Hot water pipe with low pressure

[Kenuel80]

Looks like your heater is fed by four separate breakers in the breaker panel.

Unless they are tied together with a handle bar, they can trip separately.

So turn each one fully off, then back to on.
In case one has tripped.

Yes, I checked them all individually and they all are sending power.

 

[Jeff Handy]

Yes, that little fuse just protects the brains, which controls other low voltage stuff like relays and sensors.

Besides the elements scaling up, whatever chambers they thread into will also get scaled and lose their heat transfer ability.

So the two valves idea is good, stainless lever handle ball valves.
Compression coupling valves are easy to install if your piping is copper.

You need a way to pour in straight vinegar and let some air out, so it gets to everything for a few hours.
And even better if it can be flushed out with at least gravity pressure, like pouring clean water through a funnel into a hose, and out to a bucket, or an adapter fitting on the first and last element threads.
Don’t get the controller wet.