IBUs in small boil volume

[badlee]

I tend to brew AG 95% oc the time.
I have been unsble,due to back problems,to brew for about four months. So decided that I would try a few things all at once.
I will be brewing a 17 litre berliner weisse-ish beer with a total of 1.8kg of dme.
I will be boiling 7 litres with 5gr of Nelson for 15 minutes.
Problem: when I calculate the IBUs using the full boil I get 4IBUs.
But when I calculate it with the 7 litre boil(I cannot lift more than that and will have no help) I get about 2 IBUs.
So, do I need to add 10gr of Nelson cones to get my 4-5 IBUs from the partial boil?
Cheers
Lee

 

[badlee]

No help for a poor all grain brewer?

 

[Yooper]

Sorry! I haven't done the math, but that seems right to me.

Remember that dilution is a huge factor. If you have, say 7 liters of 20 IBU wort, and add 7 liters of water (0 IBUs), you'd have 14 liters of 10 IBU wort. Since you are adding more than 50% water, it would be even more of a marked decrease.

 

[badlee]

Well,that's something to go on at least.
Thanks Yooper

 

[Jay714]

I'm a relatively new brewer myself, but I've asked the same question in the past. The answer that I received is that you get poorer hop utilization in higher gravity boils. Lower boil volume (7L vs say 16L) with the same amount of fermentables will result in lower ibu.

I hope that's correct... That's how it was explained to me.

 

[badlee]

That is how I understood it also. Thanks.
I was unsure of the workings of it. I am not a new brewer,but am new at this area of it.
Cheers

 

[schokie]

It's not the boil volume that determines your hop utilization but the gravity. I have a 5gal kettle, so I boil 3 gal and add 2gal of top off water post boil. To keep my IBUs up I only add half the extract at the beginning. I add the rest towards the end, usually with 10-15 minutes left in the boil.

Beer smith is great with calculating hop utilization during partial boils and late extract additions. I enter the recipe and planned process, then tweak my hop amounts to get the end-state IBUs I want.