Pool circulation: skimmers, inlets and drains explained

One loop, and everything depends on it

Every pool runs a single loop. Water leaves through the skimmers — or, in a deck-level pool, over the whole perimeter into the overflow channel — and through the main drain. It passes the pump, the filter and whatever else is in line (heater, salt cell, UV) and comes back through the return inlets. That is the entire system.

The part people get wrong is thinking the filter does the cleaning. The filter only ever cleans the water that reaches it. If a corner of your pool never joins the loop, no filter, no chlorine and no expensive gadget will fix it; the water there simply sits, warms, and starts growing things.

Circulation is also the part nobody photographs and nobody can change later. Tile can be replaced, the cover can be replaced, the pump will be replaced twice; the pipes and the holes in the shell are there for the life of the pool. This is the drawing to get right before anyone pours concrete.

Turnover: the number everything is sized from

Turnover is the time the system needs to pass the whole pool volume through the filter once. For a private pool the target is 4–6 hours, and two to three turnovers a day keeps water polished. An 8 × 4 m pool averaging 1.4 m deep holds about 45 m³, so a six-hour turnover means roughly 7.5 m³ an hour — and the pump is sized above that, because pipes, filter and fittings all cost head.

A variable-speed pump changes how you use that number. Power rises roughly with the cube of the flow, so halving the speed and doubling the hours moves the same water for a fraction of the electricity. Slow water filters better too: a sand bed catches fine particles at low flow that it pushes straight through at high flow.

Run time should follow the sun, not a fixed habit. Chlorine is consumed in daylight, so that is when the water most needs moving; a night-only schedule leaves the surface undisturbed all day. Split the run into a long afternoon block and a shorter evening one, and lengthen it after rain, a heatwave or a party.

Taking water off the surface

Nearly everything that dirties a pool arrives from above and floats: dust, pollen, blossom, insects, leaves, sunscreen and body oils. Harvested in the first hours it never becomes a problem. Once it sinks, it becomes brushing, vacuuming and chemistry.

A skimmer pool does this with wall units, one per roughly 25 m² of surface, with the water sitting about 15 cm below the coping. The placement rule is simple and often ignored: skimmers go on the wall the wind blows toward, so the breeze delivers debris to them. An 8 × 4 m pool needs one by arithmetic, but two is better — a pair pulls the surface more evenly and gives you a spare when a basket fills.

A deck-level overflow pool turns its entire perimeter into a skimmer: the water is level with the deck and spills continuously into a hidden channel. That is why overflow pools have the cleanest-looking water of any edge system. The price is a balance tank at roughly 5–10 % of pool volume, precise levelling and more plumbing.

A filter only ever cleans the water that reaches it — everything else is decoration.

Pulling from the bottom

A main drain at the deepest point does three jobs: it pulls the cold, still layer off the floor, it collects debris that has already sunk, and it makes fully emptying the pool possible. Without one, the bottom of a deep end is a slow, cool, undisturbed zone.

Safety is not optional here. Drains carry anti-vortex covers, and where a pool has more than one they sit about 1.2 m apart — far enough that a body over one cannot block the suction of both. If a builder offers a single small drain with a flat grate, ask what standard it meets.

In the plant room a valve usually splits suction between the skimmers and the drain — the free tuning knob almost nobody touches. Roughly 70 % skimmer and 30 % drain suits summer, when the problem floats. Move it toward the drain in spring, after a storm, or whenever the floor is the dirty part.

The inlets do the real work

Getting water out is the easy half. What decides whether your pool has dead zones is where the clean water goes back in. Plan roughly one return inlet per 15 m² of surface — two to four for a family pool — and think of them as one system, not separate holes.

Wall inlets sit 30–40 cm below the surface, angled the same way round and tilted slightly down. Done properly the pool becomes a slow carousel: the whole body turns, the surface drifts toward the skimmers, and they get fed instead of waiting. The classic error is aiming two inlets straight at each other, where they cancel out and the water stands.

Floor inlets are the usual arrangement in deck-level pools. They push filtered, heated, chlorinated water up through the whole depth, so temperature and chemistry stay even and the bottom never goes stagnant. They cost more and they are cast into the floor — no changing your mind afterwards, which is why the choice belongs on the first plan.

Dead zones, and how to find yours

Dead zones live in predictable places: behind the steps, under a tanning ledge, in the inside corner of an L-shaped pool, under an overhanging coping, and directly behind anything bolted to a wall. Every one of them is a spot where the rotation does not reach.

You find them without equipment. They are where algae appears first, where the water feels a degree cooler, and where a leaf dropped on the surface stays put instead of drifting. Do the leaf test on a calm morning with the pump running — thirty seconds tells you more than an hour of reading.

The fixes run cheap to expensive. Twist the directional eyeball in an inlet so its jet sweeps the stalled corner — free, and it solves most cases. Add an inlet if you are still building, and aim one deliberately at the worst corner even if it spoils the plan's symmetry. And run the robot, which reaches places the water never will.

Pipework decides the electricity bill

Everything between the pool and the plant room is resistance the pump pays for, every hour, for twenty years. Wider pipe is the cheapest upgrade on site: 63 mm instead of 50 mm on a long run keeps velocity down — a useful ceiling is 1.5 m/s on suction and 2 m/s on returns — and a slow pipe is a quiet one.

Sharp elbows are the other hidden cost. Two sweeping 45° bends do the same job as a tight 90° for a fraction of the head loss, and a run with six elbows can eat more pressure than the filter itself. Keep the plant room within about 10 m if the site allows, and the suction side short and straight. And have the pipework pressure-tested and photographed before anything is backfilled — those photos are worth their weight the first time somebody needs to know where a line runs.

Draw it before you dig, then tune it by season

In the designer, choose your edge type — skimmer or deck-level overflow — switch on the return inlets and set them wall- or floor-mounted, place the drains, and drag everything around your steps and ledge until nothing hides behind anything. Print it and hand it to the builder: a five-minute conversation that stops an inlet sitting behind the ledge for thirty years.

Then treat the running system as adjustable. Backwash when the filter pressure rises 0.3–0.5 bar above its clean reading, not on a calendar. Empty the pump basket weekly and the skimmer baskets after wind. Move the skimmer-drain valve with the season. And when the water goes cloudy, check circulation before reaching for a bottle: cloudy water is usually a flow problem being treated as a chemical one.

What goes wrong in practice

1. Inlets aimed at each other

   Two jets fired straight across cancel out and the pool stands still while the pump runs all day. Turn every inlet the same way round so the whole basin rotates.

2. One skimmer on the wrong wall

   Put upwind, it waits for debris the breeze is pushing to the opposite side. Check the prevailing wind on your plot before the plumbing is drawn, not after the first windy week.

3. An inlet behind the steps

   Half the return flow hits concrete and the shallow end never turns over. Draw accessories and plumbing on the same plan so the conflict shows up before it is cast in.

4. An oversized single-speed pump

   It pushes water through the filter faster than the filter can clean it, and it is the most expensive thing in the plant room to run. A variable-speed pump moving the same water slowly filters better and costs a fraction.

5. Treating a circulation problem with chemicals

   A green corner is a flow problem. Shock treatment buys a week; an inlet aimed at that corner fixes it for the life of the pool.