Just curious, what's the largest batch you've done in your 15 gal kettle?
Not the guy you asked, nor do I have a Spike kettle
I have a Northern Brewer 15 Gallon Megapot.
Last four batches or so, I have usually put nine and a half gallons into two kegs. Sometimes more, sometimes less. Nine and a half leaves about the right CO2 head space into two kegs. Better than doing the full five/ten gallons, IMO.
I just take a net five gallon recipe, and double it. Due to there being less water, the ABV will be down a little, can add more grains to the bill if you are concerned about it.
I pull the bag out (with rope and pulley), squeeze it pretty well and then sit it on/in a five gallon bucket to drain.
I then add four and a half gallons of very hot water and start boiling.
If you are going to raise the boil level this high, you need to have a needle valve adjustment on the burner and watch it carefully with a water spray bottle handy in the first fifteen minutes to knock down any potential boil over. It will settle down after the first fifteen minutes.
I usually do a ninety minute boil.
After fifteen, twenty minutes or so boiling, I pull the bag and empty it, and put the wort collected in the five gallon bucket in with the rest of the boil. The level will be down a bit by then, and if a little bit of wort is boiled for an hour instead of 90 minutes I don't see that as a problem.
I've done several slightly over thirty lb grain bills this way and they have come out fine. with my setup, I don't believe I would want to try my than about 30/31 lbs of grain.
There is a slight loss in efficiency that I have never calculated. I usually hit my intended Specify Gravities, ABV and volume pretty close into the kegs. Actually, it's harder to nail the volume due to leaving trub in the kettle and fermenter than the ABV.
Works for me, and I'm glad to fill two kegs in one brewing session instead of one and a half.
