The apartelle came with a hand lettered sign on the bathroom door with instructions on how to use the on-demand water heater: Turn on water, then turn on heater. Later turn off heater, then turn off water. The same instructions appear on the heater.
One day, after rinsing off with the heat on the lowest setting, I turned off the water without turning off the heat. While shaking water off my body the shower head gurgled, spit and then spewed. Lucky for me there is just a curtain between the shower and the rest of the bathroom so I was able to exit without getting near the HOT water. Once safe, I reached in to turn the water on high before turning off the heater.
With heat on and no water flow, the water in the heater box heats up rapidly, building temperature and pressure. No engineer would design such a system without a pop-off safety valve, otherwise the heater would blow up from the steam pressure. On-demand in-shower water heaters use the shower nozzle as the safety valve. I can see the engineers point of view, it’s better to scald the user than pepper him with shrapnel.