Originally Posted by hal2814
One thing that always throws me for a loop when brewing extract these days is adjusting boil-off calcs. Extract has significant volume. I keep forgetting that and starting with as much water as I would for an AG batch. That brings me way over on expected volume.
I can see how that would be an issue. A lot of folks on HBT want to quantify boil-off as a percentage, which would mean that the amount of boil-off would change relative to the amount of wort being boiled. I don't find that to be the case.
Given the same atmospheric conditions, equipment, and process, you will boil off the same volume of liquid whether you are boiling 2 gallons or 10. Variations in boil-off volumes are determined by the aforementioned factors of atmospheric conditions
(ambient temp, barometric pressure, wind, etc.) , equipment
(wort surface area in your boil kettle, lid or not, etc.), and process
(duration to get wort to boil, duration of boil, boil vigor, etc.) and maybe a few others I haven't considered like a leaky weldless kettle spigot or something stupid like that. I'm sure that there is or could be a mathematical calculation that takes all of these factors into account to give you the beginning boil volume to absolutely nail your post boil volume, but I believe that most of us without a doctorate in quantum physics and brain surgery initially make an educated guess on what our boil-off volume will be for the duration of our boil and the equipment we are using and adjust from there with a little R&D (trial and error).
I would suggest this as a starting point...
1. Fill your kettle with water (and grain steeping liquid, if applicable) to the volume that you intend to put in your fermenter.
2. Bring that volume of water to a boil and turn off the flame.
3. Add your extract and more water, if necessary, to bring you pre-boil volume to 1.25 gallons more than your intended fermenter volume. Record these volumes for future reference.
4. Bring that to a boil and proceed with your recipe as usual.
5. Every 10-15 minutes, check your kettle volume and record the remaining boil time and volume.
6. If you haven't already, fill your fermenter with water to the volume that you intend to fill with wort and mark a fill line with tape or sharpie. (You could really mark graduations in quarts +/- 1 gallon of your target fermenter volume. That will help you determine how close you came over or under your target.)
7. At the end of your boil, measure and record your post-boil volume.
8. Cool and transfer your wort as usual. Be sure to transfer the entire contents of the kettle; of course minus trub/hops that you may normally leave behind if that is a part of your process.
9. Record the final volume collected in the fermenter.
Use the amount +/- that you missed your target volume in the fermenter (unless you nailed it the first time) to adjust, up or down, the initial 1.25 gallons added to the kettle beyond the target volume.
It will take some experimentation to home in on the beginning volume you will need to start with to get consistently near your target volume with your
equipment and process, since those are the two factors of the three that you can control. In my brewery I've found that if I start with 7 gallons in my kettle, I finish the boil with 6 gallons in the kettle, and transfer approximately 5.5 gallons of cooled wort into the fermenter. Good luck!!!