If you're worried about an element touching the wort directly, use a HERM coil. Going with a double-jacketed system for mashing seems like a ton of expense for something that probably doesn't warrant that much complexity.
jmo, of course.
EDIT. Sorry. I misread. You want this for the boil. I still think double-jacketed for heating is overkill.
Muddy Creek Brewery
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Skinny Cow IPA: In India, cows are sacred, that doesn't mean they feed em...
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No Paddle Amber Ale: Up the creek with No Paddle!
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