Whether the drain can carry the return pump, how deep the water runs over the weir, and whether the emergency line copes.
| Nominal pipe | Full siphon | Durso or open channel | As emergency |
|---|---|---|---|
| 3/4 in | 366 | 225 | 169 |
| 1 in | 650 | 400 | 300 |
| 1.25 in | 1,016 | 625 | 469 |
| 1.5 in | 1,463 | 900 | 675 |
| 2 in | 2,600 | 1,600 | 1,200 |
A full siphon carries far more than an air entrained standpipe of the same size, because it runs full of water instead of half full of air. That is the whole idea behind the Herbie and BeanAnimal designs, and it is also why a siphon has to be valve tuned and needs a backup.
capacity ≈ 650 GPH per in² × (1.00 in)² = 650 GPH · weir head = (Q ÷ (179.77 × 24))^(2/3) = 0.268 in
Drain capacity scales with the square of the pipe diameter, because it scales with cross sectional area. Moving from one inch to one and a half more than doubles what the pipe can carry.
The style matters as much as the size. A full siphon runs completely full of water and carries roughly 650 GPH per square inch of nominal diameter in a conservative design. A Durso or open channel standpipe entrains air, runs part full, and manages about 400. An emergency line that only engages when the primary blocks runs as an open channel from dry, so it gets about 300.
These coefficients are fitted to published hobby drain capacity tables and hold within about six percent across the common sizes. Treat them as design figures with margin built in rather than the last word.
Weir depth comes from the Francis weir formula. In US units, Q = 179.77 × (b − 0.2h) × h^1.5 with flow in GPH, weir length in inches and head in inches. Inverting it gives the depth of water flowing over the teeth.
That depth matters for skimming. Under about an eighth of an inch, the sheet is so thin that small level changes make the flow uneven. Much over half an inch and the overflow is carrying more than it was designed for, which is where noise and surging start.
Size the drain for at least one and a half times the pump. Drains do not stay clean. Coralline grows, snails wander in, and a drain running at its calculated limit on day one is running over it by month six.
The strongest argument for the BeanAnimal design is the third pipe. A dry emergency standpipe sits doing nothing until the primary blocks, and then it carries the flow instead of the floor. Size it for full pump flow rather than treating it as a token.
Test it. Block the primary drain deliberately with the tank running and watch what the emergency does. Better to find out at a time you chose.
Sources: BeanAnimal, silent and fail-safe overflow · Synergy Reef overflow tuning