The HLT is basically the same as an electric water heater. I have a converted keg, with SS couplings welded on in 4 different places. One is for the exit, two are for the heat-exchange coil  and the other is for the heating element.The heat exchange coil is made of 25' of 1/2" copper tubing, all other piping is 1/2" rigid copper.


I originally used a 1650W, 120 volt element, Grainger part #2E754, to heat the water, and a standard water heater thermostat to keep the  tank heated to 170° F. I've since changed the heater to a 4500W 240V element and have also installed a PID controller for the HLT temperature. This made it easier to change the HLT temperature if needed, and the higher wattage made mash step easier. The 1650W element just barely had the power for a 10 gallon batch, but mostly struggled providing heat for step mashes.


Seen here are the two valves that will control the heat exchange coil: These are V1, and V2 on the diagram. The piping shown here connects the outflow of the heat exchange coil to the next series of valves, which determine whether the liquid flows back into the mashtun, or to the boiling kettle. These are V3, and V4. Also seen on the side is the standard water heater thermostat, which has since been removed. Shown in the 2nd view is where V5 attaches. V3 has been replaced with a ball valve, and the solenoid valve is now used on the Lauter Grant.


Here it is completed.