degrub

Don’t forget surge protection (MOVs) on the AC feed to the property. Cheap insurance. Replace every decade or so or when the indicator light goes out.

And when it does go wrong with a closed kitchen tap, the bursting heater internal line or joint will spray whatever volume present in the unit of superheated water followed by hot water. Any flesh in the way of the water/steam mixture will get instant 2nd and 3rd degree burns. How likely depends on the details of the unit’s control design and built in safeties. That is what the thermal pressure relief valve guards against. Works the same for an ordinary tank type water heater with the only difference being the volume.