Quote Originally Posted by chem geek View Post
The faster flow rate will heat more quickly, but not by the factor of the flow rate. It's simply more efficient to transfer heat by a small amount that keeps the temperature differences small. A slow flow will be less efficient because the water gets heated hotter and therefore accepts getting more heat a little less (due to a smaller temperature difference between the flame/pipe and the water) and also loses more heat down the pipe (due to the greater temperature difference between the water and the pipe/ground).

However, I don't know how big a difference this is and you are welcome to experiment with both flow rates and let us know what you find out!

Richard
HOORAY! I couldn't have said it better!
In fact, I couldn't have said it nearly as well!