Originally Posted by seabass07
Boiling 7 down to 5 is not the same as boiling 5 down to 3. You need to calculate your boil off with the amount of water you will be boiling off. I can do about 1.2G in an hour if I start with 7.5. If I start with 5, I'll boil off about 2.
I think your boil off rate is going to be pretty consistant regardless of volume. If you can get it to a rolling boil it doesn't matter if it has 3 gallons under it or 10 gallons under it. Obviously it will take longer to get the larger volume to a rolling boil, but if your burner is strong enough, it won't really matter on the volume. I think the surface area is probably more of a factor.
All my batches are 10G. On my messed up one, I had started with just over 14 in the kettle and boiled off 2.5 before starting my "normal" boil time. Just FYI - at full tilt - I can actually boil off 3 gallons per hour.