How do Tankless / On demand water heaters heat so fast?
I have never understood how they can heat enough water instantly to wash my hands, much less heat enough water to fill a bathtub instantly.
If I put a pan of water on my gas stove and turn the flame to high, it takes quite a few minutes to bring that pot of water to a boil. (And that's just one gallon or water or less)
If I turn on the power to a standard 4500W 240V 30 gal. electric water heater filled with cold water, it takes a good hour or more to heat that water to 120deg or so.
A 30 gal gas water heater, heats faster than electric but it still takes at least a half hour if the water is cold to begin.
I have an electric element called a "bucket heater". It uses 1500W at
120V. It's made to heat a 5 gallon bucket of water, in a barn or where there is no hot water at the tap. That thing takes over an hour to heat
5 gallons of water to around 100 deg.
With that said, I cant understand how a "tankless" water heater can possibly heat enough water INSTANTLY to fill a bathtub, or even a half gallon to wash my hands. Whether they run on gas or electric, how can they surpass the speed of heating water above any of the methods I mentioned above? (And do this continually)?