A skimmer is that unremarkable window in the pool wall that draws off the very top layer of water — and it is precisely there that the vast majority of the dirt entering your pool stays. If the waterline keeps leaving a greasy mark on the walls, the pump is sucking air or the filtration is losing performance, nine times out of ten the answer lies in the skimmer or in the height of the water level. This article covers how a skimmer works, exactly where the water level should sit, how many skimmers a pool needs and what to do with them every week.
How does a skimmer work?
Pollen, dust, grass pollen, fine ash, fibres, insects, sunscreen and skin oils — all of it is lighter than water or is held up by surface tension. Both the technical literature and everyday practice agree that almost all contamination first sits in the top few centimetres of the water column and only sinks to the bottom once it has soaked through. The goal of circulation is therefore not to “vacuum the pool” but to continuously draw off that surface layer while it is still up there.
The skimmer achieves this with one simple trick. The intake opening sits at water level and is covered by a hinged flap (technically a weir, or weir door). The flap floats on the water and lifts according to how hard the pump is pulling. This creates a narrow slot through which only a thin film from the surface flows over — not water from the depths. The flap also acts as a check valve: when the pump switches off, it swings shut and the trapped dirt does not float back into the pool.
Behind the flap comes the basket — a coarse strainer that catches leaves, insects and blades of grass before they reach the pump impeller. Below it is the suction port leading to the pump, on better models supplemented by a suction control: a flap or gate valve that splits the suction between the skimmer and the main drain. A ratio of roughly 70:30 in favour of the skimmer is a good starting point for normal operation; with cloudy water you draw more from the bottom.
The skimmer is thus the first link in the circuit: pool → skimmer → pump → filtration → heating and disinfection → return jets. What happens further along the circuit is covered in the article on sand and cartridge filtration; the skimmer itself filters nothing, it only pre-cleans and determines where the water is taken from.
Where should the water level be? This is the most important number in the whole article
A skimmer only works properly within a narrow water level range. The rule is: the water level should cross the skimmer mouth at roughly half to two thirds of its height. On a typical skimmer pool this corresponds to about 10–15 cm below the top of the coping.
When the level drops below the bottom edge of the opening or right down to it, the flap stops floating on the water and the skimmer starts drawing in air. Anyone can spot it: you can see a vortex in the basket, the pipework gurgles, foam appears in the pump pre-filter and the pump gets noisy. Air in the circuit reduces the flow rate, overheats the shaft seal and in extreme cases leads to dry running — meaning damage to the seals and the motor. What to do once the pump is already humming or will not start is covered in a separate article on a pool pump that hums and will not start.
The opposite extreme is just as damaging, only quieter. When the level is too high and covers the entire opening, the flap has nowhere to swing to and the skimmer draws water from the depths instead of from the surface. The pool filters quietly, the pressure gauge shows normal values — and yet an oily film sits on the surface and the typical grey ring forms on the walls. This is the most common “invisible” mistake after rain or after topping up.
The level changes on its own, too: evaporation in a hot week can easily reach 3–5 mm a day, while heavy rain will raise the level by several centimetres. Checking the water level therefore belongs in the weekly routine, not in the once-a-season list. Anyone who would rather not keep watch at all should go for automatic top-up with a float or level sensor.
How many skimmers does a pool need and where do they belong?
The rule of thumb used by both designers and manufacturers of pool equipment is one skimmer per approximately 25 m² of water surface. A smaller family pool of 3 × 6 m (18 m²) therefore gets by with one, a 4 × 8 m pool (32 m²) already deserves two, and a 5 × 12 m pool calls for three. That figure is a conservative compromise; some technical documents allow up to 35 m² per skimmer, others recommend denser fitting for pools surrounded by trees.
What matters is not just the number, however, but the placement. The skimmer is fitted on the side towards which the wind pushes the dirt — that is, downwind, on the leeward side of the pool. The wind naturally drives the surface film across the water, and at the end of that journey the skimmer simply “collects” it. A skimmer placed upwind works at half its capacity, because the wind constantly pushes the dirt away from it.
