Herms systems work on the principle of heat exchange. They either come with a copper wire immersed in the hot liquor tank (HLT) or another vessel filled with hot water. The mash water goes through the coil, picks up the heat, and returns to the mash lauter tun (MLT). The cycle keeps repeating until the mash begins to break down compounds and ferment the sugar present in the grains to finally produce beer.