We live in Texas, we’ve had Geothermal for 20 years, and we’re happy with it. The unit generates plenty of hot water in the summer and winter, but there are a few weeks in the spring and fall when the system doesn’t run, and we get no hot water. To handle that, we have a pair of 50-gallon water heaters arranged in serial. The back tank is just storage. The front tank is storage and fill-in heat for the spring and fall.

We use propane for the hot-water heater and for the fire-starters in the fire-places (about 5 fires/year). My annual propane bill is about $300, so I’ve got very little to complain about. Well, there’s always something, and it this case, it’s the fact that the propane tank needs to be replaced, so I’ve started thinking that if I can switch our supplementary hot-water to electric, I can remove the tank entirely.

I thought about a conventional electric water heater, but the water heaters are in the attic, and getting a new tank up there is a substantial undertaking.

It would seem to me that the best thing would be an in-line tankless water heater.
* The incoming water temperature would be between 60 and 80 degrees
* Our well pump can only supply 5 gallons/minute, so there’s a natural limit on water use and multiple simultaneous showers.

The problem is that the in-line water heaters that I’ve found in the US seem to be triggered only by water flow, not temperature. I want something that I can leave on at all times and not think about. From a safety perspective, I certainly don’t want a unit that puts a 40 degree rise on top of 135 degree water.

So, I’m looking for
* In-line hot water booster
* Electric
* Temperature and water-flow triggered
* Enough to handle two showers at a time.

Of course, I’m open to other suggestions as well.