[asylumdown]

Herbie style overflows definitely have their place in this hobby. When they are set up right, they give you all sorts of control over the noise level in your tank, and prevent your overflow from becoming a sludge filled accidental refugium. However, I've discovered that they're not appropriate in every circumstance. When I was researching how to set up my tank, I never found a thread that explained why they wouldn't work for me specifically (and thus my current predicament), so here's hoping I can help avoid this PITA for someone else.

When Herbie overflows work great:
If you have one overflow on your tank, or if you have two overflows that are completely separate. It also helps if your auto-top off system adds water relatively slowly, and if your return chamber is large relative to your sump.

When Herbie overflows are not the right choice for a system:
If you have two overflows that are plumbed together before they exit in to your sump so that you only have one pipe depositing water in to your sump, and when your return chamber isn't very large relative to your sump. My plumbing is set up this way, for a few reasons that aren't relevant to discuss here, but one of them being that I only need to use one filter sock. Originally, I believed that placing the gate valves that controlled the herbie's above the point where the pipes met would give me individual control over the height of the water in each overflow chamber. I was mistaken. Even though the pipes connect downstream of the gate valves, the whole system still functions like one unit. I can set the height in the overflow chambers for maybe a day, then over the course of a few days, one overflow chamber will progressively fill up, while the level in the other will fall. Once that happens, adjusting the height of the water in the overflows is extremely difficult, as adjusting one overflow will have secondary effects on the height in the other. I gave up months ago trying to have both overflows set at the level I wanted and accepted that only one would be below the main tank's water level. They are also prone to freak out if you put your hands in the tank for maintenance. I've had my herbies (whose dials hadn't been touched in months) fill up spontaneously and start pouring down my emergency drain simply because I started turkey bastering my rocks.

I then added a single gate valve downstream of where the overflow pipes met to see if I could 'Herbie the herbies'. This works, but you still have no individual control over the water level in each chamber.

There also seems to be a relationship between the height of the water in the overflow chambers, and the height of the water in the return chamber of your sump - that isn't as obvious as it sounds, I promise. For example, if you have your herbie set and running perfectly, then you add a few inches of water to your sump, the water level in your sump will eventually fall back to where it was initially set. This has the side effect of filling up your overflow chambers by the same amount of water that was added to the sump. This isn't a huge problem unless, like I did yesterday, you add a more powerful pump to your ATO system (the R/O reservoir is in the basement and the pump I had wasn't cutting it). When the Tunze osmolator brain kicks on, it runs for a set number of seconds before it can shut itself off. In a sump with a relatively small return chamber, and a powerful ATO pump, this means that the osmolator actually overfills the return chamber slightly (water completely submerses the sensor) before the osmolator turns itself off. It did this all day, overfilling the return chamber, which then filled up the overflows in the display tank by a small amount, then evaporation triggered the sensor again, which led to overfilling of the return chamber again. Process repeat until water was pouring down my emergency drain standpipe.

Anyway, I'm going to get the parts together for a DIY Durso this week. In my application, herbies are more trouble than they are worth, though I'm sure for someone with a much simpler plumbing set up, they would work beautifully.