Oilpan,

I am designing the complete energy package for them (geothermal heat pumps, radiant flooring, PV solar, etc) and the new home will have a total of 600 amp service. Current to a tankless electric unit is not a concern.


Steve

Then 5kwh a day sounds like a good estimate even if preheat doesn't work as well as we would like to see.

OK, so for the cheapest rig in the world, just buy a recirculator pump without a timer. Buy a clip-on snap-disc aquastat and clip it onto your return line. Wire the aquastat in series with the pump. Obtain power from the light switch in the room nearest the end of the hot water line. Tada, enjoy. Not quite as precise or adjustable as a valve-style aquastat, but it will do the job.
http://s3.supplyhouse.com/images/pro...om/563-2-1.jpg
563-2 - Taco 563-2 - Temperature Aquastat for "00" Series Circulators (95°F - 115°F )

I got some normally closed 45°C type KSD301 thermal switches. They are cheap $5 for 5 of them.

The cheapst listed hot water recirculating pump I can find is the watts pump at lowes.

If cheap is the goal, go with what Mike said earlier. A brushless DC motor wired to a wall wart will start out around 20 bucks. A real, plumber approved, industry standard recirculator pump will begin above 100 bucks. Make sure if you get a cheapie that it can handle the temp and pressure.

Yeah I'm going with the listed pump.
We have up to 100 psi of water pressure here and a leaking pump could cause thousands of dollars in damage so I don't mind spending the $ on an appropriate pump.

Here's a high pressure Laing D5 going for just under $180.
https://smile.amazon.com/Solar-Pump-.../dp/B007ETI63I

I always tell my customers who are considering one that it's not the unit that saves that much money or energy, it's how you use it. If a teenager can take an endless shower they will!