Could you not just use the water jacket to maintain/adjust the mash temps? Where I am at there are more than one or two outdoor boilers with a few extra circuits to pull 180+ degree water off of. I am just wondering why to bother with HERMS at all with such a kettle.
I use a simular setup for making cheese using 180 degree water to heat the milk. Figured the same setup could be used for mashing.
Perhaps this discussion should take place elsewhere though.