I had a Rinnai tankless water heater installed a few years ago and love it. It has a circulation pump built in that you control through schedule or a manual button on a display that I have mounted on the wall in our master bathroom. I automated this with HE via dry contact relay that “pushes” the button and then have a temp sensor and power metering plug to monitor power of the heater to determine on/off state. This mostly works but there are many edge cases where power spikes when the pump is off because of cold temps outside or laundry is consuming hot water, etc. So in some cases I cannot detect the proper state.
I saw on the ST community that there is an official integration now via an optional Wi-Fi module. I see that it also has Zigbee but unsure if that is to integrate with their own motion and other sensors.