Every pool is a system: water flows continuously from the pool through several components and back again, and each component has a specific role to play. When you understand how the parts work together, diagnosing a problem becomes straightforward. Is the water cloudy despite correct chemistry? Check the filter and pump run time. Is there poor circulation in one corner? Check the return jet alignment. Does the pump lose prime repeatedly? Check the skimmer basket and the pump strainer.
This guide covers every major pool component from the skimmer to the return jets, explains how they function, what can go wrong and what basic maintenance each one needs.
Overview: how water flows through a pool system
Pool water circulates in a continuous loop. Understanding the direction of flow is the key to understanding why each component is where it is.
Water leaves the pool through two collection points: the skimmer (at the waterline) and the main drain (at the deepest point of the floor). Both are connected to the inlet side of the pump. The pump creates suction that draws water in from these points. Water then passes through the filter, which removes suspended particles. From the filter, water can optionally pass through additional treatment equipment such as a heater, UV system, salt chlorinator or automatic dosing unit. Finally, clean, treated water returns to the pool through the return jets.
| Component | Function | Location |
|---|---|---|
| Skimmer | Draws surface water and debris into the system | Pool wall at waterline |
| Main drain | Draws bottom water into the system | Pool floor, deepest point |
| Pump | Creates suction; drives circulation | Equipment area, outside pool |
| Filter | Removes suspended particles from water | Equipment area, after pump |
| Heater / heat pump | Heats water before return | Equipment area, after filter |
| Salt chlorinator / UV | Generates or activates sanitiser | Equipment area, after filter |
| Return jets | Returns treated water to pool | Pool walls, below waterline |
The equipment is typically installed in a sequence: skimmer and drain - pump - filter - optional treatment - return jets. The entire cycle repeats continuously as long as the pump runs.
The skimmer
The skimmer is the pool’s primary surface collection point. It sits at the waterline in the pool wall, and as the pump draws water in, the surface layer is pulled through the skimmer opening. This removes floating debris (leaves, insects, pollen, sunscreen film) before it sinks to the floor.
Inside the skimmer housing you will find two key parts: the basket and the weir.
The basket is a removable plastic cage that sits just inside the skimmer opening. It catches debris drawn in from the surface, preventing it from travelling to the pump and filter. Empty and rinse the basket at least weekly, and daily during periods of heavy leaf fall. A blocked basket restricts water flow, makes the pump work harder and reduces filtration effectiveness.
The weir is a hinged flap at the skimmer opening. It allows water to flow into the skimmer when the pump is running but swings back to block the opening when the pump stops. This prevents debris that has already entered the skimmer from floating back into the pool. The weir should move freely on its hinge - if it sticks in the open or closed position, clean the hinge and check for warping or cracking.
Common problems: cracked skimmer housing is a frequent leak point, particularly in older pools or where freeze-thaw cycles have affected the structure. If you notice water loss around the skimmer area, inspect the housing for hairline cracks. A skimmer that does not draw well usually has a blocked basket or a collapsed or kinked suction line.
Tip
The skimmer works best when the water surface sits at the midpoint of the skimmer opening. Too high and surface debris flows over the top instead of into the skimmer. Too low and air is drawn in, which can cause the pump to lose prime. Check the water level weekly, especially in hot weather when evaporation is significant.
The main drain
The main drain sits at the deepest point of the pool floor and serves as the second water collection point for the circulation system. It draws in the bottom water, which tends to be the coolest, the most contaminated with sinking debris and the least circulated. Including the main drain in the circulation loop improves both water quality and heating efficiency.
Most residential pools have a single main drain, though modern pool codes in many countries require a dual-drain system to eliminate the risk of suction entrapment. The main drain must always be covered with a certified anti-entrapment cover (also called a VGB-compliant cover in the US, or equivalent in European standards). These covers are designed to break suction if a body part comes into contact with the drain opening.
Never remove the main drain cover or allow swimming with a missing or damaged cover. Suction entrapment at an uncovered main drain is a life-threatening hazard.
In normal operation, the main drain valve is partially open alongside the skimmer valve. The ratio depends on the pool and personal preference - most pool owners run approximately 70% skimmer and 30% main drain during normal operation. When vacuuming manually, some pool owners close the skimmer valve completely to direct maximum suction to the vacuum hose.
The pump
The pump is the heart of the circulation system. It creates the suction that draws water from the skimmer and main drain, and the pressure that pushes it through the filter and back to the return jets. Without the pump running, there is no circulation, no filtration and no effective chemical distribution.
Single-speed vs. variable-speed pumps
A single-speed pump runs at a fixed speed (typically 3,000 or 3,450 RPM depending on regional electricity frequency). It is the most common type in older residential pools and is cheaper to purchase (EUR 150 to 350) but less energy-efficient to run.