The second rule: skimmers and return jets belong on opposite sides. The jets should push water along the surface towards the skimmer and create a gentle circular flow. When they sit next to each other, the water “short-circuits” and half the pool is left without circulation. How long the whole circuit needs to run for the volume to turn over is covered in the article on how long to run the pool filter.
The difference between this arrangement and a pool where the water overflows around the entire perimeter into a channel is described in the comparison of a skimmer pool vs. an overflow pool. Put simply: an overflow collects the surface continuously, a skimmer does it at single points — and that is why, in a skimmer pool, the position and maintenance of those few openings matter so much.
Skimmer maintenance: what to do every week
The skimmer is one of the few pool components where a few minutes of work a week directly saves both electricity and chemicals. A full basket throttles the flow just as effectively as a clogged filter.
- Empty the basket at least once a week — and always after a windy day, a storm or after mowing the lawn nearby. During the birch and conifer flowering season, every other day is fine.
- Check the flap. It must swing freely and must not be jammed, broken or blocked by a leaf. A jammed flap = the skimmer draws from the depths, which means almost nothing from the surface.
- Watch the seals and the flange. Hairline cracks in the skimmer body or a leaking flange are a common cause of slow water loss; the difference between a leak and evaporation is explained in the article on water loss vs. evaporation.
- Before winter, fit a winterizing plug (blanking off the suction port) and place a freeze protection float in the skimmer — a polystyrene or hollow plastic block that absorbs the expansion of freezing water and protects the skimmer walls from cracking. The full procedure has its own guide: winterizing a pool step by step.
The seasonal load of leaves, pollen and insects can be reduced considerably at the surface itself — the less organic matter reaches the basket, the less chlorine is consumed breaking it down. Practical methods are summarised in the article on leaves, pollen and insects in the pool.
Accessories: what belongs in a skimmer and what definitely does not
A skimmer sock (a fine nylon or microfibre stocking pulled over the basket) is the cheapest upgrade to your entire filtration. It catches pollen, fine fibres and particles that would slip through an ordinary basket all the way to the filter. During pollen season it can significantly extend the interval between backwash cycles. Change it conscientiously, though — a saturated sock throttles the flow very quickly.
A floating chlorine dispenser does not belong in the skimmer. A tablet dispenser dropped into the skimmer creates an extremely concentrated solution with a very low pH inside the basket, and that solution flows straight through the pump and filtration. On steel and stainless steel parts — screws, flanges, heat exchangers, the frame of an above-ground pool — this means rapid pitting corrosion, and on plastics it means embrittlement. Tablets belong in a floating dispenser on the surface or in a separate dosing unit; an overview of the options is given in the article on automatic chlorine dispensers.
A vacuum plate (adapter) is a flat part that is inserted in place of the basket and lets you connect the hose of a manual vacuum directly to the suction port. Without it you push the hose into the basket and coarse dirt then goes straight into the pump. If you vacuum regularly, the plate is one of the first things to buy — as is a separate shut-off valve on the skimmer pipework, which lets you deliberately split the suction when you have several skimmers.
Diagnostics: what bubbles, slurping and falling flow mean
The skimmer is the best diagnostic point in the whole circuit, because it is the first to signal a problem on the suction side. The logic is simple: bubbles and noise = air in the suction, falling output from the jets with rising pressure = a blockage after the pump.
