Originally Posted by Bobby_M
Partial boil all grain is very inefficient. You'd be better off doing a partial mash, then augmenting with some extract.
I think Bobby_M is right, but if you fly sparge, you may be able to get away with it.
For many years, I fly sparged with a 5g Rubbermaid cooler. With that setup, I always needed to collect a full boil volume from the sparge. A few years ago, I changed to a 10g Rubbermaid cooler. With this, I find that normally I need to stop the sparge before I have collected the full volume. For simplicity, I make up the volume prior to the boil, but it would certainly be possible to do a partial boil, and make up the volume post boil, without losing any efficiency.
With the 10g cooler, If I sparge for 40 minutes, I need to collect the full volume, but if I sparge for 90 minutes, I can stop the sparge at about 85% of the full volume with virtually no loss of efficiency.
This would indicate that I could do a partial boil with about 85% of the full boil volume. With a different MLT geometry, it may be possible to reduce this %age even further, and make a partial boil with AG possible with minimal efficiency loss.