I think I've really screwed this up... I'm trying to wrap my head around what happened.
It's my first AG brew. HERMS system. Mash went beautiful. Added the 3.4 gallons to MLT (I have a sight glass on HLT). Temps were within 1-2 degrees for the full 60 mins. Mash out. Transfer mash to boil kettle. I then sparge with two halves of the 1.4gal (stir, vorlauf etc). There's barely enough liquid to circulate with my pump.
I for some reason checked the sparge SG after the second half, and (temp corrected) it was 1.05 and still just as dark. I thought... well, there's still a ton of sugar left. So I did another 1 gallon sparge (which I probably was in the wrong to do?).
I measured my pre-boil SG in the BK (1.050), which actually was almost the same as the calculated I had done (1.058). So now I'm at roughly 5.5 gal in the BK (I don't have a sight glass on my BK yet, but I did use a measuring stick). I go ahead and complete the boil. I chilled it to 70F (best I could do), and transferred to carboy. It was at roughly the 4 gallon mark. I reaally should have measured SG at this point, but I assumed it would be high because my volume was low. I added more water to get to about 4.8 gallons, and check my SG just to be sure I wasn't adding too much.... and it was 1.031....
Should my sparge liquid have been a lower gravity after the second half? When I added the .75gal, it did not rise over the grain bed.
How vigorous should your boil be? Initially I had both 1500W elements on and I was getting a pretty violent boil. I dropped down to one of my 1500W elements and it was steaming maybe half the amount. It was a rolling boil, but just over the one element's side of the kettle.
My assumption is I did not get enough extracted during sparging, underestimated my preboil volume, and added too much water in the fermeter. Any thoughts? I'm a bit puzzled.
Edit: took another gravity reading. It was 1.039. I'm thinking the water I added initially was not mixed up well. Regardless, its still pretty low. I took a taste and it is bitter, like a very hoppy ipa.