If we are still talking instant here. Small diameter coil of soft copper bends easily without having to be annealed. Batch heating is no where near instantaneous. Physics has some crazy constants. Heat rises, volume matters, not only when we're talking heat exchange but changes from liquid to a gas state. My Toyotomi on demand water heater not only heats domestic water but also has a closed loop radiant floor heat system. If it's been offline (radiant heat portion) as it is in the warmer months it takes a bit of time to get back online it is not instant heat like a conventional forced air heater or wood. When working with hot water always think relief valve, better to know where it is going to pop-off than have an unintended failure in a surprise location.