How a Pool Skimmer Works (and How to Fix One That Isn't Skimming)

How does a pool skimmer work?

A pool skimmer works by using the pump's suction to constantly draw off the top inch or two of the water, which is exactly where floating debris and oily film collect before they sink and become harder to remove. The pump pulls water from the skimmer through the underground plumbing, the filter cleans it, and it returns through the wall jets. As long as the pump runs, the surface is being pulled toward the skimmer mouth, which is why a skimmer that is set up right keeps the surface looking clean on its own.

Inside the skimmer you have a few parts that each do a job. The weir is the floating flap across the mouth; it rides on the water and creates a thin, fast film of water spilling over it so debris gets sucked in and cannot float back out. Behind it sits the basket, which catches leaves and large debris so they never reach the pump. At the bottom are usually two ports: the main suction line to the pump and an equalizer line lower on the pool wall that lets the skimmer keep drawing water if the level drops, so the pump does not gulp air.

Because the skimmer is on the suction side, it is also where you connect a manual vacuum hose, and it is the first place to look when circulation goes weak. Good surface skimming is part of what keeps a pool clear; when water stops moving across the top, debris sinks and the pool can cloud, which is one of the first things to rule out in the cloudy pool guide.

What water level should the pool be for the skimmer to work?

Keep the water level at roughly the middle of the skimmer opening, somewhere between one third and two thirds of the way up the mouth. That is the single most important setting for a skimmer, and it is the one most often wrong. At that level the weir can ride the surface and skim a thin top layer, which is what you want.

Too low and the skimmer sucks air. When the level drops below the bottom of the opening, the skimmer pulls a mix of air and water, the pump basket runs half empty with bubbles, and you lose prime. That is the same air-leak symptom covered in the pool pump repair guide, except the fix here is simply adding water with a hose. Evaporation and a few pool parties will drop the level faster than people expect, so check it weekly.

Too high and the skimmer barely skims. When water rises near the top of the opening, the weir floats up and lies almost flat, so instead of a fast skimming film you get slow, lazy flow and debris drifts right past. After heavy rain, drain the level back down to mid-skimmer. Keeping the level right also protects the pump, because a starved skimmer is hard on the impeller and the seals.

Why is my pool skimmer not sucking or not skimming?

A skimmer that stops pulling water comes down to a short checklist, and you can work through it in a few minutes. Start with the easy stuff: is the water level at mid-skimmer, and is the basket full? A low level makes the skimmer suck air, and a basket packed with leaves chokes the flow before it ever reaches the pump. Empty the basket and top up the water first, because those two fix most cases.

Next, check the weir and the valves. If the weir is stuck shut, jammed by a leaf, or has fallen out, the skimmer either cannot pull surface water or pulls so hard from deep that it stops skimming the top. If your pool has more than one skimmer or a main drain, there are diverter valves at the skimmer or the pump that split the suction; a valve turned the wrong way can starve one skimmer entirely. Make sure the suction is actually directed to the skimmer you are looking at.

If the level, basket, weir, and valves are all fine and the skimmer still will not pull, you are likely looking at a clog or an air leak farther down the line. A blockage in the underground pipe between the skimmer and the pump kills suction; so does air being drawn in through a cracked fitting, a loose pump lid, or a bad o-ring, which shows up as bubbles in the pump basket and weak returns. That is suction-side troubleshooting, and the step-by-step is in the pump repair guide.

What does the weir (the skimmer door) do?

The weir is the hinged, floating flap across the mouth of the skimmer, and its job is to make the skimming work better and hold on to what it catches. Because it floats on the surface, it forces the incoming water to spill over the top of the flap in a thin, fast sheet, which pulls floating debris in efficiently instead of letting the suction draw from deeper water where the junk is not.

The second job matters just as much: when the pump shuts off, the weir swings closed and traps the debris in the basket so leaves and bugs do not float back out into the pool. A skimmer with a missing or broken weir still moves water, but it skims poorly and lets debris escape every time the pump cycles off. If yours is cracked, stuck, or gone, it is a cheap, snap-in replacement matched to your skimmer brand, and well worth doing.

A weir should swing freely and sit upright on its own. If it is sluggish, check the hinge for grit or a warped flap, and confirm you have the right model, because a weir that is too tall jams against the opening and one that is too short never reaches the surface.

How do you clean and maintain a pool skimmer?

Skimmer maintenance is mostly one habit: empty the basket regularly, at least weekly and more often during pollen season or leaf drop. A full basket is the most common reason flow drops, and leaving wet debris to rot in there feeds algae and uses up chlorine. While the lid is off, glance at the weir to make sure it still moves and the basket is not cracked, since a broken basket lets debris through to the pump.

One thing that does not belong in the skimmer is chlorine tablets. Dropping trichlor pucks in the skimmer sends a stream of acidic, highly concentrated water straight through your pump, filter, and heater every time the pump runs, and it keeps corroding metal parts in stagnant water when the pump is off. Use a floating dispenser or an inline feeder instead; the reasoning is laid out in the chlorine tablets guide, and the trade-off against liquid chlorine is in liquid chlorine vs tablets.

Beyond that, keep the skimmer on your normal rounds: check it as part of a simple maintenance schedule, and at season's end protect it from freezing by lowering the water below the skimmer or using a skimmer plug or cover, the way the winterizing guide describes. A cracked skimmer body from ice is one of the more annoying spring repairs.

How many skimmers does a pool need, and can you replace one?

A common builder rule of thumb is one skimmer for every 400 to 500 square feet of water surface, so most residential pools have one or two. Two skimmers placed on different walls skim a large or oddly shaped surface far better than a single one, because surface debris tends to collect downwind, and a lone skimmer on the wrong wall fights the breeze all day. If your pool has two, balance the suction between them with the diverter valves rather than running everything through one.

A worn or cracked skimmer can be replaced, but how involved it is depends on the pool. In a vinyl-liner or above-ground pool, the skimmer is a bolt-through faceplate with gaskets, and swapping it is a doable DIY job. In a concrete or gunite pool, the skimmer is set in the deck and shell, so replacement is a real excavation and usually a pro job. Either way you do not drain the whole pool; you lower the level just below the skimmer to work on it.

If you are adding equipment or rethinking circulation, remember the skimmer is only the intake. The water still has to pass through an adequately sized pump and filter to actually clear, so pair any skimmer work with a look at the pool filter and how it is sized.

Frequently asked questions