OK, you have to heat water to sparge with, right? What do you do with all that heat?  You just wait until the end, and pour it on the mash. Why not use that heat for the mash? This is easily accomplished, in theory, by having a copper coil running through the HLT. But, how do you control the temperature of the mash? Do you have 15 different  thermometers in different spots, and do complex calculations do determine how long to leave the ball valve to the coil open? Well, I guess you could, but that seems pretty crude to me (no offense intended if that's what your system uses). There are controls designed specifically for keeping a temperature at a set point, without severe overshooting.

But how do you go about doing this? All you're doing is cycling the wort through the hot water, and picking up heat from it. My design is to have solenoids switching the wort  between cycling through the HLT, and being diverted around it. The PID control determines the states of the solenoids, and controls the mash temperature accurately.