Originally Posted by 37OliveStBrews
I brew exclusively through kits (AHS, NB, MW, and local HBSS) and up until now have always done partial boils with my partial mash kits. But I want to do a full boil since I have the capacity and I hear the beer tastes better.
Here is my question, why can't I just mash with more water to get my boil volume and follow the recipe as is since all I do is top off with water anyways?
In short, no.
Now, comes the explaination and still how to do a full boil.
Mashing requires enzymatic conversion, from starch to simple sugars. To do that, you need a certain pH in the mash and a certain temperature. That generally requires a mash of 1.25-2 quarts of water per pound of grain. Most people find that 1.25-1.5 quarts of water per pound of grain works best. The temperature range is variable, but 153-154 is a good basic temperature.
Then comes the sparge. Sparging is "rinsing" the grain of the rest of the sugars that were converted during the mash. You can sparge with up to .5 gallons of sparge water per pound of grain, without risking oversparging.
So, for a typical PM, say you have 5 pounds of grain.
5 pounds of grain x 1.5 quarts of water per pound= 7.5 quarts of water.
The grain will absorb about .125 gallons per pound- so you should get out 1.25 gallons of wort out of this mash.
For the sparge, you can use up to 2.5 gallons of water for the sparge. You won't have any more absorption, so you'll end up with 2.5 gallons out of the sparge.
At that point, you'd have 1.25 gallons (mash), and 2.5 gallons (sparge)= 3.75 gallons in the boil kettle. You can then top up to 6.25 gallons with water (or whatever your boil volume is), add your DME or LME and start your boil.
Adding water to the boil is fine, rather than oversparging your grains.