If you see bubbles in the pump pre-filter or foamy water coming out of the return jets, the circuit is drawing air somewhere — most often because of a low water level, a leaking pre-filter lid or a cracked skimmer flange gasket. If, on the other hand, the jets run weaker and the filter pressure gauge is rising, the problem is in the filter, not in the skimmer; this pair is covered in the article on filter pressure and reading the gauge. When both pressure and output drop at the same time, look for an obstruction before the pump: the basket, the sock, a throttled valve.
| Symptom | Likely cause | Fix |
|---|---|---|
| Slurping sound from the skimmer, vortex in the basket | Low water level, the suction is taking in air | Top up the water to 1/2–2/3 of the opening height; check the automatic top-up |
| Bubbles in the pre-filter and from the jets | A leak on the suction side (lid, flange, gasket) | Tighten and lubricate the pre-filter lid, replace the O-ring, check the skimmer flange |
| Weak output from the jets, pressure still low | Clogged skimmer basket or sock | Empty the basket, replace the skimmer sock, check the valve on the pipework |
| Weak output from the jets, pressure high | Clogged filter media | Backwash the sand filter / clean the cartridge |
| Greasy ring on the walls, the surface does not get clean | Water level too high or a jammed flap | Lower the level, free or replace the flap, redirect more suction to the skimmer |
| The skimmer is drawing, but the dirt stays at the opposite end | Wrong jet orientation or position relative to the wind | Turn the return jets along the surface towards the skimmer, or add a second skimmer |
| Corrosion of screws and metal parts around the skimmer | Chlorine tablets dropped into the skimmer | Remove them immediately, switch to a floating or automatic dispenser |
| Cracked skimmer body after winter | Missing float and winterizing plug | Replace the body, winterize properly next season |
Above-ground pools and hang-on skimmers
On above-ground pools the skimmer is not built into the wall but handled differently. The simplest option is a hang-on skimmer, which hooks over the pool edge and draws water through a floating flap much like the built-in type. The second option is a through-wall skimmer fitted into a cut-out opening in the wall with an inner and outer gasket; on liner pools this is an intervention that has to be done precisely, otherwise you create a permanent leak point.
Hang-on skimmers on cheap kits tend to be undersized and sensitive to waves — while you swim, the flap gets swamped and the skimmer loses its function. When choosing a pool it is therefore worth checking what skimmer and return jet fittings it has; the parameters to look at are summarised in the guide to choosing an above-ground pool. With seasonal pools it is also true that the skimmer is dismantled along with the pool, so there is no need to worry about a winterizing float.
FAQ
How high should the water be in the skimmer?
The water level should cross the skimmer mouth at roughly half to two thirds of its height, that is about 10–15 cm below the pool coping. Any lower and you risk drawing in air and running the pump dry; any higher and the skimmer stops collecting the surface film.
How often should the skimmer basket be emptied?
At least once a week, after every windy day or storm, and during the tree flowering season every other day is fine. A full basket reduces the flow just as effectively as a clogged filter and puts unnecessary strain on the pump.
Can I put chlorine tablets in the skimmer?
It is not recommended. A highly concentrated acidic solution forms in the basket and flows straight through the pump, the filtration and the heat exchanger. On steel and stainless steel parts it causes corrosion, and on plastics embrittlement. Use a floating dispenser on the surface or a separate dosing unit.
How many skimmers does a 4 × 8 metre pool need?
With a water surface of 32 m² and the rule of thumb of one skimmer per roughly 25 m², the answer is two skimmers. One would work, but the surface would clear noticeably more slowly, especially at the end furthest from the suction.
Why does the skimmer gurgle and the pump hum?
It is almost always air in the suction line. First check the water level, then the pre-filter lid gasket and the skimmer flange. If the problem persists even with the correct level and a sealed lid, look for a leak on the suction pipework.
In conclusion
A skimmer costs almost nothing compared with a pump, filtration or heating — and yet it decides whether your equipment works with a clean surface layer or with water from the depths. A weekly basket check, the right water level and a working flap will do more for the cleanliness of your pool than any amount of extra chemistry.
If you are dealing with the replacement of a cracked body, flap, basket, winterizing plug or vacuum plate, you will find these parts in the pool structural parts category. Further practical guides to equipment and maintenance are published regularly in our Guide.
