Don't think that will work the way you want. Tankless water heater depend on a flow sensor to turn on and off so the way you want to set it up would require you to pipe in cold water to the loop before it gets to the TWH. Because of that, you run the risk of scalding because of the water not moving in the pipes and overheating, not to mention the PVC or PEX melting because it exceed the design temps. You would be better off with a drainback system going into a storage tank with the cold water supply for the TWH looped through a heat exhanger in the tank.
Another thing to consider is that tankless water heaters have a MINIMUM tempurature rise before they come on. If your setpoint is 120 degrees then the max incoming temp necessary for the unit to run is generally 40-45 degrees lower than that. Thats because the minimum BTU input is about 12k. That means that at minumum flow the outlet temp on the unit won't exceed the setpoint.
At the moment I can't think of any setup that you can use with a tankless water heater to cut down on energy usage.
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