Hi CW, I'm building a HERMS and had similar questions. Digging through the various posts on this subject, it's my impression that measuring the temp in the MT to control temp in the HLT creates feedback problems because the HLT temp is likely to overshoot while the MT is coming up to temp. In other words, if you're shooting for 153 in the MT, the HLT is going to keep right on heating up until the MT reaches that temp. This may mean that you have to stop recirculating through the HEX to avoid having the MT go above 153. That, of course, would defeat the purpose of the HERMS (I've seen systems where there's a cutoff valve so you can recirculate without going through the HEX, but apparently you don't need this if you have the TC in the HLT). Anyway, these are my impressions. Others may have other, better, smarter explanations!