Advice on adjusting your pre boil volume for lengthy boils

I have been curious about this since I started brewing bigger batches and using a new boiler which has a substantially higher boil off rate than my previous one.

Is there anything you need to be careful about when adjusting your pre boil volume to account for boil off? Particularly in the case of long boil times. For example, if I want to brew a 30 litre batch over 120 minutes and I know that on my set up I will loose about 10 litres over the two hours is it really just as simple as sparging to a pre boil volume of 40 litres? This makes sense to me but maybe I'm wrong. Little concerned about any issues that may occur with oversparging or gravity being compromised somewhere.

Thanks in advance! Still very much finding my feet with all grain!

Graeme

Graeme said:

For example, if I want to brew a 30 litre batch over 120 minutes and I know that on my set up I will loose about 10 litres over the two hours is it really just as simple as sparging to a pre boil volume of 40 litres?

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Yes, that's about right. You want to make sure you aren't oversparging (keep your runnings above 1.010 and a pH under 6) but that's not that common in a normal sized grainbill anyway.

You just need to know your boil-off rate. The water sparge calculator I use assumes a rate of 10% boil-off per hour, but based on experience my system is about 13%, so I use that value to estimate pre-boil volume.