump
       (my current ASHP has a heat recovery unit and if the unit comes on while showering you can tell)
   b) good - no heating of the additional water by the resistive elements
   c)  not optimal - there is only the amount of water in the buffer tank to capture heat from the desuperheater
   d) bad - I like the idea of a Marathon needing less maintenance but the cost of 1 tank let alone 2 is too high

1 80 gal purpose built tank
   a) good - separate plumbing for desuperheater circulation reduces mixing with resistive heated water
   b) good - 1 mid tank heating element less effected (than a single standard tank) by circulated water
   c) better - more water to store energy from desuperheater
   d) good - maybe lower cost for material and labor (could be said for single standard tank as well
   e) bad - 1 mid tank element does not have much "recovery" capacity when usage really spilkes.
   f) needed - if desuperheater can raise entire 80 gal to 130 degrees a tempering valve is needed on HW out.

1 80 gal standard tank
   similar to purpose built tank, but more mixing of water due to circulation pump, but I like the idea
   of bottom element set at 100, though in Florida it might never come on, nice to know there would
   be a lo probability of very low water temps at HW out.

 I also thought of a tankless unit + buffer tank, but I don't have the circuits available in my panel
(an no NG) hence the cost would be prohibitive.

Jim
  

Yes, once a buffer tank, or a single tank, is at capacity the hot water needs to be used to make room for additional DSH heating of the hot water. Since April 2009, I recall only one time that our geothermal unit indicated that our tank was at 130 degrees and shut off the DSH. I reduced the timer settings in 30 seconds, and the four of us were happy (geothermal system, DSH, water heater, and me!)

So, in 9 months, the DSH has always had a place to put hot water from 2 DSH's.  Of course, geothermal run time needs to be factored in as well.  Our cooling season was relatively mild this past year.  It was only this winter (a cold winter for us) that I finally got to see what the DSH's can do.

Unfortunately, we don't have time of use electricity rates either. The timer allows plenty of time for the DSH to work without the heating elements going on. It allows more "room" for the DSH to put water it heats. We have both heating elements set for 120-130f.

Our hot water usage is relatively low. We sized our Marathon to be larger than we needed to allow for more cool water at the bottom of the tank while keeping more hot water at the top for use. It may be difficult to estimate hot water usage, but it is possible. For example, you know how long it takes to shower. The maximum rate of the shower head can be found (possibly 2.2 or 2.5 gallons/minute.) Maybe mix of 75% hot to 25% cold water used, and so on.

I'm not really sure that a Marathon is less maintenance. For us, the benefits are the lifetime tank warranty, very low heat loss, and knowing it won't leak from rusting out. With high hot water use, I'd consider a buffer tank.

Will you have an open pump & dump or closed loop?

Until someone comes up with a better idea, I recommend sizing the primary tank for the household's worst hour of hot water use, and sizing the buffer tank for a full day's use of hot water.

That's question is still being debated.   I am looking for a lab to test my water quality for use in an open loop system.
I have half of that loop in the form of a 4 in well into the aquifer.  This well is currently used for irrigation and would
continue in that service while also being used as the dump for an open loop.  A new well would have to be drilled
for the source, but that one well would be cheaper to install than the proposal for a vertical closed loop.
Well water temperature is 76 degrees.

Thanks Engineer, that sounds very practical.  I need to do my homework and see if I can determine those values.

A little more data to look at.

A 3 ton unit DSH will put out a little over 3kbtuh with 76F EWT in cooling mode.

A 2 ton DSH is a little over 2kbtuh same conditions.

Good information.  Given WF manual has "partial" (stage 1?) btu about 70% of "full", would the heat output of the DSH be proportionately lower?
Say, 2.3K btuh for the 3 ton at the lower speed, and maybe 1.5K btuh for the 2 ton (2 stage compressor, Envision models 038 and 026)?

The measurement I took today (01/26/2010 @ zip 34683) surprised me.   The potable water from the county water system was 66 degrees at
my kitchen tap.  So we are looking at a 64 degree rise for water added to the buffer tank (130 degree DSH cut off - 66 degree inlet temperature).
That's a little more than 500 btu per gallon (make up for hot water used).  In one hour of operation of the 3 ton unit at stage 1, just 4 gallons
would be back up to the 130 gal max temp, and more usage and the buffer tank temperature would still be below the DSH cutoff. 

That provides some perspective.  Hopefully the units wont be running continuously, and I certainly don't expect that the DSH would cover any
demand over an hours period.  Per Engineer's suggestion on sizing I am trying to determine my peak hourly hot water use, and an average
daily hot water use.  With those numbers I'll need to have an idea of unit run time in order to have a handle on how often I might expect the
active tank (assuming a 2 tank install, one active, one buffer) to be consuming power from the grid.

Thanks for the information and taking the time to respond.

Yes - low stage reduces DSH output, in fact by more than 1/3. That's because both refrigerant flow and dishcharge temps fall in low stage. Note that published DSH heat transfer rates assume 90F water into the DSH. Actual DSH output will vary substantially with different entering water temps.

You can browse WF's site and download manuals with extended range operating condition tables detailing the power used, heat transferred, and hot water heat available

In fact the system won't in 1 hour's operation heat 4 gallons by 64 degrees. It is more likely to heat 64 gallons by 4 degrees, depending on plumbing arrangement. Each pass through the DSH coil adds a few degrees to the water, on the order of 5-10.