You guys have any sights or simple equations you use to account for boil-off? I’m doing my first full wort boil soon :mrgreen: pretty excited, but I want to do it right.

I’m not aware of a formula that accounts for the variables (humidity, kettle geometry, boil vigor, etc.), but one gallon per hour seems to be the generally accepted starting point. When doing a full volume boil, your boil-off rate should the be same as your partial volume boil-off rate unless you have changed one of the variables. I don’t believe starting volume is a variable. Just note that you may need to adjust your heat source in order to maintain the same boil vigor as your volume decreases.

I’m generally at or slightly higher than one gallon per hour. I use a 10 gallon Blichmann kettle with a propane burner near the opening of my garage.

As kcbeersnob stated, pay carefull attention to your rate of boil. I have boiled off a gallon in 15 mins once because I cranked my burner all the way up and have a very wide kettle. Also, you’ll want to continuously monitor the volume you have in your kettle. If you have a liquid level gauge, awesome. If not an easy solution is to take a piece of cpvc pipe or a metal rod and cut notches in it for each gallon mark (1 gal, 2, gal, 3, etc). Then just stick that stick into your kettle periodically to see where your volume is.

Too many variables for any equations to help you. Any time you break in a new kettle you’ll need to either do a test run and measure it or just do a batch and adjust next time. IME, the most significant variables are boil vigor and kettle diameter. I have a 34qt kettle that loses 1.25g/hr, and a 100qt kettle that loses 2g/hr.

I think the best way to know is trial and error. After 4 batches with my current kettle I am pretty dialed in on how much water I need to begin my boil with. I lose about 1.25-1.5 gallons during a brew session (which is around an hour and a half from starting the flame to turning off the burner) in my 9 gallon BK. My advice is to start with a gallon more than you you want your final volume to be and if you lose too much water then top off your fermenter to get the correct volume. by the third or fourth batch you will be hitting it right on.

If memory serves, I boil off at about 1.2 gallons/hour. This is true regardless of batch size. The main variable that really matters is the surface area of the boiling wort. If you use the same kettle for every batch, and you boil with the same vigor for every batch, then your boiloff rate should remain perfectly constant from batch to batch, plus or minus maybe 0.1 gallon. If you start playing around with different kettle sizes, or split a batch and boil part of it on your stovetop and part on a burner, stuff like that will mess with your boiloff rate calculations. But by and large, boiloff should be very constant from batch to batch for the same brewer on the same system.

Thanks guys. I’ll be making a pale ale here this week sometime. I’ll post results.

It’s also useful to know what your typical target volume looks like in your kettle at boiling temp (water expands when heated) so at the end of the boil you can adjust your boil-off to match your hop additions and still end up at the right volume in the end.

It definitely depends on your setup. I’ve done two full boils with a 10 gal. bayou classic kettle (14" dia.x 16" tall) on a Blichman burner & only lost just over 1/2 gal. both times. Not a real vigorous boil, just rolling. Not sure if outdoor temps play a big role in evaporation.

And as they’ve already said, do it by gal/hr, not percentage.