Well, after a promising start, I'm not sure the thing is working.
I plugged it in, and could hear & feel the coolant sizzling & bubbling in the tank, and within
10 minutes the coolant temp was up 20* F above "cold". Woohoo!.
But 10 min later, it was only up another 5 F. 10 after that it was
down a couple.
Not sure what happened (didn't have time to troubleshoot this morning).
Also I can't be completely certain that my cooling system is set up to do convective circulation through the coolant pump return line. Maybe it only works if the pump is turning.