Hi, Interesting project...
So, I think I understand this:
1. Reed switch / magnet senses water is flowing. (Does this cycle on-off like a rotating magnet, or stay on??)
2. Thermistor senses output water temperature
3. SSR is cycled to control water temperature.
So the Arduino has a pretty simple job:
- IF Water is flowing enable the temperature control (Otherwise make sure no heating happens!)
- IF Temperature control is enabled, monitor temperature with thermistor, cycle SSR appropriately.
(This implies you have a way to set the desired temperature. Can be a simple external Potentiometer that you later calibrate.)
Or "His" and "Her" buttons? (You have to decide what to do if "Both"
As in that saying from the 60's "Save Water .. Shower With a Friend".
Later you can control your wood stove...
Let us know how it goes....