Plumbing HOB overflow

[Sticker shock]

Quick question regarding HOB overflows...

I read many threads about these overflows and in pretty much every one people discuss eventual failure leading to floods and how you should just drill the tank. If it is set up correctly, shouldn't his be a non-issue. With the correct water levels why would there ever be a flood?
Suppose there is a power outage...if you keep the correct water level in the sump, in theory it will fill up to where there is no more drainage allowed from the overflow as the tank level drops. On the return if you break the back siphon with air holes or high placement, then that also will not be an issue. A simple power outage test can cover that scenario. In the case of a broken siphon the return pump should only be able to pump in what the return chamber allows. Worst case would be a burnt out return pump (probably could be mitigated with a simple float switch).

The only item I haven't quite wrapped my head around is auto top off. Since I haven't run a sump before I'm not quite certain what the effect would be here with either a broken siphon or power outage. My thoughts are that a power outage would fill the sump so no issue there. Broken siphon on the other hand would drain the sump, but only in the last chamber as the other chambers should have a fixed level...correct? If that is true then ATO in any chamber besides the return chamber then this again is a non issue.

So my question is why are these floods happenning with HOB overflows?

 

[Silent]

People have ran those for years without issues. Also, their are many horror stories. Some have used an aqualifter pump attached to the top of the tube for the overflow to help with the siphon.

 

[beaslbob]

The reason there are floods with HOB (or any overflow/drain) is that they are not properly setup.

Part of any sump setup is these three critical adjustments:

1) insure no floods (sump) with power outage. Turn the power off and make adjustments so there are no floods.

2) insure normal operation returns when the power comes back on. Turn the power back on to check. With hob overflows make sure the siphon is broke before turning on the power and the siphon is reestablished with the power returns. I just use a powerhead with a 'bubbler" function to suck the air out and reestablish siphon.

3) (and the one I forgot to test in my first setup). Fail the drain (siphon break, blockage, etc) and insure the sump runs dry before the display floods. Then restore the drain and mark a line on the sump for the water level. From that point on never never under any circumstances add water if 1) the pump is not running, and/or 2) to above that line.

Any ATO you add needs to be setup to where it is turned off if the display level rises. For instance, during a drain failure, the sump level goes down, display up and the ato pumps more water into the system. Very likely causing a flood.

All of those things apply to any display and sump setup. The only additional failure mode for an over the top system is the siphon break. Which is mitigated by the power head with a bubbler system sucking the air out of the hob siphon area. But even with a drilled tank those same adjustments are necessary,

my .02