BIAB, Can I use lees water for mashing and then top it for cooling?

Question

I am new to homebrewing, I made only 2 BIAB batches. I am using a 25l keg and a burner.

My pre mash batch is 21-23l since I don't want the wort to overflow when boiling. In a biab method since there is no recirculation and sparging, I think we loose 3-5l until the fermentation bucket. Due to that loss and that I can't chill the wort to the desired temp (I brew for 2 hours and try to chill for 3 hours😂) before adding the yeast, can I boil and freeze 3-5l of water and add them in my fermenter to cool my wort down?

If this is possible, can I use the grains and hops for a 25l batch in a smaller water ratio (15l) and then proceed with sparging (5l) to reach 20 l before boil and then add 5l of ice before fermentation?

Answer 1

Yes, it will work and I believe that doing so is a common trend among BIAB brewers. What you're describing is pretty similar to extract brewing (except you're sort of making your own extract) so there are some considerations around brewing with a semi-diluted wort... mostly nitpicky things around IBU calculations that probably aren't going to make-or-break you.

Just make sure you're doing your calculations so your dilutions will still hit your numbers. If you have a refractometer on hand and can check your gravity mid-brew it may help to have a bag of dry malt on hand to shore up in the event you come up short.