I agree, as my limited understanding goes, in many cases. In an open room, where those 3" returns can run level and not have to go up, through a bulkhead and back down to get to the res, it obviously can and does work great. But for us tent growers, from an engineering standpoint, we (tent) growers all run into the same problem he was appearantly trying to avoid. That "caterpillar" he uses to get up to and through the bottom ducting port/sock sucks, IMO. Yes, it works, but only up to a point. ANY tiny air bubbles that get pulled into that return will collect at the apex of the caterpillar and eventually can cause the syphon action needed for it to work properly to be broken by an air lock. That is the very issue I was running into with my trainer tomato grow. (and as I recall,
@SweetLeafGrow had that same issue with it, the caterpillar) My return line also had a high point in the middle where it went through the tent sock, ...and the rest is history. I regularly had to 'burp' that 1" hose line by raising the grow bucket about 10-12", until the air bubble traveled UP the hose and out. I could do that because of the flexable return hose. I have no idea how SLG burped his caterpillar.
I fixed that issue with my current RDWC by raising the buckets up off the floor, about 12+", so that the return hose was higher on the ends and lower in the middle, going through the tent wall sock. That keeps all the bubbles out of the return line and works in my 8' tall tent. Loosing 8-12" of vertical space by raising the pots in a standard 6'5" tent to keep that 3" return nice and level is not as much of an option.
So, for the sake of us that grow in tents (and those tents need some major upgrades, IMO) and don't want to cut additional holes in them, is there some obvious reason not to reverse the flow that I'm just not seeing and overlooking? By running it with the return on top and the recirculating pump pumping from the res into the bottom of the grow bucket, I'm THINKING you would eleminate 2 issues. The caterpillar bubble issue, and any potential root clogging issues, minimal as they may be with the 3" return. Just thinking out loud and wondering where the problem(s) would be, ...are?
Draining? yes, I'd want a small handy spare submersable to empty the system, like I have now. So that's no problem.
Filling/topping up? yes, I understand the basic water hydraulics of it. Only the res bucket water level will go down. OK, so.... is that a problem?
What am I missing here, honestly? Seriously?
