Early Problems in the System
As I said I had ran into a lot of problems early on with the system. I had planned on breaking this out separately but I realized it was too hard to determine what was caused by what, so I will go ahead and lay them out here one by one with the final changes I made to get me through this run.
When the plants were very young we noticed the stems had turned entirely purple. More purple than I had ever experienced before. Magnesium was the suspect, but I knew my water / nute mix was balanced because I have grown monsters with this same solution before.
What I determined was I was having problems keeping the pH balanced, causing nutrient lock-out. Since I was using a small res and still tweaking the timings with D2W I was going through a lot more water changes than normal. With no recirculation and low EC for the clones I was having a hell of a time keeping the pH in check.
My initial timing had me running 3s ON and 5m OFF -- but the girls were showing severe underwatering. Going much higher flow rate helped eliminate the problem, but that also caused more water use than I had expected. The small res was needing filled daily at this point.
High Root-Zone Temps
One day when checking on the roots I opened the lids and it literally felt like a sauna in there. I went ahead and bought some thermostats with remote temp sensors so I could monitor things closer.
I was surprised to find that the root zone was running nearly 20° hotter than the room itself! Since the grow beds are well insulated and sealed I figured the only thing I could do would be to cool the water down, which in turn would cool the root zone.
Excessive Water Usage
With everything else going on I found that the D2W was just becoming annoying and I was having to manage water a LOT more than I ever did with my E&F setup. Instead of checking water once every 3 days I was having to check things 2x a day. This wasn't going to work the way I had expected, so I needed to find a solution.
In an effort to solve multiple problems at once I went ahead and picked up a chiller and decided to make the lower 1/4th of the grow beds into an ebb and flow setup.
I still had the old submersible pump from my previous iteration and my loop siphon was still in tact so what I ended up doing was feeding from the submersible through the chiller, then splitting that out into feed hoses into each grow bed. I filled the bottom portion of the beds w/ hydroton to help retain some water (and lower the amount of water required to hit the siphon).
The drains from the grow beds were then rerouted, instead of draining off to waste they would instead converge into my siphon and feed back into the res. Since I was going to have more water flowing I also swapped out my small res with the larger res I used last time.
These changes helped me get root temps under control via the chiller and recirculating water, eased the burden of managing pH by giving me a larger volume of water to balance, and allowed me to switch to a 1s ON/1m OFF cycle w/o fear of losing the water to waste.
I'm still not 100% sure which route I'll take with my next run yet, but I will not be going back to D2W. Right now in my mind there are 3 ways I can run this next time around.
With some of the changes I've made and lessons learned I still think full HPA is possible. By running the drains direct back to the res w/ no siphon I could keep the benefits of a larger res and hopefully be able to keep pH in check. My only concern here is the underwatering problem I ran into.
HPA + E&F
Keeping the system how I have it today with the lower portion filled with hydroton and the upper 3/4 running true HPA. I need to reroute some fill/drain lines and make a few other changes, but this eliminates the fear of underwatering by letting them heavy feed from the lower portion while getting great nutrient uptake in the upper portion of the roots.
HPA + NFT
In this type of setup I would run a film of water down the 4 walls of each grow bed and since they are actual sinks it keeps the film straight down the drain. Then the middle of the beds would be HPA, but the plants could heavy feed off the bottom film once they reached it. (And I could set this up on a separate timer, if I wanted.)
There's a few things I have went ahead and purchased for the next run, knowing I want to change some things. First up is the way I have the water cooler plumbed into the system. Right now I'm running my nute soup through the chiller and to the plants. In the future I will add a second separate res w/ it's own water. That will use the submersible pump to feed through a chiller coil that is submerged in the nutrient res.
Since I will be scavenging my submersible pump I went ahead and picked up the parts I'd need to eliminate the pump in the nutrient solution. I was also having issues w/ small roots being flushed back and clogging up the filter bag, or having to stick my hand down in the soup to change the speed of the pump. I picked up a controllable external pump for my needs as well as all of the filters I would want in-line. This will cover me if I do decide to keep the E&F or NFT systems in place next time around.
The two filters I picked complement each other (40 and 100 micron screens) and contain external cleanouts to help keep the system in tip-top shape.
While I was picking up parts I also upgraded my air pump to a stronger one. The previous pump would bubble the water slightly, but this new one churns the water like it's boiling -- way more air getting mixed into the solution.
When this run is completed I will be looking closely at the roots to help me decide which route I want to take next, but no matter which way I go I have all of the parts on hand to change over and restart.