My brewing buddies and I tried to do a late LME addition this weekend. It didn't go as we expected.
We were brewing a 10 gallon batch from two identical kits (NB Honey Brown Ale) and the kits came with 2 containers of LME (1 for each kit). We added the first container at beginning of boil and then added the second one at flameout, along with the 2 containers of honey for the batch. We stirred for a while and then added distilled water to bring the batch up to 10 gallons. Then we hooked up the CFC and hoses and opened up the spigot. What we thought was wort flowed through the clear hose to the chiller and then...nothing.
Minutes later, we finally figured out that it was mostly undissolved LME that was trying to get through my CFC. We closed the spigot, cleaned out the hoses and CFC, heated up the kettle for a few more minutes while stirring then tried again, with success.
After cleanup, we discovered that we had a goodly amount of scorching taking place under our built-in SS mesh screen that was fitted over our diptube.
So...two possibilities, I guess.
1) The LME just sank right down into our SS mesh screen and sat there, not being affected by the stirring taking place outside of the screen, waiting to be sucked up into the diptube.
2) We didn't stir long enough before adding the cold distilled water.
I think if we try this again, we'll probably add the last batch of LME 10-15 minutes before end of boil just to make sure the LME has had time to dissolve into the wort.