'it "makes the most sense for use with "on demand" hot water heaters," which is absolutely true; anything that reduces the temperature difference between what goes in and what goes out will save a lot of gas.'
---This is not exactly true. The vast majority of on-demand gas water heaters use a fixed flame - when the flame lights, it always burns the exact same amount of gas per minute. The incoming water temperature does nothing to change this. In my house I have an on-demand gas water heater designed for use with solar thermal pre-heating of water. This heater modulates the amount of gas burned depending on the temp of the water coming into the heater to ensure a constant 60 degree Celcius water temp output. This is the kind of heater that would be needed to make good use of this recovered waste water heat. However, I had to special order my heater from Junckers. They don't just sell it in any store. You can't walk into a store, look at a model hanging on the wall, and walk out with it. They are sold, but almost always they have to be ordered and one has to wait several weeks or more for it to arrive. The OVERWHELMING majority of on demand gas water heaters here in Europe and in the US are NOT designed to be used with pre-heated water. So the waste heat picked up and fed into these demand heaters ends up simply making the water hotter than what it would otherwise be - which also ends up being a form of waste.

You know, these things have some theoretical merit to them, but real world is another story. The amount of BTUs they scavenge from the drain water is so totally minuscule, it will never offset the embodied energy in making that copper heavy device.

Remember, most of the drain water is not touching the walls of the pipe, and it is also moving very quickly.

Some years from now, these devices will be looked back upon as silly gizmos that took more energy to produce than they ever saved.

Willy: Yes, I'd love to have one of these, but it's hard to be convinced that it's worth using that much scarce copper for it. Of course it depends on how many showers it gets--certainly in a gym or dorm it makese a lot of sense. A real analysis of the energy savings vs. the environmental impact of the copper use would be valuable.

On the match with on-demand vs. tank heaters, I think that's a red herring: a tank heater benefits just as much as a modulating tankless heater; and as Houston explains, many tankless heaters don't benefit from it at all.

There are electric on demand water heaters that do modulate, but can't recall the brand at the moment. The one I have does not.

One could always install a tempering valve to solve the problem.

That design looks very inefficient. A tube-in-tube heat exchanger would have been a much better design. Thin walled aluminum also could have been used with a minimal drop in performance.

I agree that this device would take a long time to pay back its embodied energy in an average home and makes more sense in commercial applications.

Because of water's adhesive properties the majority of it does run down the sides of the pipe, not falling down the middle.

Copper is most likely used because of existing plumbing codes, which rarely allow aluminum pipe. (Although aluminum is not as rare or expensive as copper is is also very energy-embodied)

I would like to see the thing wrapped in insulation too...

I decided to run the numbers on payback. It's way better than I expected! For a very miserly shower by a single user, I calculated a 2 year overall environmental impact payback for one of the smallest ones; 4 year for a big one. I used the Okala impact factors.

I think that the reason this is better than our intuitions indicated is that showers are way more energy intensive than they seem. I think I'll get one after all! And then invite my friends to shower here to make maximum use of it.

To clear up a few items of confusion:
Drain-water heat recovery is not now--is been used and tested over ten years. ENERGY STAR permits adjustments to the energy factor used to calculate HERS scores on new homes built with DHR. On-demand or tank isn't that important here--was is is simultaneous use and discharge. That is water is running down the drain while you're pulling it out of the tap. Showers are an example of this. Dishwashers and clothes washer and other batch uses don't work--the inlet water isn't pre-heated because there is nothing going down the drain as it comes in. You can find a lot of studies on these. Start with the GFX that has been around for longer. A summary from Home Energy magazine from 10 years ago:

"The GFX system has been through five technical evaluations in different studies. Of these, the most extensive and most recent was done by Old Dominion University in Virginia. The system was tested with three types of electric water heaters. Under three different usage schedules, GFX preheated supply water by 20°F-30°F, depending on inlet temperature, at flows of 2.5 to 3.0 gallons per minute (gpm). Total energy savings by the GFX ranged from 47% to 64%, and averaged 57%. The GFX increased the EF on each tested heater by 57%-73%.

Researchers caution that actual savings depend highly on whether the home tends to use batches of hot water that are drained later, as in the case of washing machines, or whether the big users drain continuously, as do showers. Other factors that can affect savings are the cold-water inlet temperature and the difference between supply and drain flow rates."

Look for other credible sources including DOE, LBL, EPA, RESNET, etc.

I would absolutely put one in any new construction project. It's sometimes more difficult in existing homes because they have to be installed vertically--that can require some demolition and reconstruction

Houston isn't exactly right on how water drains--water doesn't pour down the middle of the drain--it tends to go down in a thin film on the walls--perfect for this application.

As Griffin's payback numbers indicate, these do make sense. Not a fad or silly gizmo.

Lots of little steps can take us a long way in the right direction.