Originally Posted by flananuts
That's very helpful. So it safe to assume that the return temperature of the circulating wort is less important as the average grain bed temp? My thought is I just set my PID to hold temp at 159 for an average grain bed temp of 156 for example.
Yes, IMO that is the best way to do it. Let the grain bed temperature govern. The optimum difference between the two will depend on the system design, what you are trying to do and the ambient conditions. A number of variables must usually be considered. The heat source, flame intensity, circulation rate, insulated or not, ramping up or simply maintaining temps etc, etc, must all be considered and only you can figure out what works best for your system.