Sparging is the rinsing of the grain bed to extract as much sugar as possible from the grain without extracting the objectionable tannins from the grain husk. Typically, 1.5 times the amount of make-up water is used for saccharification (e.g., 8 lbs. of malt, 2 quarts/lb. = 4 gallons of saccharification, so 6 gallons of make-up water is needed). The temperature of the refill water is important. The water temperature should not exceed 170 degrees F because, depending on the pH of the wort, hull tannins are more likely to dissolve above this temperature. This can lead to an astringent taste in the beer.
Wort should be drained slowly for optimal extraction. The refilling time depends on the amount of grain and the refilling system, 0.5 – 2.5 hours. Overflow means “sparge”, which explains why you may have seen or heard discussions about “overflow arms” or overflow devices that sparge on the grain bed. There is no reason to use such a thing. There are three main methods of dosing. English, batch and continuous.
In the English method of refilling, the wort is completely drained from the grain bed, then more water is added for a second mash and drained again. These wort is then combined. In addition, the first and second wort are often used to make separate beers. The second run-off has a lighter gravity and is traditionally used to make a small beer, a lighter-bodied, lower alcohol beer that is suitable for drinking in large quantities over a meal.
Batch sparging is an American homebrewing practice in which the full amount of sparge water is mixed into the malt slurry. The grain bed is allowed to settle and then the wort is drained. The recirculation step in this process is done a few minutes before the water is added. You can use more than one batch of water if needed. This method differs from the British method because the wort does not remain at saccharification temperature for very long before being emptied.
Continuous refilling usually produces better extraction results. The wort is recirculated and emptied until about an inch of wort remains on top of the grain bed. If necessary, sparge water is added gently to keep the liquid at least at this level. The goal is to gradually replace the wort with water, stopping the addition of water when gravity reaches 1.008 or when enough wort has been collected, whichever comes first. This method requires more attention from the brewer, but can produce higher yields.