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SUMP FLOOD CAPACITY

Whether the sump can hold what drains back when the power goes out. Test it before you find out the hard way.

Measure in
Display length
Display width
Drop from water line to overflow teeth
How far the display level falls before it stops draining. Usually a quarter to one inch.
Overflow box internal volume, gallons
The water sitting inside the overflow box that drains out when flow stops.
Return nozzle depth below water line
How deep the return outlet sits. All of this siphons back unless a break hole stops it.
Return pipe internal diameter, mm
Return pipe length below display water line, ft
Sump
Sump length
Sump width
Free height above running level
From the normal running water line to the lowest point it could overflow.
Sump capacityadequate with a siphon break
7.27gallons of free space
Needs, with siphon break
3.46gal
Fits, with 3.81 gal to spare.
Needs, without one
10.94gal
Floods without a siphon break hole.
Display drains
1.87gal
0.50 in off the display before it stops
Return siphons back
7.57gal
A 3 mm hole drilled just below the water line at the return nozzle cuts this to 0.09 gal.

When the power fails, the display drains down to the overflow teeth, the overflow box empties, and the return line siphons backwards until air reaches the nozzle. All of it lands in the sump. Lower the sump running level until the free space above it exceeds that total.

MethodFormula and sources

drain down = display 7.1 L + overflow box 5.7 L + siphon 0.3 L = 13.1 L against 27.5 L of free sump

Three things drain into the sump when the return pump stops. The display falls to the level of the overflow teeth. The overflow box empties through the drain. The return line siphons backwards until air breaks the siphon at the nozzle.

The display contribution is footprint times the drop to the teeth. It is usually small per inch and large in total, because the display footprint is large. Half an inch off a 48 by 18 tank is 1.7 gallons.

The return siphon is the one that catches people. Without a break, water siphons back through the return line until air reaches the nozzle, so everything above the nozzle depth across the whole display surface comes down. Drilling a 3 mm hole in the return pipe just below the water line stops the siphon almost immediately and reduces this to the pipe volume alone.

Check the siphon break hole regularly. Coralline algae grows over it, and a blocked break hole turns a tested safe system into a flood without any warning.

The real test costs nothing. Fill the system, run it normally, then unplug the return pump and watch. If the sump takes it without spilling, it works. Do this before the tank is stocked, and do it again after any plumbing change.

Set the sump running level low enough that the free space above it exceeds the calculated drain down with margin. Losing a couple of gallons of sump volume is cheaper than replacing a floor.

Check valves are not a substitute. They fail on debris, and the failure mode is the flood you were preventing.

Sources: BeanAnimal, silent and fail-safe overflow · Synergy Reef overflow tuning

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