What Does a Pool Skimmer Actually Do?

A skimmer is the rectangular opening set into your pool wall at the waterline. It continuously draws in the top inch or two of water, which is exactly where floating debris, sunscreen oils, pollen and surface film collect, and routes it into your circulation system before any of that has a chance to sink.

Roughly half to three quarters of your pump’s suction typically comes through the skimmer, with the rest from the main drain. That surface layer matters more than people expect, because anything you catch floating is something you never have to vacuum off the bottom or treat as a stain.

How a skimmer works

Water enters through the throat of the skimmer and passes a hinged flap called a weir door. That door floats up and down with the water level and creates a small waterfall effect, which accelerates surface water into the opening and stops debris from washing back out when the pump shuts off.

Behind the weir sits the skimmer basket, which catches leaves and larger debris. Water then continues through the suction line to the pump, where a second basket catches anything that got past, and finally into the filter.

Many skimmers also have an equalizer line low in the throat. If the water level drops below the skimmer mouth, that line lets the pump draw water from lower in the pool instead of sucking air and losing prime.

Water level is everything

A skimmer only works within a narrow water level range. The correct level is roughly one third to one half of the way up the skimmer opening, which usually means the middle of the tile band.

Too low and the skimmer gulps air, the pump can lose prime, and in a bad case the pump seal overheats and fails. Too high and the weir door is submerged, the waterfall effect stops, and debris just drifts past the opening instead of getting pulled in. This is why a pool can look dirty after heavy rain even though nothing is broken.

Maintaining the skimmer

1. Empty the basket at least weekly, and every few days in heavy debris season. A full basket kills flow to the pump.
2. Rinse the basket rather than banging it. Cracked baskets let debris reach the impeller.
3. Check that the weir door swings freely and is not stuck, warped or missing. A missing weir door noticeably reduces skimming performance and is an inexpensive part.
4. Look at the skimmer throat and lid for cracks. A cracked throat can pull air into the suction line.
5. Run the pump long enough each day to turn the water over. Skimming only happens while the pump is running.

If you have two skimmers and one clearly outperforms the other, you can balance them with the diverter valve so both pull evenly.

Skimmer accessories worth using

A skimmer sock or fine mesh liner over the basket catches pollen, fine silt and suntan oil that the basket lets straight through. They are cheap and they meaningfully reduce filter loading. Just check them often, because they clog faster than the basket does.

Why skimmers work harder in the Florida Keys

Almost no pools here are screened, so everything airborne lands directly on the water. Palm frond fragments, seed pods, bougainvillea petals and salt dust all arrive continuously, and our skimmer baskets fill several times faster than a caged pool on the mainland would.

Bougainvillea in particular is a problem worth catching at the surface. Those petals leach color quickly and will mark plaster if they sink and sit, so a skimmer that is doing its job is your first line of defense against staining.

Salt air also degrades skimmer lids and weir doors faster here. Plastic goes brittle, and a lid that shatters underfoot is a real injury risk, so replace them when they start to chalk or crack.

Check your water level and empty your skimmer basket on the same visit every week, because a skimmer that cannot breathe makes every other part of pool care harder.