Constand decay rate is not a real world situation, is it? Using dt/L and claiming it to be real world example would demand that the material surrounding the pipe drops in temperature exactly like the water inside all the way along the circuit, which would be a very odd situation imo.
For a counterflow heat exchanger I could possibly expect something similar to that result, but I am pretty sure it will never happen in a real floor.
As such, I am now even more firmly sure that the example is not a real world example, but a simplified model.
The part 2 paper does not as far as I can understand, measure repeatedly along the tubes to verify or disprove that a fixed dt/L is very close to, or equal to, a variable dt/L as a function of dt between water and air. Part 1 is more a theoretical model from what I understand. Unfortunately I cannot find a free version of it, and I will not pay for it just to see what it sais.
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