Boil Off rate's effect on Original Gravity

Well, today was my first experience with All Grain brewing. I loved every second of it, and I think I did pretty well overall (thanks in large part to this forum). I did have a couple minor issues, and I'm hoping for a little clarification on boil off rate and how it affects OG.

I ended up boiling off more than BeerSmith had predicted (about 1/2gallon more than expected). I also ended up overshooting my OG (target- 1.057, actual 1.064). Are these two things related?

I'm thinking (hoping) that boiling off eliminates H2O from the wort, but not sugars, unfermentables, etc., thereby raising the OG as water is lost to evaporation. Am I on track here? I just want to understand why I missed the OG so I can plan better next time. Thanks!

You hit the nail on the head. Water evaporates, but not the sugar. This is why your pre-boil OG will always be lower than your post-boil OG.

Yup, essentially, the more water you boil off the higher the concentration of sugars, resulting in a higher OG.

I would recommend doing a test boil to determine your boil off rate as well as make other adjustments for wort left behind in the kettle, etc.

The other thing you will notice when you do AG is there is WAY more trub. You should shoot high on your preboil, get the volume and OG you need, and leave the trub in the kettle.

Thanks so much for the quick reply, guys. I sure did notice a lot more trub. That in conjunction with the boil off left me with about 4.5 gallons in the carboy.

I had to refine my recipes when I got a new BK. My old one was tall and skinny and my new one was shorter and wider. The larger surface area had a higher boil off than the smaller pot.

KNOTSANE said:

I had to refine my recipes when I got a new BK. My old one was tall and skinny and my new one was shorter and wider. The larger surface area had a higher boil off than the smaller pot.

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Which makes sense, I have a 10 gallon BK that's short and wide. I wonder if the cold weather has an additional effect on boil off rate.

Nothing stopping you from adding back enough water to bring your OG back to 1.057

Actual Gravity / Desired Gravity * Actual Volume = Desired Volume

64/57 * Actual Volume = Desired Volume

Next time, just modify your equipment profile to have stronger boil-off rate