How to make a larger batch on same equipment

[LovesIPA]

Right now I do 5 gallon batches. I only purchased an 8 gallon kettle (oops) so getting much more than 5.5 gallons in the fermenter is tough. I keg so 5 gallon batches are perfect.

I owe a buddy a case of beer and he likes high ABV, hoppy beers. For an upcoming batch I want to brew this beer. I would like to bottle a case for my buddy and then keg the rest but I don't want to keg half a batch. I'd like to get the keg as full as possible. I have two fermenters that will hold close to 9 gallons so I'm going to use one of them to ferment.

What I am thinking of doing is scaling the recipe up to 8 gallons, but only collect about 7.5 gallons of wort (my kettle would be full) so I'd end up with about 5.5 gallons in the fermenter. Then I'd top it off with purified water to bring it up to 8 gallons (blowoff tube for sure).

Two questions:
1. How is this going to affect the quality of the beer? I'm particularly concerned about hop utilization, especially with an aggressive hop schedule like this one.

2. With a grain bill this large (~24 lbs) how much will my efficiency be impacted by only sparging enough to collect 7.5 gallons of wort? I typically get low/mid 80's.

I don't mind boiling for longer than necessary and adding more wort to the kettle before the first hop addition to up the gravity points if that's necessary.

I use a 10 gallon igloo cooler for a mash tun. I know it's supposed to do 24 lbs of grain. I guess I'll find out for sure. The grain bill would be:

-20 lbs of 2-row
-2 lbs of C80
-1 lb each of Carafa III and chocolate
-1.25 lbs dark candi sugar

 

[bja]

Replace some of the base malt with DME and add it at the end of the boil. Top off to 8 gallons after it's in the fermenter.

 

[abrix]

We've started doing something similar to what you're planning, also with an 8 gal. pot. We design the recipe for 7.5 or 8 gallons, collect 9-9.5 gallons of wort, and move some of the later runnings to a second boil pot (a 5 gal. pot from the extract days).

All hopping occurs in the larger kettle, but we have not done more than a 40 IBU beer in this way, so I can't say how a big IPA will go utilization-wise.

Later in the boil, we'll transfer the boiling wort from the smaller pot into the large kettle. With room for the immersion chiller and with trub/hop gunk left in the bottom of the kettle, we can usually transfer 7 gallons to fermentors a few points above our calculated OG, at which point they can be topped up with boiled and cooled water as needed.

It's kind of a pain, and we don't have it totally dialed in, but it does work.

As for mashing 24 lbs in your 10 gal. Igloo, we've done it (27 lbs at a grist thickness of 1 qt/lb) and our efficiency dropped from our usual 75-85% to low 60%-ish. A longer boil can help. Here's one calculator for mash volume.

For 1.100 beers, we usually do two separate mashes or a re-iterated mash. In this case, I would consider a late-boil DME addition to reduce worries about hop utilization.

 

[LovesIPA]

Hmmm... I do have a second boil kettle from my extract days as well. I'd have to run between the back patio (gas burner) and the kitchen (electric stove top) but it sure would solve some problems. If I actually collected ~10 gallons of wort that should mostly solve the efficiency problem, right? If not, I can easily calculate and add some DME to bring it to where it should be.

I can also hop both kettles and combine them towards the end. That should mostly solve the hop utilization problem too.

I think we have a winner except I'll have to lay off the homebrew so I can keep all this straight in my head on brewday.

I plan on brewing BM's Centennial Blonde Ale either tomorrow or Saturday (with WLP001) depending on my schedule. After a week or so in primary I plan on racking it to a keg and using the yeast cake to ferment this monster.