I built this exchanger to regulate my HERMS temperatures. Basically, there are five one-inch copper pipes at the bottom, each has a 4500W water heating element in it that I run at half voltage. (I think it totals around 6000W in the wiring configuration I’m using.)

The heat exchanger water is heated in those pipes and pumped into the outer tube of the Chillzilla counter-flow wort chiller above it (they work so darn well chilling the wort, why wouldn’t they work as well heating it up?), which the recirculating wort is counterflowing through.

The white PVC at the top of the down pipe is just a little reservoir for the exchanger water. The heating elements are controlled with an Omega Engineering CN9000A and some big solid state relays, and the temperature is read at the wort outlet of the mashtun.
The pump moves the exchanger water through the tubes and up into the outer coil of the exchanger, which then gets dumped into the reservoir that drops back down into the pump.

It’s kind of an expensive setup since it requires two pumps, and a second Chillzilla, but it is capable of bringing the mash from dough-in to 154 °F (68 °C) in less than ten minutes with absolutely no risk of scorching. Not that that’s always what you want, but it can.