Why does the kettle overlap work?
If you think about the process, the bottom ‘grant’ tank will never see a full 15 gallons at once. I mash in at about 8 gallons. I use the boil kettle (at the very top not shown) to gravity sparge which you only need about 10 gallons. When the sparge water is done from the boil kettle, there will still be 3-4 final gallons of wort left sparging in the mash tun, and about 12-13 gallons that have collected in the grant. Once this happens, you can pump all collected wort form the bottom grant/HLT to the boil kettle that just got finished sparging. I preserve gravity draining from the mash-tun as you know that the liquid line in the grant/HLT will never exceed the bottom of the mash-tun. Hence you can overlap the kegs. Hence no stools and hardly any tippy-toes
Let me distill it down even more (no pun intended), as long as there is the weight of the liquid left in the top container, it will ‘push’ liquid out though your pipe to the bottom container as long as you maintain a difference in liquid line heights. You can even loop the hose up to the ceiling and back down – it will still siphon.
Take advantage of this easy law of physics to reduce the vertical and horizontal footprint of your home brewery. Conventional 3 tier beer stand trees don’t take advantage of this…they assume every kettle will be 100% full at some time!
This diagram is a snapshot of the very end of our mashout/sparge right before you pump the wort from the grant to the very top boil kettle (not shown).