There are two issues with showering on Shabbat.
- Bishul - heating up or causing water to be heated up.
- A Rabbinic gezera of bathing on Shabbat.
If the boiler is set below Yad Soledet - 130°F then there’s no Bishul problem.
If the bathing is not for pleasure (you’re uncomfortable because you’re sweaty…) then there’s no gezera by mitztaer