Converting to Full Boil

I brewed a partial mash IPA kit recently. The boil size was about 3.5 gallons. I plugged the recipe into ibrewmaster and the IBUs came out to about 43, which seemed low for an IPA. I changed the boil size to 6.5 gallons and the IBUs went up to 73, which I expected due to better hop utilization. The question I have is if I actually went to a 6.5 gallon boil would it be watered down since the grain bill and extract amount was formulated for a smaller boil? You have to add water after the boil anyway so I didn't know if it mattered.

Full boil is one of the best changes you can make as a new brewer. The water added in the boil will make no difference in gravity as it only replaces the water added to the fermenter. You'll get better hop utilazation, have a reduced risk of contamination, and could also have more fermentable wort from slightly less kettle carmelization.

That is what I thought. I have been doing AG fit a while and like the full boil minus the extra time it takes.

Do you think it is right that the IBUs would nearly double just by adding 3 gallons to the boil? Seems like a large jump.

remember too that the added gallon is for boil off. To do a full boil your going to need big BTUs to get there. In turn boil off.

chanson16 said:

That is what I thought. I have been doing AG fit a while and like the full boil minus the extra time it takes.

Do you think it is right that the IBUs would nearly double just by adding 3 gallons to the boil? Seems like a large jump.

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That is definitely correct. Better hop utilization is part of that because with more volume you are able to dissolve more iso-alpha acids into the wort resulting in higher IBU's.

The other part is that when you add top off water to a partial boil you aren't just diluting the wort gravity, you are also diluting the IBU's, hop aroma, hop flavor, etc. So you are getting all of your IBU's in about 2.5 or 3 gallons of wort then you are diluting them with water and effectively reducing them by about half. This can be accounted for by just using more hops in a low hop beer, but in something like an IPA you max out the IBU's in the partial boil (many people think wort can only hold about 100 IBU), then you dilute that by half resulting in the maximum amount of IBU being around 50ish. And 50 IBU or less doesn't make for a great IPA in my opinion.