What happens when/if the H5i runs out of water (say for example a patient didn't fill the tub up completely)? What temperature does the plate reach?

What is the maximum heater plate temp for the H5i?
The heater plate is controlled by software that limits the maximum heater plate temperature to 65°C).  Additionally there is also a hardware circuit that shuts off power to the heater plate before it’s temperature exceeds 74°C.

What will happen if the H5i is run without a water tub or the water tub is empty?
Obviously without water the system will not add humidity to the air.
Running without a tub or water will not cause damage to the system as the maximum heater plate temperature is limited to 65°C.
The maximum temperature of the air leaving the device without a tub or water is 41°C and hence there is no potential risk to the patient.