Reading all this makes me glad that all I have to worry about is accidentally flooding the customer’s bathroom, kitchen or hallway. Lol.
Now on a practical level, why is there not an on-demand heater out there? Gas or electric models exist for residential use, and for exhaust heat exchangers, it seems a simple flow diverter to direct heat into the coils or to bypass them would be child’s play to fabricate. Similarly, a flow diverter could send water into an air-cooled coil to keep the heat exchanger temperature below any danger level, without dumping any water.
There.