A variable-speed pump can operate across a range of speeds. For routine background circulation, it can run at a low speed that uses a fraction of the electricity of full speed. For vacuuming or backwashing (where you need maximum flow), it can run at full speed. The energy saving over a season is typically 40 to 60% compared to a single-speed pump of equivalent size, which translates to EUR 100 to 300 per season for most residential pools.
Variable-speed pumps cost EUR 400 to 900 but typically pay back the price difference within 2 to 3 seasons.
Pump sizing
The pump must be sized correctly for the pipe diameter, filter and pool volume. An oversized pump creates excessive pressure that stresses fittings and the filter. An undersized pump provides insufficient circulation. The general rule is to size the pump for a full turnover of the pool volume within 8 hours.
Maintenance: clean the pump strainer basket every 2 weeks and before each backwash. The strainer basket (inside the pump lid, on the inlet side) catches any debris that passes through the skimmer basket. A blocked strainer basket is one of the most common causes of the pump losing prime. Check the pump lid O-ring annually and lubricate it with silicone grease.
The filter
The filter is where the actual water cleaning happens. The pump pushes water through the filter media, which traps suspended particles too small to be caught by the skimmer basket.
Sand filter
Sand filters are the most common type in residential pools. Pool-grade silica sand (grain size 0.4 to 0.8 mm) fills a fibreglass pressure tank. Water enters from the top, percolates down through the sand bed and exits through a laterals manifold at the bottom. Particles are trapped in the spaces between sand grains.
Over time, the trapped particles increase the resistance to water flow, which shows as a rising pressure reading on the gauge mounted on top of the filter. When pressure rises 0.5 bar above the clean starting pressure, backwash the filter by reversing the water flow to flush the trapped debris out through the waste port.
Sand lasts 5 to 7 years before the grains wear smooth and filtration efficiency drops. Filter glass (crushed glass media) is an alternative that lasts longer and filters to finer particle sizes.
Cartridge filter
A cartridge filter uses a pleated polyester or fibreglass cartridge instead of sand. Water flows through the cartridge fabric, which traps particles. Cartridge filters require no backwashing: you simply remove the cartridge, rinse it with a garden hose every 2 to 4 weeks and replace the cartridge annually (or when it can no longer be cleaned effectively). They filter to a finer particle size than standard sand (10 to 15 microns vs. 20 to 25 microns for sand) and use no backwash water, which is an advantage in water-restricted areas.
DE (diatomaceous earth) filter
DE filters use a coating of diatomaceous earth powder on a grid framework to filter water to approximately 5 microns - the finest filtration available for pool use. They are common in commercial pools and high-end residential applications but require more maintenance than sand or cartridge filters: the DE powder must be recharged after each backwash.
Filter comparison
| Type | Filtration size | Maintenance | Relative cost |
|---|---|---|---|
| Sand | 20 to 25 microns | Backwash weekly, replace sand every 5-7 years | Low |
| Cartridge | 10 to 15 microns | Rinse every 2-4 weeks, replace annually | Medium |
| DE | 5 microns | Backwash and recharge DE powder | High |
Return jets
Return jets push filtered, treated water back into the pool through fittings set into the pool walls. Their alignment determines the circulation pattern in the pool, which affects how evenly chemicals and heat are distributed and whether dead spots form where algae can establish.
Correct alignment: aim return jets slightly downward (about 10 to 15 degrees below horizontal) and at an angle to the pool wall - not directly across the pool. This creates a circular or rotational flow pattern that covers the entire pool volume without creating a direct short-circuit from the return to the skimmer.
Adjustable eyeballs: most return jets have adjustable eyeball fittings (a ball-and-socket joint) that let you direct the flow. Experiment with the angle and direction to eliminate dead spots in corners. A simple way to check: add a small amount of dye (food colouring or pool dye) near a suspected dead spot with the pump running and observe how it moves.
Maintenance: check the eyeball fittings annually for cracks or loose fit. A fitting that spins freely or is cracked can allow air into the return line, which can cause foaming. Replace damaged fittings promptly.
Optional components
Many residential pools include one or more additional components beyond the basic pump-filter circuit.
Heat pump or heater: extends the swimming season by heating the water. A heat pump (the most energy-efficient option) extracts heat from the air and transfers it to the pool water. A gas or electric heater warms water directly. Both are installed in the circulation line after the filter.
Salt chlorinator: electrolyses dissolved salt (sodium chloride) in the pool water to generate chlorine. The pool still contains chlorine, but it is produced continuously on-site rather than added manually. Maintains a consistent chlorine level and reduces the need to buy and handle chlorine products. Requires a salt level of approximately 3,000 to 4,000 mg/l.
