Is the liquid flowing out the bottom and going through the grain bed or is it flowing out the sides near the top?
Don't know and honestly don't care as long as the liquid is recirculating somehow and there are no flow restrictions. I don't need it to go through the grain bed because BIAB does not rely on the grain bed for filtering. (That's what the bag is for.) I am recirculating simply for the purpose of maintaining an even temperature and avoiding heat stratification during the mash.
Fair enough. I always have been a tried and true BIABer and have never worried about it before.
Since I am upgrading to a PID electric system I have been watching videos of the "3 vessel" systems and I was wondering if it would be possible to use the grain bed to filter like they do.
One of the guys ("Beachbum") in the Aussie BIAB forums recently used a plastic bucket instead of a bag and modified the bucket with a false bottom to lauter through like a more conventional 3V system. Link, if you are a member: http://www.biabrewer.info/viewtopic.php?f=49&t=1057