Main drains look like one of the most mundane parts of a pool, yet a poorly designed one carries a real suction-entrapment risk. Much of international pool safety standardization (VGB, ANSI/APSP and similar) focuses directly on main drain design — here's the technical reasoning behind it.
Why is a single suction point dangerous?
If a pool has only one main drain and the grille cracks, comes loose, or gets fully blocked by hair, a swimsuit or a limb while the pump is running, the pump's entire vacuum concentrates on that single point. This is the root cause of suction-entrapment incidents, which disproportionately affect children and can cause serious injury.
This risk is why, since the 2000s, many countries have legally restricted single-point main drain designs and require multiple drains instead.
How does multi-drain design work?
The recommended modern approach is to split total suction flow across at least two main drains, ideally spaced at least 1 metre apart. That way, if one point is fully blocked, the other still carries the flow and the dangerous vacuum concentration never builds up at a single point.
The grille's open area (free-flow surface) also needs to be sized generously against the design flow rate — a grille with a wide, well-designed hole pattern produces a much lower suction velocity, and therefore a much lower risk profile, than a small, densely perforated one.
What to check when specifying a grille
The core criteria are: the grille must be screw-fixed so it can't be removed without a tool, made from an impact-resistant material (ABS plastic or AISI 304/316 stainless steel), and sized correctly for the pool's flow rate (square or round options from 250mm to 700mm are common).
Side suction grilles (Ø250-Ø400mm) can serve as a second suction point in deep or narrow pools where a floor-mounted grille alone isn't enough, making it easier to apply the multi-drain principle.
A quick checklist for existing pools
A practical checklist for operators: is the grille cracked or broken, are all the fixing screws in place, could a single-drain pool be converted to a second suction point, and does the grille match the manufacturer's rated flow? These four questions should be checked on a regular basis — at least once a year.
Browse Main Drain Grilles
Square and round main drain grilles from 250mm to 700mm, plus Ø250–Ø400mm side suction grilles — in ABS plastic and stainless steel (AISI 304/316).
