I added a safety interlock to the instant hot water shower system.
This heater has such a huge delta T, it is possible to reach dangerous water temps almost instantly. And because my kids will be using this shower I wanted a very high safety factor.
Using a Watlow 96, I measure the output temp of the shower water and if the Temp exceeds 125 deg F (adjustable) the water pump is shut off.
Once the water temp returns to an acceptable level a manual reset can be allowed to restart the pump.
The Water heater has its own safety that shuts the propane gas feed off if no water flow is present. Once water flow is reestablished the heater self ignites and hot water is produced.