I have no idea how modern tankless hot water heaters work, HOWEVER,
64 years ago (1960-1961) when I lived in a suburb of Nicosia Cyprus,
My rental house had a tankless water heater that heated kitchen sink, shower and bath tub. Water entered from bottom, in copper coil. When water turned on, natural gas flame lighted in center of coil. Faster the water flowed, higher the gas flam. Worked great the entire two years I lived in Nicosia.
Again, don’t know how modern units work.
64 years ago (1960-1961) when I lived in a suburb of Nicosia Cyprus,
My rental house had a tankless water heater that heated kitchen sink, shower and bath tub. Water entered from bottom, in copper coil. When water turned on, natural gas flame lighted in center of coil. Faster the water flowed, higher the gas flam. Worked great the entire two years I lived in Nicosia.
Again, don’t know how modern units work.