Thickness of the grain bed affects the flow dynamics during fly sparging, but is irrelevant for batch sparging. The integrity of the grain bed only matters during the fly sparging process, or run-off when batch sparging. The grain bed can be reset by vorlaufing - running off some wort and then pouring it back on top of the grain bed. This is repeated until the wort runs clear. At this point you can start fly sparging, or running off if batch sparging.This sticky was super useful to read through. Thanks for pinning it.
There is one part that confuses me, at least when applied to my home setup. Why does the thickness of the grain bed matter if stirring is okay during the mash out?
Until recently, I had always thought the rule was "don't touch the grain bed" because it needs to set and act as a filter (despite having a false bottom is doing a lot of the filtering). But if stirring is okay, why not even stir during the sparge and avoid channeling altogether?
I'm sure there's good reason, I just think I'm overlooking some critical fact.
Thanks
You don't want to stir during fly sparging, as the efficiency depends on having a vertical concentration gradient of extract (basically sugar) thru the grain bed. Stirring during the sparge will destroy the concentration gradient, and at that point you are basically doing a batch sparge.
Hope this helps. Let me know if you have additional questions.
Brew on