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I just moved my (first) batch to my secondary fermenter. While doing so it became apparent that I did not top off to five gallons like I thought and instead it seems closer to four gallons. I now have relatively concentrated brew.
Should I boil and cool some water to top it off or will this ruin the batch?
When racking from a primary fermenter to a secondary vessel, you will leave behind a non-trivial amount of "stuff" so the volume in the secondary will be less than the volume in the primary. If you start with five gallons in the fermenter you won't have five gallons left to bottle, but it isn't any more concentrated than when you started.
If your OG and FG were about where you expected them to be, you probably should leave it alone. Otherwise, you're just diluting your beer. Quality over quantity!
But yes, if you want to top up, you can boil and cool some water and gently add it without causing harm (other than dilution).
You can top up if you want, but you don't have to.
Boiling the water and cooling is necessary, since boiling both sanitizes and releases dissolved oxygen, which would prematurely stale the beer.
To save time, I top off my brew to 5 gallons just before bottling (with water that has been boiled and cooled, of course). I'll be adding the priming sugar at this stage anyway, so I just bump up the amount of water used to dissolve the priming sugar enough to top off my brew.
