Originally Posted by Got Trub?
Boil off rate can make a big difference. In my system a slow boil results in 0.8g/hr evaporation, a strong rolling boil 1.1g/hr.
My other thought is have you calibrated all your equipement so you know exactly how much water you are using and collecting?
YES!! thank you! the single most useful thing i've done to fine tune my system so that i always hit my volumes: boil some water. i filled my kettle to x gallons and boiled it for an hour. let it cool over night (while covered so you dont lose evaporation) and measured it. take your volume measurements at room temperature because that's the volume that matters going into your fermenter and that way you don't have to mess with cooling loss. because of this i know that i need 4.75 gallons pre-boil to get 3.2 gallons after 90 minutes. 3 goes into my carboy and .2 swims with the fishes (and hops and break). the other thing, like got turb? said, is be consistent with how strong your boil is. i start my flame after i've added my first runnings, and my last amount of sparge water usually just quenches the boil. i set my flame so that the boil is JUST rolling. any stronger and you're scorching sugar and wasting propane. if you're super concerned about getting a good hot break then put the flame on turbo until you break, then bring it down, but do it the same every time so your notes and boil off measurements are useful.
i realize that this question was about batch sparging, so i digress!