UV system: a UV lamp installed in the return line exposes water to ultraviolet light, which destroys bacteria and reduces chloramine formation. UV does not replace chlorine entirely but reduces the chlorine dose needed to maintain disinfection. Common in pools where chloramine odour or eye irritation is a recurring concern.
Ozone system: injects ozone gas into the return line to oxidise organic compounds. Similar role to UV - reduces chlorine demand and chloramine formation.
Automatic dosing system: monitors pH and chlorine continuously via a probe and doses the correct chemicals automatically. Ideal for pools where chemistry is difficult to keep stable or for pool owners who want minimal manual intervention.
Pool robot: a self-contained robotic cleaner that is entirely independent of the filtration system. See our guide to the best pool robot for a full comparison.
Understanding pump run time
The pump must run long enough each day to turn over the full pool volume at least once. This ensures all the water passes through the filter and receives even chemical distribution.
The formula: pool volume (litres) divided by pump flow rate (litres per hour) equals the minimum hours needed for one complete turnover.
For example: a 40,000 litre pool with a pump rated at 10,000 litres per hour requires a minimum of 4 hours for one complete turnover.
However, one turnover per day is the minimum. In practice, during the swimming season - especially in summer - run the pump for 8 to 12 hours per day. Higher water temperatures, heavy use and intense sunlight all increase the rate at which chlorine is consumed and algae can grow. Longer pump run times compensate for this by increasing filtration frequency and improving chemical distribution.
For variable-speed pumps, you can run at low speed for 10 to 12 hours per day and achieve the same filtration benefit as a single-speed pump running at full speed for 4 to 6 hours, with significantly less energy consumption.
Product recommendations
Variable-speed pool pump - 0.75 kW
✓ Our pick: Energy-efficient variable-speed pump suitable for pools up to 50,000 litres. 5 pre-set speeds, timer function and transparent strainer lid. 40 to 60% energy saving vs single-speed equivalent.
Pool filter sand - 25 kg, 0.4 to 0.8 mm
✓ Our pick: High-quality silica sand in the correct grain size for residential pool sand filters. Washed and graded. One 25 kg bag fills most standard residential sand filter tanks (check tank capacity before ordering).FAQ
How do I know if my pool pump is the right size? Your pump should be able to turn over the full pool volume once every 8 hours. Divide your pool volume (in litres) by 8 to get the minimum required flow rate in litres per hour. A 40,000 litre pool needs a pump capable of at least 5,000 l/hr. Check that the pump flow rate also matches the pipe diameter and filter capacity - an oversized pump paired with narrow pipes or a small filter creates excessive pressure and wear.
How often should I backwash my sand filter? Backwash when the pressure gauge on the filter reads 0.5 bar above its clean starting pressure, or approximately once per week during the swimming season. During heavy use or after an algae treatment, backwash more frequently. Always run the filter on the rinse setting for 30 seconds after backwashing before returning to normal filtration.
What is the purpose of the main drain? The main drain sits at the deepest point of the pool floor and draws water from the bottom into the circulation system. This ensures the entire water column circulates, not just the surface. It also improves heating efficiency by drawing in the coolest water from the bottom and sending it to the heater. By law in most countries, the main drain must have a dual anti-entrapment cover to prevent suction entrapment.
Should I run my pool pump at night or during the day? Running the pump during the warmest part of the day is most effective because chlorine demand is highest when water is warmest. In summer, run the pump for 8 to 12 hours, starting in the morning. For energy savings with a variable-speed pump, you can run at high speed during peak hours and at low speed overnight to maintain circulation without the full energy cost.
What is a variable-speed pump and is it worth the extra cost? A variable-speed pump can operate at different speeds rather than just full power. Running at lower speeds for routine circulation uses significantly less energy than running a single-speed pump at full power. The energy saving is typically 40 to 60% compared to a single-speed pump over the course of a season. Variable-speed pumps cost more upfront (EUR 400 to 900 vs EUR 150 to 350 for single-speed) but typically pay back the difference within 2 to 3 seasons through electricity savings.
How long does filter sand last and when should I replace it? Filter sand typically lasts 5 to 7 years in a residential pool. Over time, the sand grains become smooth and rounded from constant water flow, which reduces filtration efficiency. Signs that the sand needs replacing include: water stays hazy despite correct chemistry and adequate run time, the filter requires backwashing more frequently than usual, or it has been more than 7 years since the last sand change. Replace with pool-grade silica sand (0.4 to 0.8 mm grain size) or consider switching to filter glass for better filtration and longer life.