Wort Boil Volume question

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Jan 12, 2011
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I tried a Porter, my 2nd ever AG batch. My pre-boil volume was spot-on, but I didn't get anywhere near the amount I supposed to boil off. Naturally, I missed both my OG and volume.

I had a 60, 30, 15 hop schedule. My question is, what are the ill effects of having that oh sh*t moment, realizing you did not boil off nearly as much as planned, and just keep going? i.e. your 15 minute hop ends up in there for 40 minutes. Do you pull the hops and boil to reach target? Do you keep them in? Do you stop all and start calling your buddy with the giant fermenter and pray he's not using it?

Incidentally, despite what the Internet says, IMHO a 9 gallon pot does NOT provide enough headspace for an AG full boil. We had it at a pretty low boil, which I'm sure is how this all happened.

Any help appreciated! Thanks!



Sep 13, 2010
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There's a lot more to factor in on this. (Kind of Hops used and their AA (Alpha Acids), Amount of hops used, and the time thing)

Pulling the hops and boiling could help but probably don't want to leave them in. It may only raise your IBUs 4 or 5 points with 40 minutes extra boil time, but with beers that aren't very hop heavy (Such as Porters) you want to be careful you don't bitter them up to bad.

Do a test next time to find out your Evaporation rate. Take 2 to 3 gal. of water and boil it for an hour then measure what's left. Say you started with 2 gallons and an hour boil left you with 1 1/4 gallons. Your Evap. rate it .75 gal an hour. From that you can adjust your start boil volume to hit you end boil volume.


Your Evap is .75 gal an hour. You want to hit 5 gallons on the nose. Your boil time for your recipe is 90 min or 1.5 hr. (1.5 x .75 = 1.125 gal lost during boil) so take 5 gallons plus 1.125 and this gives you your desired start boil volume 6.125 gal or 6.2 to be safe.

Hope this helped and sorry if you knew all this already.


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Feb 5, 2008
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Evanston, IL
First, I think it's a better bet to boil for the same amount of time that you planned. Otherwise, the hops will get too bitter and the later hops will be too dull to add flavor or aroma. It's not as big a deal usually if the gravity is off by 10% due to slow boils.

Second, how is a 9 gallon pot not big enough? How big of a batch are you doing and how big are the boils? I find an inch or two of headroom is usually enough, especially once you pass hot break.

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