If the level dropped below the top of the cylinder, then it would probably merely get a bit noisy.
If the level dropped below the overheat sensor (if it still has an active one, because many have been bypassed), then there would be a risk of the water boiling.
If the level dropped below the top element, then that element would probably overheat and go faulty in a burny/melty kind of way.
If the level dropped below the primary flow take off, then the pump would bugger itself. At this point you'd probably notice that the water coming out of the hot taps is cold.
If you didn't notice, and the level dropped below the bottom element, then that element would probably overheat and go faulty in a burny/melty kind of way.
If you didn't notice the smell at this point, then nothing else would happen.
... which is grammatically correct.
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You say that you've read the requirement to add an inhibitor in the "instructions", but there's no such requirements in the Pulsacoil 2000 instructions.
You say that an inhibitor will prevent corrosion, but you've neglected to explain what you think would corrode if a dose of inhibitor weren't added.
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