Bosch on Demand water Heaters
Got mine at Home Depot - they work, but the gravity pressure problem is real, and I had to buy a booster pump ($1800 pesos) which has to be turned on and off by hand (supposed to be automatic, but isn't).
Then a guest forgot to turn off the booster pump, so had to buy a new one (pump, not guest) - the part that 'blew' was the impeller, a plastic gizmo that looks like it costs about 50 cents, but isn't available here (or anywhere else, either, apparently) and not available from the manufacturer's rep in Nuevo Leon, or where-ever, or from the manufacturer in Denmark (world's largest pump maker!), so had to buy a new pump (another $1800) just to get the 50 cent part - now have two pumps, one working impeller.
Frankly, the whole thing has been a pain in the ass, overall (the concept, however, is great). Makes the expense of gas for a water heater pale by comparison. Oh, and they silt up with the calcium scale in the water here (but so does everything else, so that cancels out - sort of).