Connecting a Marathon to a desuperheater (DSH) is straightforward. Typically the cold water line close to the inlet of the water heater has a "T" installed. The water goes into the desuperheater from this point. The DSH's output is connected to the water heater through another "T" installed near the drain valve at the bottom of the water heater. The drain valve will still remain and will still be functional. There are a few shutoff/bleeder valves installed too. I may have missed something, but that's most of it except for the piping.

I would buy a full port 3/4" ball valve with a hose bib connection to use as an upgraded drain valve at the bottom of the water heater and have it handy for the installation. This valve makes it easier for any debris in the water heater to be passed out when the water heater is drained (this valve has a larger internal opening than a standard boiler valve). Unfortunately, we could not find one at the time of our installation, but I have since bought one at Lowes for about $8.00 I believe. Just waiting for a good time to install it.

We really like our water heater timer, and it works great for us. If you get just one tank, I would recommend the digital variety of water heater timer over the older style (looks like an old style lamp timer, but it's designed for water heaters). If you can program an alarm clock, you can set this timer just as easy. The digital model gives us multiple program settings, and keeping the DSH in mind, we program it accordingly. Engineer is correct with the points he made, but most of them are not relevant to our situation. The bottom line is that having a buffer tank is better and requires little thought to get the full benefit of the DSH. With no facts available, I believe we are getting 75% of the benefit of our desuperheater (the way we use the timer and with our hot water usage pattern) over the course of a year compared to having a second tank. The same will not be true for everyone. The ballpark 25% we are giving up is fine by us as we do not want a second tank taking up additional space.

For what it's worth, I recall reading about someone that had a Water Furnace storage tank in another geothermal forum that lost about 10 degrees over night.  I have no idea if this is accurate or not, or if the unit was installed or used correctly.  The Marathon claims a 5 degree loss in 24 hours.  I would find out what WF claims their storage tank heat loss is if this is important to you.

All you do to use a standard electric tank for buffer is remove the nipple containing the backflow preventer on the cold inlet since the desuper withdraws water at the cold inlet to the buffer tank. the desuper is plumbed to inject water into the buffer at its drain, and the buffer's H outlet goes to the main tank's C inlet. Look for Bergy's sketch here

Dkubarek,

Being on a tight budget and still needing a very well insulated building, you might want to price cellulose instead of fiberglass.

I agree.

Here is a link to a site that makes most of the avail desuperheaters. If you poke around there are some good diagrams and they also sell valves that replace the faucet. My desuper heater is piped from hot water side to faucet at bottom with a single tank. If I add a tank I will just use the existing DHWH as a buffer. I will install a 2nd tank fed from Hot side of the existing tank into cold of new tank. Which will feed house from hot side. I will move electric to second tank and have no power to 1st. The timer could be used but I think it is unneccesary. The point is to feed the powered tank with water that is hot enough to not kick on the elements in the 1st place. I currantly have a 50 gal whirlpool DHWH with a smart circuit that learns when you use water and keeps it hot at those times. I think it works well even when the DSH was turned off.

http://www.trevormartin.com/hru.asp

I agree. I would not recommend a timer with 2 tanks. A timer would only be of benefit with 1 tank, and even then it may not benefit everyone in every situation. A second tank should - if both tanks are sized correctly, installed correctly, and the buffer tank is not powered (except during the infrequent occasions when more hot water is required).

Interesting points about the insulation. I thought Rs were Rs but I see how performance can make a big difference. The company uses blown fiberglass insulation and the GC praised it because it doesn't contain fireproofing additives found in cellulose and thus does not settle. What do you guys make of this?

Also, I'll probably go two water tanks. 80 gallon (nonpowered, Rheem, comes with house) and 66 gallon Kenmore 12 (because of pricde, R24 foam insulation).

I noticed some models offer what appears to be a smaller heat pump that's dedicated to water usage. Runs like an air source heat pump water heater. Is this a viable option for water heat, too?

Anyone see the Rheem HP-50? Comes out soon. Not sure on price but that sounds like a decent option. You can run air source heat pump only or switch to use the electric elements. I could buy one of those instead of two electric water heaters and power it down in the summer (or not if the heat pump is pretty efficient.) Says it costs about $250 a year to run. Sounds very efficient to me.

Been busy not ignoring you...
Doc case was water source heat pump (open loop, 2 stage). Check valves are not common place in DSH installs (with one heat pump). Circulators do not completely stop flow when turned off. Line loss was one of the possible problems.....

If I sounded like the "holier than thou" pro on my last comment, it was unintended I was using my Blackberry with big lumbering thumbs (causing un intended tersness). Yours and a few others work well without buffer. We've noticed R-22 (vs R-410) is not uncommon in these success stories (but not exclusive). Mitigation of flow (when heat pump not running) would seem to be most important.
Truth be told once I find a way that works in every application I spend little time or energy studying others.
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