Boil Timing / OG Question

I brewed my second all-grain batch this weekend with some new equipment I received as gifts (propane burner, 10 gallon pot). All went very well! I achieved a 75% efficiency with my mash/batch sparge, so I was excited about that.

Prior to brew day, I did a test boil with my new equipment and some water to figure out the evaporation rate. During my actual brew day, I didn’t burn off as much as I thought I was going to (still playing with the burner settings).

All in all, I ended up ~4 points less than my targeted OG. Not a huge deal, but could this be because I didn’t boil off enough? Am I correct in understanding correctly that if I had boiled a little longer, that would have evaporated more water, reduced the total volume and ultimately raised the OG?

If so, I considered doing that, but I was worried that it would throw off my hop schedule. For instance, if I was boiling for 60 minutes and had a 15 min hop addition, wouldn’t extending my boil an additional 15 minutes throw that hop schedule off?

You do the extra boil time, prior to adding the first hops. I usually do a 90 minute boil, so after 30 minutes I add in my 60 minute hop addition.

Thanks -that makes sense, but how do you know how much boil time to add, or if you'll need the extra boil time to begin with?

The best thing to do is to measure your boil off using water. Every system is going to be different. Add a known amount of water to your kettle and boil for one hour at the same rate of boil that you use during the brew day. Now measure what's left. Let's say you're making a 5 gallon batch. You probably want about 5.5 gallons of wort at the end of the boil. So do you have more or less than 5.5 gallons. If more then subtract how much more from your original level. If less then add. Ending up with the correct amount of wort is what your after.

After you have this figured out then brew a beer and see what your gravity is. If it's low or high you will need to adjust your recipe to your system. Using brewing software will make this easy and I recommend it highly.

We boil all our beers for 90 minutes and add our bittering hops at 60 minutes left just like Mdsutton and we get the same amount of wort everytime and we hit our numbers exactly. It's really nice once your system is fine tuned!

There are things that will affect your OG like the ph of the mash and how fine your malt is milled. If you change any of those things you might have to adjust your recipe again. Basically, dial in your system and do things the same way every time and you will get consistent results!

I have 3 kettles in my brewhouse, depending on the size of the batch I'm making. I've boiled water in each of them for an hour and measured how much got boiled off, but when it came to boiling wort, the boil off rate for each of them was different from the water test, though they were certainly in the same ballpark. I attribute that to a difference in boil vigor between water and wort, because boiling water looks a lot different than boiling wort to me.

What really helped me nail down my boil off rate was keeping track of it over the course of 5-10 batches. Now that I know it, I hit my fermentor volume within 8-10 ounces (I do mostly 3-gallon batches).

The important thing is that you were so close to your target OG! I wish I had been that close on my first AG batches.

dierythmus said:

Thanks -that makes sense, but how do you know how much boil time to add, or if you'll need the extra boil time to begin with?

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In your situation, I would have checked the volume before the 15-minute addition to see how close I was to hitting my target post-boil volume. If I didn't look like I was going to hit the volume, then I would have either increased the heat and dropped in the 15-minute hop addition, or I would have boiled for an extra 15 minutes then dropped the hops in for the 15 minute addition. Yes, it would increase the bitterness some, but the aroma hops would have been done right. Hitting your OG is more important than getting the correct bitterness, in my opinion. I can't usually tell if a batch of beer is slightly more bitter than it should be, but I can usually tell if a batch is more watery than it should be.

Here's how I make sure I hit my volume on brew days:

Working backwards...
I want 10 gallons of finished beer.
I usually lose around a half gallon per carboy in the fermentation process, so I need 11 gallons of wort going into the carboys at the end of brew day.
I lose about a half gallon to trub and hoses and my chiller, so I need 11.5 gallons of wort.
11.5 gallons of wort will expand about 4% going from room temp to boiling (about a half gallon), so I need 12 measured gallons of boiling hot wort at the end of my boil. 12 gallons is my target end-of-boil number.
My crazy wide pot boils off about 2.25 gallons of water per hour, so my pre-boil volume is about 14.25 gallons, by the way.

Anyway...I monitor my volume during my boil with my sight glass. At the 30 minute mark I know my volume should be right around 13 gallons. If my volume is lower than 13, I reduce the heat going to my BK. If it's higher than 13, I increase the heat.

At the 15 minute mark, I check the volume again to see if I'm right around 12.5 gallons and again make adjustments if necessary.

If I'm not distracted on a brew day and carefully monitor my volumes like this, then I always end up with the correct volume. Provided that I hit my pre-boil SG, then I'm golden.