Pool skimmers: how many you need and where

How many skimmers does a pool need?

One per 269 sq ft (25 m²) of water surface, rounded up, and never fewer than one. A 20 × 10 ft (6 × 3 m) pool (194 sq ft (18 m²)) takes one. An 26 × 13 ft (8 × 4 m) (344 sq ft (32 m²)) takes two, because 1.3 skimmers' worth still means fitting two. A 33 × 16 ft (10 × 5 m) (538 sq ft (50 m²)) manages with two, and a 39 × 20 ft (12 × 6 m) (775 sq ft (72 m²)) takes three. The figure is surface, not volume: a skimmer harvests what floats, however deep the water underneath.

Every plan in the plan library prints its count worked out this way, from the real water surface of its shape — which is why a 30 × 16 ft (9 × 5 m) kidney, just over 269 sq ft (25 m²), gets two, while a 23 × 11 ft (7 × 3.5 m) rectangle at 264 sq ft (24.5 m²) gets one. The plan pages stop counting at three. A 52 × 20 ft (16 × 6 m) pool's 1,033 sq ft (96 m²) is really 3.84 skimmers' worth, so read that three as a floor — or, at that size, weigh skimmer or deck-level overflow, where the whole rim skims and the count stops mattering.

Treat the count as a minimum. Shape, wind and the way the pool is used can each argue for one more; none of them ever argues for one fewer. The table below gives the count for nine plans across the library's sizes and shapes.

How a skimmer works: mouth, weir, basket and pipe

A skimmer is a plastic box buried behind the pool wall, under a round lid in the coping. A short passage through the wall — the throat — ends in a rectangular opening at the water line, the mouth. Across the mouth hangs the weir, a flap hinged at the bottom that floats on the surface, so water spills over its top edge in a thin, fast sheet that draws a leaf in from metres away. When the pump stops, the weir floats up and shuts, keeping the catch inside.

Inside, a basket stops leaves before they reach the pump's own strainer, and a suction pipe, usually 2″ (50 mm), leaves the bottom of the box for the plant room, where on most pools a valve splits the pump's pull between it and the main drain; the guide to circulation gives the settings by season. Two extras turn up on some pools: an equaliser line, a second pipe to an opening lower in the wall that keeps the pump fed if the water falls below the mouth — seen mostly on older North American pools — and a vacuum plate, which sits on the basket and lets a cleaner's hose borrow the skimmer's pull.

Does an L-shape, a kidney or a long pool need one more?

Count first, then look at the outline. A skimmer reaches the surface in an open sweep; anything round a corner or across a narrow waist drifts to it slowly or not at all. An L-shaped pool is two pools sharing water and wants a skimmer in each leg — the plan pages give the 33 × 20 ft (10 × 6 m) L two, one per leg, and if yours counts one, fit two anyway. A kidney is the same in curves: the 30 × 16 ft (9 × 5 m) counts two by a whisker, so set one in each lobe. On a long lap pool, say 49 × 13 ft (15 × 4 m), the wind decides more than the count: if it blows down the length, a skimmer in the far end wall catches everything herded down the lane.

Steps, benches and tanning ledges make dead corners of their own — the pocket beside corner steps is the classic — and floating debris parks in all of them. A skimmer cannot go in a wall the steps are built against, so the fix there is a return inlet aimed across the pocket. And in a sheltered garden with no prevailing wind, add one: two skimmers on the wall opposite the inlets catch whatever the circulation brings.

Count skimmers by the square metre, place them by the wind, and keep the water at the middle of the mouth — the rest is detail.

Where should the skimmers go?

On the wall the prevailing wind blows toward. The breeze herds pollen, dust and leaves into the corner where it runs out, and a skimmer there is fed all afternoon; placed upwind, it watches the same debris pile up at the far wall. To find your wind, look at where leaves gather in your garden in October rather than at a forecast. If two walls qualify, take the one nearer the plant room: a short, straight suction line costs the pump less for twenty years.

Then arrange the rest around them. Return inlets go on the opposite side, angled the same way round so the surface drifts toward the mouths — never right beside a skimmer, where clean water goes straight back into the pipe. Keep skimmers out of the walls where people get in and sit: the steps, a bench, the side of a ledge. With an automatic cover, the roller end belongs to its bench or pit.

Now draw it. Open your pool in the designer and choose the Skimmer edge: it draws as many skimmers as this page counts, on a long wall toward the deep end — a sensible default where no wind decides. Drag the return inlets to the opposite wall and the steps clear of them, and send the builder the share link with one sentence: which wall the wind blows toward.

How high should the water stand in the skimmer?

At the middle of the mouth; anywhere from a third to two thirds of the way up still works. Too low, and the skimmer gulps air: the pump loses its prime, runs loud and can run dry, which costs it its shaft seal. Too high, and the weir has nowhere to drop, so the surface stands still. In a standard skimmer pool that middle is about 5 7/8″ (15 cm) below the coping.

The level drifts. An uncovered pool loses 1/8–1/4″ (3–5 mm) a day to evaporation in warm weather, so a fortnight away takes off 1 5/8–2 3/4″ (4–7 cm) — enough to drop the water from mid-mouth to the lower lip — and a night of heavy rain does the opposite. Top up weekly, let some water out after a storm, or fit an automatic top-up valve.

