Hi guys This is my first post here. I studied physics about 15 years ago and some weeks ago I realized that I'd forgotten most of it. So these days I'm playing around with physics just for the fun of it. So, I got to the subject of heat transfer, and though I would like to learn some more than what's in my old books. So I thought of the following problem: Imagine you have a tank of hot water and a tank of cold water. There is a tube going into the bottom of the hot tank, coming out on the top of the tank, then it goes into the top of the cold water tank, coming out at the bottom of the tank and then back to the bottom of the hot water tank. In this tube water is circulating. The water circulating in the tube is moving heat to the top of the cold water tank, while it's also "moving cold" from the bottom of the cold water tank to the bottom of the hot water tank. Now, what I'm curious about is: How can the temperature at any given point at any given time in the tube (or the tanks) be calculated?