There isn’t much about water heaters in Home Assistant. I’ve recently worked out I can add a wall controller to my hot water system (Rinnai) rather easily.
Unfortunately nothing ‘smart’ is available, so what I’ll be doing is creating my own interface with an Arduino, which effectively just presses the buttons on the wall controller. I’d like to use MQTT to interface Home Assistant with this Arduino. From there I can program it to track and control the temperature (i.e. when I’m in the shower, drop the temp to X degrees).
I’m sure I can get around this for now using other components (probably MQTT climate), but it would great to be able to make use of the actual water heater component.