A high-level skimmer moves everything up. Its mouth is wide, shallow and tucked under the coping, so the water stands about 1 5/8″ (4 cm) below the edge instead of 5 7/8″ (15 cm) and the pool reads nearly full without an overflow channel. The margin is thin, so the automatic top-up is not optional, and splash lands on the deck. In the designer it is the Water level choice under the Skimmer edge: switch between Standard and High-level and see what 4 3/8″ (11 cm) does to the look.

Concrete, liner or fiberglass: which skimmer body?

Skimmers are made for a type of shell and are not interchangeable. In a concrete pool the box is set before the walls are poured or sprayed, so the concrete closes round its throat; setting it level and at the right height fixes your water line for the life of the pool. In a liner pool it is bolted through the wall, the liner clamped between gaskets and a faceplate, and the mouth is cut only once the water stands just below it. A fiberglass shell works the same way, with the opening cut by the shell maker. Faceplates and gaskets are what wear, and every new liner gets new gaskets.

Then choose the mouth. A standard one is roughly 5 7/8–7 7/8″ (15–20 cm) wide; wide-mouth models run to about twice that and suit water that stands high, under a slatted cover or in a high-level design. Pick a lid that takes an insert of the coping stone — a white disc in a travertine deck is the most visible thing a skimmer does — on an adjustable neck, so it sits exactly flush.

Robots, suction cleaners and slatted covers

A robot cleaner shares nothing with the skimmer: it has its own motor and filter, works the floor and walls on a cable, and leaves the surface to the skimmer. A suction cleaner on a hose is different. It plugs into the skimmer through a vacuum plate and borrows its pull, so while it runs that skimmer stops skimming and its basket fills faster. With one skimmer the surface goes unharvested meanwhile; with two, one keeps working — a fair reason to round one up to two if you plan on that kind of cleaner.

A slatted cover floats on the water, so it cares about the level as much as the skimmer does. Its maker specifies how far below the coping the water must stand, and the skimmers have to hold it there; get that figure before they are ordered, because it decides between a standard mouth and a high-level one. With the cover closed the skimmer still draws water from under the slats, so circulation carries on — only the debris now lands on the cover.

Suction safety: drains, equalisers and lids

The danger with suction is a body, a limb or long hair sealing an opening the pump keeps pulling on. A skimmer mouth, half in air with a weir and a basket in front of the pipe, is the lower-risk opening. The serious ones are submerged suction points a person can cover completely: a main drain, or an equaliser opening. That is why drains carry anti-vortex covers, domed so they cannot be sealed flat, and why a pair sits about 3′ 11″ (1.2 m) apart, so one body cannot block both. With drain and skimmers plumbed to the pump together, no single opening takes its full pull — so never run the pump with a lone drain as the only suction line open.

An equaliser opening needs the same kind of cover, and many builders now leave the line out or plug it. Rules differ by country and change over time, so ask your builder which entrapment standard the covers and the layout follow, and check with your local authority. At the skimmer itself: keep the lid seated, since an open hole in the deck is an ankle-breaker; never run the pump without the basket; and if a drain cover is missing or cracked, the pump stays off until it is replaced.

Keeping skimmers working: baskets, leaks and winter

Empty the baskets weekly and after every windy day: a full basket throttles the suction long before it looks blocked, so check that the weir still swings freely while you are there. If the surface stops moving, check in order: the water level, the skimmer basket, the pump strainer, the filter pressure, and the valve between skimmer and drain.

Skimmers are among the most common places for a pool to leak, almost always at a joint: where the throat meets a concrete shell, as a hairline crack around the mouth; at the faceplate gasket and screws of a liner or fiberglass pool; or at the pipe connection under the box. The test is free. Switch off any automatic top-up and let the level fall for a few days; if it stops just below the mouth, suspect the skimmer, and a drop of dye beside the joint, with the pump off, shows a crack drawing water.

Winter is when skimmers break: water left in the box freezes, expands and cracks it or its throat, and the leak appears in spring. If you winterize fully, take the level below the mouths, blow out and plug the lines, and set an ice compensator — a squeezable bottle or a foam insert — in each skimmer to take the ice's push. With active wintering, pool full and pump running on frost nights, the skimmer stays in service: keep its basket empty and its lid on.

What goes wrong in practice

1. Rounding down

   344 sq ft (32 m²) is 1.3 skimmers' worth, and one skimmer on an 26 × 13 ft (8 × 4 m) pool leaves the far half of the surface to drift. Count from the real surface and round up.

2. A skimmer next to a return inlet

   Clean water goes straight back into the pipe it came from, and the far side of the pool stands still. Put the inlets opposite, angled the same way round.

3. Letting the level wander

   Below the mouth the pump sucks air and can run dry; above it the weir stops and the skimmer only sips. Two summer weeks take 1 5/8–2 3/4″ (4–7 cm) off, so top up weekly or let a valve do it.

4. Water left in the skimmer over winter

   Ice cracks the box or its throat, and in a concrete pool the repair means lifting the coping. Take the level below the mouth and fit an ice compensator, or keep the water moving all winter.