Originally Posted by alexnharvey
How would it? A smaller FB would have less dead space. It might make draining slower.
Assuming he's looking for 18" because that's the diameter of the mash tun, then use of a 12" FB would disrupt the laminar flow down through the grain bed. How much? No idea.
Space under a false bottom really isn't considered dead space (which is defined as the amount of liquid remaining in the vessel after a full draining). I don't know of an existing term for the area under a FB so I've been calling it slack space or foundation volume. The only time it would effect efficiency is if it forces you to use a mash thinner than you'd normally want. It would take quite a bit of slack space to do that.