You can certainly do it -- but it's a lot of watching and controlling. A better approach might be to watch the temp at the HEX-out (as opposed to the HLT temp). This, too, you can do manually, but a Ranco temp controller makes it pretty much effortless -- set it and forget it.
I found that my temps were much more accurate when I did three things with my HERMS:
a) Recirc the water in the HLT.
b) Install a thermometer near the HEX-out (so I know that my temp in my MLT will never go above my HEX-out temp). In my case, this meant installing the Ranco probe (via thermowell) right where the HLT tubing goes into the MLT (through the lid of my Blichmann 15 gallon). There's a 1 degree differential between my HLT water temp and my MLT-in temp. (This used to be 5-7 degrees when I didn't recirculate the HLT.)
c) Automate the HEX-out monitoring with a $99 ranco. I realized that I really don't care about the HLT temp, so long as my HEX-out equals my mash-temp. So if I want to mash at, say, 154F, I simply set the Ranco at the HEX-out at 154F, and I'm done with it. I know it won't ever go over 154F and I won't have crazy temp swings. The Ranco turns on and off my electric element in my HLT to maintain my mash temp. This also means that I never have to worry about calculating a differential between the HLT and MLT. I don't care about the HLT temp, and I don't really care about the internal MLT temp. What I really care about is the water coming *out* of the HLT and into my MLT -- that's it. It makes it wonderfully simple, actually. I said this before, but I'm surprised more folks aren't turned on to HERMS for this very reason.
And the fourth thing -- to preserve my sanity -- was to forget about monitoring the mash. So long as the HEX-out is where I want to be, I don't really care about the mash itself since the top is much hotter than the bottom. Sure, I occasionally check it with a thermopen, just to make sure everything is calibrated and my Ranco isn't reporting something totally wacky, but I know that with a thicker mash especially, the temp at the top of the mash may be much different (hotter) than the bottom. A thinner mash is more accurate throughout -- but it's still not uniform all the way through. I usually mash for 90 mins.
I recirc the mash (MLT --> HERMS coil in HLT --> MLT) for the entire length of the mash (with Pump 1) and rercirc the HLT (HLT-->HLT) (with Pump 2) for the entire mash.
