I usually use an average number of 2 kPa friction loss for normal tap water in pipes. Does anyone know if this number changes significantly up or down for water which is around 80 degrees C instead of 5?
Well, you're looking at only a 3% change in density but a factor of 4 change in the viscosity. Looking at Reynolds numbers (with a constant velocity and ID), this results in a very similar friction factor. I get about .01 vs. .012 for a smooth pipe. I didn't really go any further with the numbers. It looks though like you're not going to see anything noticible. You might do a couple of quick calcs adjusting your density and viscosity to match the two temps.