So, a few batches ago, I purchased a bag to try BIAB after seeing how much I could save over extract by going all-grain. My brew pot is only 21 quarts, so I was only boiling about 4 gallons in it at a time, but had a second, 4 gallon pot going beside it to keep it topped off. I'm doing all of this on my kitchen stove, and since my apartment complex won't allow propane accessories, I don't have another option.
For that first batch, I did an ordinary bitter, and drained the bag into the second pot after it stopped dripping vigorously. I was all over the place with my mash temp, because I heated the water with the grains already in it. My efficiency was somewhere below 70%, and I missed my target ABV by about .5%. Not terrible, of course, and the beer turned out really, really tasty, so who cares about the alcohol?
On my second batch, a Vienna lager (the lagering equipment is why I haven't sprung for more expensive AG supplies -- I'm just not ready to sink that much equipment cost yet), I'd heard that you could move your bag into your bottling bucket and use that as a lauter tun. I tried that, and had a really hard time getting a vorlauf to work, but was eventually satisfied that the result was close to what I wanted. Tons of break material (this was also my first batch with a wort chiller), but the beer looked fairly clear in the fermenter. I had a few stuck sparges, and finally found a balance point of how to clip my bag onto the bucket to get proper flow. In that process, I lost a lot of temperature, though, given the time I had to fidget. This time, I'd also added the heated water to the grains in the mash kettle, and it worked like a charm -- just wrapping it in a sweatshirt kept the mash at my target temp for the whole hour.
For the third batch, an experimental dark wheat beer with oats and honey, I basically improved upon my successes with the bottling bucket lautering, but attempted to get to mash-out temperature, and scorched a hole near the top of my bag (not sure why it was that high up, but it doesn't make the bag entirely unusable). The lauter worked great (though, with as dark a beer as this was, it's a little hard to be sure I got the clarity right in vorlauf).
Looking ahead, I've got an idea. Would it be reasonable to mash in my brew pot without the bag and then transfer the grain to the bag inside my bottling bucket as a lauter tun? Since I'm lautering, it seems like it would be less of a mess and make mashing easier to only involve the bag in the bucket, rather than having to lift the bag and transfer it (I got splashed a lot on batch three when it hit the bottom of the bucket).
No, a bottling bucket isn't the ideal lauter tun (especially for heat retention), but I've already mashed for an hour before I'm transferring there. But it's something I've already paid for, and it means less equipment taking up space in the apartment. Has anyone else used a process like this?