Heat Exchanged Recirculation Mash System
In a HERMS system, the mash is circulated through a coil in the hot liquor tank to maintain a certain temperature. The temperature of the HLT can be controlled in a few different ways, the most common are manual control, electric heating, or electronically controlled gas. With all 3 methods, the goal is to maintain the water in the HLT at a temperature high enough to get the returning mash to the desired temperature. The main benefits of a HERMS system are a much cleaner wort than manual recirculating and consistent mash temperatures.
In a manual controlled hot liquor tank, you would have to monitor a burner to maintain your desired HLT temp. This can be tedious and difficult to be consistent, but is fairly easy to implement. All you would need is a HERMS coil, a pump, and 3 hoses, in addition to your normal all grain equipment.
In an electric hot liquor tank, you would have a heating element in your tank. A PID control intelligently switches the element on and off as needed to maintain the temperature, much better than you could do manually. This is my preferred system. The benefits of this method over a manual HERMS system are very consistent temperature control, an inexpensive heat source, and very fast heating of water. In addition to the equipment needed for a manual HERMS system, you would also need an electric heating element in your HLT, a temperature sensor in the HLT, and a controller to tell the element to switch on and off.
An automated gas HERMS system is very similar to an electric, but the controller turns a gas valve on and off. A pilot light stays lit throughout the process. This route is really a matter of preference of gas heating over electric, but the equipment required may be more expensive. Instead of an electric element, you will need an electronic gas valve, a pilot light, and a burner. The controller will be similar, but will need to be wired up to control the 24volt valve and not a 240 volt heating element. A benefit is the you wont need a high amperage circuit wired in your house or garage.
No matter wich HERMS system you choose, you will need still need the same 3 vessels as all grain brewing. You will need to choose one of the ways to heat your HLT, and you will also need a way to heat the boil kettle.