Hop utilization?

[Flashman]

Hi, I'm thinking about brewing a extract full boil in readiness to moving on to AG brewing. I usually do a partial boil and top up the fermenter with cold water etc. In doing a full boil do I have to reduce the weight of hops I use to get the correct IBU level. According to the brewersfriend recipe calculator if I use the same amount of hops in a full boil as in a partial boil the IBU rating goes up dramatically and to get my target IBU I have to reduce my hop weights, is this right?. Also if I want to end up with a 21 litre batch size would I need to have a starting boil volume of approx 25 litres?
I must say I'm a bit stuck on this one and any advice would be greatly appreciated.

 

[Nosybear]

Right on all counts. Full boils use less hops than partials and you have to allow for evaporation in the boil. The Recipe Builder on this site is helpful for estimating hops usage and you'll have to determine evaporation from your system but the same thing applies here as to partial boils, if you boil away too much, add back water. In fact, if you're trying for consistency, you'll probably want to overshoot the sugar in your wort then thin it back to where you want it to be.

 

[Flashman]

Hi Nosybear thanks for your advice. When I do a half boil I use half the DME and get all the hop additions in before adding the other half of the DME and getting the right IBU I have aimed for. Bearing this in mind, when doing a full boil and using all the DME from the start the wort gravity would be the same as my half boil. In this case would I still need to reduce my hop amounts to get the same IBU?
Cheers Nosybear

 

[Ozarks Mountain Brewery]

longer you boil hops the more it bitters, its just a fact