If I'm making a 5 gallon batch of beer, how much water should I use for my wort? Papazian's book says to use 1.5g and add it to 3.5g of room temperature water already in the carboy. I see how this would be helpful because it would help to cool the wort down quicker to pitch, and I wouldn't have to worry necessarily about the glass breaking. I've also seen other resources (including something, I think, through this site) recommending that for a 5g batch you boil all 5g in your wort. What are the pros and cons of the latter method? Thanks in advance
Okay, well there are a few things to consider.
How big is your pot? If you don't have the headroom to handle the inevitable foam, you will have a mess.
Can you easily chill five gallons of wort without using a cold water additive? If you don't have a wort chiller, this can be a big issue.
The amount of wort you boil, the specific gravity of that wort, and when you add hops will greatly vary the bitterness you obtain.
A simple beer with 6lbs of light DME, 1.5oz of Norther Brewer hops @60min and .5oz of Saaz at 1min in a three gallon boil gives us 34.4 IBU.
If you change ONLY the boil size, to five gallons, the IBUs go up to 47.3. Fermentables, boil size and hop AA can all effect the final bitterness. I recommend using software or something like Beer Calculus (free, online) to adjust your recipe to the boil size you want. So in short, it does matter how much you boil and you have to adjust your recipe to account for it. Or you will miss your IBU targets.
Doing a full-wort boil (all 5 gallons) offers a few technical advantages over partial-wort boils. There are a number of reactions that depend on the concentration of wort.
First, the wort-darkening reactions are more pronounced at a smaller volume meaning your wort will come out a little darker than you expect.
Second and more importantly, the rate of hop utilization is diminished in a partial-wort boil. You may have to use more hops to get the desired bitterness level. Most brewing software takes boil volume into account when calculating IBUs. In rare cases of high bitterness, small wort boils the wort can become saturated with iso-acids, putting a cap on IBUs.
The biggest drawback to this method is the extra equipment you need. A good heat source and a chiller are important.
Partial-wort boils are convenient, and for the most part, you will make fine beer without a full boil. I recommend you sterilize the added water by boiling and cooling (maybe overnight).