Grain bill/efficiency

So I asked this in the all grain forum but got no answer, so I'll try here. Does the size of your grain bill have any effect on conversion efficiency? Will fifteen lbs of grain convert more completely than seven lbs? I ask because I've been brewing 2.5 gal batches which are consistently coming up with low OG's. I do not have this problem with 5 gal.batches, so I'm thinking it's not a crush issue. Any insight would be greatly appreciated.

it all depends on recipe, water amounts, temp, process etc. what does your sparge process look like? I suspect it has more to do with something in that process than in conversion.

brewstaedt said:

Does the size of your grain bill have any effect on conversion efficiency? Will fifteen lbs of grain convert more completely than seven lbs?

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In theory, the size of the grain bill doesn't matter. At the right temp, the enzymes will convert starches to sugar and a kernel of grain doesn't care if it is alone in the tun, or with thousands of friends.

However, in practice there can be a significant variance in the efficiency, especially if you are brewing in the same equipment. Your efficiency is based on how completely your sugars are rinsed away by the sparge water. If you use a small grain bill in a larger tun then the grain bed is thin and wide. This decreases the contact with the sparge water and could decrease the efficiency. A taller, narrower tun will change the way the water flows through the grain bed.

Stirring vigorously can help to overcome some of this and increase efficiency.

If everything else is the same, no, there is no difference. However everything else is possibly not the same. For example, it is easier to hold a larger mass (15 lbs of grain plus water) at constant temperature than a smaller mass (2.5 lbs of grain plus water).

Thanks for the input. When I brew 5gal batches I use a 10gal cooler; when I brew 2.5gal batches I mash in a 5gal cooler, and batch sparge with one step. I usually just halve my recipes using brewpal or beersmith. Measured temps are are usually within a degree or two. Volumes are measured pretty carefullly.
I don't know,....I guess I'll try having them double crush my grains at the LHBS next time. Again thanks for the input.