I’ve been all grain brewing for about 6 months now and keep having an inconsistent problem. Some batches boil off too much. i.e. I put 13 gallons into a 15 gallon kettle and only get 8 gallons into the fermenters. Most recently I did a 5 gal batch of Marzen and had a 9 gallon pre-boil volume and ended up with only 3 1/2 gal post boil. Yet the batch before was a stout that I had an 8 1/2 gal pre-boil volume and ended up with 6 gal in the fermenter. I use a 15 gal stainless kettle and a bayou classic 10" propane burner. Am I using too much heat? Anyone else had a similar issue?
Wow, I have to assume there are some measurement errors on that Marzen. 5.5 gallons boiling off would take like a 3-4 hour boil depending on the geometry of your kettle even at a huge rolling boil. Are you sure you measured 9 gallons going in and only 3.5 coming out? Do you leave any wort in the kettle to leave behind hops and break material?
I believe your variable is the humidity in the ambient varies which affects how much boil off is possible. I see that with my 25 gallon pots.
I have to say mine is random as well, but I have never boiled off 6 gallons in 60 minutes. I do believe I boiled off 3 once though.
One thing for you to try. When you set the burner and getting the boil you want…Leave the knob on the burner alone and just shut off the gas on the tank. That way you have the same valve setting each time you boil.
I keep meaning to mark mine with a permanent marker but I always forget. This would also be another solution
My measurements can’t be off by much. The first time this happened I marked the kettle in one gallon increments. Measured each gallon by weight and marked the waterline with the kettle on a level surface. Even if I miscounted the marks I wouldn’t be off by THAT much on more than one batch. I try to keep the lid on until the boil begins to limit my losses but with only occasional success. Does anyone else monitor boil temps? I haven’t been doing this since getting the larger burner because I didn’t figure it was an issue. Has anyone noticed a significant loss at say above 225 deg? I never had an issue with my smaller burner that only managed 215ish. (the reason for the much larger burner.)
Temp/humidity will definitely have an effect on your boil off, but your numbers are really an extreme. I have recorded boil off anywhere from 15-25% depending on the time of year. I suspect you are getting different numbers due to weather, but that can only be part of the equation. I have to assume your numbers are off somewhere. I don’t see how you can boil off 6 gallons in an hour.
Final volumes may be a bit off since I don’t actually measure them. I leave about a quarter gallon in the kettle to keeps most of the hops out of the carboy. Still I’m only filling my 6 1/2 gallon carboy a little over half full. Even figuring I started with say 8 gal preboil and lost 1/2 gal to hops/trub I’m still losing 3 1/2 to 4 gal to boiloff. Beersmith says I should only loose 2 so I’m at a loss (pardon the pun) for the rest. Weather has been very bi-polar in Texas all winter so sure that could be some of the issue. Anyone know how much shrinkage I can expect when using a wort chiller? is it enough to bother measuring?
Beersmith can only predict boiloff based on the information you give it. If you haven’t changed any of these settings then it’s just using default values for boiloff which may or may not represent your systems actual boiloff. I too experience some difference in boiloff which seems to be directly related to the vigor of my boil but it’s only a difference of 0.5gal per hour.
Dang!
I guess that means I gotta brew my next 2 batches sober and mark everything so I can compare results. I know it’s something in my system I haven’t idiot proofed yet. Just had to see if anyone else had a similar issue.
[quote=“Valentines12”]Dang!
I guess that means I gotta brew my next 2 batches sober and mark everything so I can compare results. [/quote]
That might be your answer. 
It’s easy to make mistakes when you’re drinking.
I marked the tube from my MT, so I hit the desired volume every time. Some people mark a spoon or dipstick.
I’ve noticed boil off rates do vary. I have chalked it up as SG, humidity, and temp of flame and room.
