[jeff5may]

Since you already have your water heater chosen, why not use a similar volume of water in your trials? A Styrofoam 12 can cooler is close, and is like a dollar or 2. You may already have a little playmate cooler lying around somewhere already to use. That way, you can see all about recovery rate, temperature maintenance cycle time, etc. before you rig a hx in your new tank. Every startup cycle, the rig has to build up heat and pressure before any useful heating begins.

With this rig especially, you may not need the unit to start immediately when hot water demand initiates. For larger (longer) flows, the local tank will only see as much cool water is in the supply piping before hot water arrives from the main water heater. In this situation, the local tank will short cycle and waste energy. A delay-on-make timer would be a wise investment if the local tank is connected to the hot water line. If it feeds from the cold water supply, this doesn't matter, as the thermostat will trigger and stay on until the cold water is heated.