Decoction

This is the guide for planning a decoction mash: how much of the mash to pull for each step, and what the pulls cost in time and water.

The functions to start with

decoctionPull answers "how much do I pull to get from this rest to the next?" for one decoction.
planDecoctions answers the same for a whole schedule, sizing each pull against the mash the one before it left behind.

DECOCTION_SCHEDULES has the traditional schedules ready to use (Hochkurz, double and Bohemian triple), and findDecoctionSchedule looks one up by id. A recipe whose mash steps include a decoction step gets its plan from calculateRecipe in decoctions.

One thick pull

5 kg of grain in 15 L of water, resting at 52 °C, with the next rest at 64 °C:

const pull = decoctionPull({
  mashWaterL: 15,
  mashGrainKg: 5,
  currentTempC: 52,
  targetTempC: 64,
  spec: { thickness: 'thick', boilMinutes: 20 },
});

Pull 6.1 L, about 3.7 kg of it grain. It's out of the tun for 37 minutes (rest, ramp, a 20-minute boil and the return), during which the tun drifts to 50.8 °C and the pot boils off 0.5 L, and the pull is sized to make up for both. Brought back, it lands the mash on 64 °C. The pull comes with one issue to show the brewer: "A pull this thick scorches on a direct flame. Stir it the whole time it heats."

The working

pull.explain is the heat balance with its numbers filled in: the mash's heat capacity in water-equivalent liters, the temperatures, the evaporation and the grain's specific heat. Its notes say how thick the pull was taken and how far the tun drifted, and one says that the boil's effects on color, efficiency and pH aren't modeled.

Traps

Other calculators give a smaller pull. Most use Palmer's classic formula, which assumes the tun doesn't cool while the pull is away, the pull comes back at 100 °C, and the boil costs no water. Each of those makes the pull too small. The method page works through why, with numbers, and the classic form is still here as decoctionVolume if you need to match it.
A thin pull carries no grain. It's liquid only, like the traditional last pull of a triple, and it's there to raise the temperature. It adds little of the character a decoction is usually done for.
The rig matters. Standing loss, burner ramp time and pot boil-off are on the equipment profile, and decoctionPull takes them as DecoctionRigOptions. The defaults assume a domestic burner and a tun that loses 2 °C an hour.

Going further

Decoction, the method page, derives the heat balance, covers pull thickness and pot volume, and lists what the model leaves out and why.