Various recipes call for “decoction” mashing, this is a process that involves pumping about 1/3 of the mash from the vessel it was pumped into, to another vessel where it is boiled and mixed, then returned to the original vessel and mixed in with the other 2/3. Ideally, a cereal cooker is used for this process. It is usually 1/3 the size of the brewhouse and is essentially a small Mash/kettle. There are less ideal ways to perform a decoction, if one didn’t wish to involve a 3rd vessel. You could have the Kettle/Whirlpool converted to a Mash/Kettle/Whirlpool and a separate Lauter Tun as your second vessel. In this case, you would Mash in, then pump 2/3 of the Mash to the Lauter Tun and heat the remaining 1/3 in the M/K/W. Then pump the original 2/3 of the Mash from the Lauter Tun back to the M/K/W to mix with the heated 1/3. This process can be repeated as often as is necessary. It’s not as efficient, as you can never quite get all the “stored” mash from the Lauter Tun back into the kettle.