When we first introduced the Smart Recirculation Control tankless water heaters were not nearly as popular as they are today. Installing the flow meter on a tank water heater simply entailed disconnecting the flexible cold water hookup line, threading the flow meter onto the tank and reconnecting the flexible hookup line.
But tankless water heaters are many times installed with hard copper piping right up to the water heater not allowing any way to easily disconnect the cold water feed and install the flow meter. In these installations we recommend that the flow meter be installed in the copper piping with union fittings on each end to facilitate easy removal and replacement rather than threading it onto a NPT fitting and soldering it into place. Not that we have had failures of the flow meter, the failure rate is exceedingly low, but they are a mechanical device with a spinning magnet if there is an issue with debris getting stuck in the flow meter and it needs to be removed from the line, you will be very happy that there are union fittings to facilitate the easy removal.