Cleaning a Pool Filter with Backwashing - How to Do It

Pool water that becomes cloudy despite correct water values is often due to the filter not being able to operate at full capacity. Cleaning the pool filter by backwashing is therefore one of the most important routines for anyone with a sand filter or glass filter. When this is done at the right time, you get clearer water, more even circulation, and less strain on the pump, heating system, and water chemistry.

Backwashing is simple in principle, but the details matter. If it is too short, dirt remains in the filter bed. If it is done too often, you waste water and risk disturbing the filtration that builds up in the filter media. The goal is not to backwash according to the calendar, but to do it when the pool system shows that it is needed.

How backwashing works in a pool filter

During normal filtration, the pool pump forces the water through the filter tank from top to bottom. Small particles, pollen, skin residue, and other organic material are trapped in the sand or filter glass, while the purified water returns to the pool. Over time, more dirt accumulates in the filter media and the resistance increases.

When you set the multiport valve to the Backwash position, the water flow is reversed. The water then passes through the filter from below, lifting the dirt out of the filter bed. The dirty water is instead discharged to a drain or another approved location for wastewater. In this way, the flow is restored without you needing to open the filter tank.

Backwashing applies to sand filters and glass filters. If you have a cartridge filter, it is cleaned differently, usually by removing the cartridge and rinsing it off carefully. Always check which type of filter your pool has before changing the valve position.

When should you clean the pool filter by backwashing?

The most reliable indicator is the pressure gauge on the filter housing. Note the pressure the system has immediately after a thorough backwash and rinse. When the pressure then rises by about 0.2-0.3 bar above the normal operating pressure, it is usually time to backwash.

In a well-maintained family pool, this can mean approximately every other week to every fourth week during peak season. However, the intervals vary depending on bathing load, the amount of pollen, the weather, the pool's location, and whether you use a pool enclosure. A pool enclosure Often reduces the amount of debris and evaporation, which can result in both cleaner water and longer intervals between cleanings.

You may also need to backwash after heavy rain, intensive bathing, or an algae treatment. However, do not wait until the water has become noticeably cloudy. By then, the filter has already been operating under poorer conditions, and the water chemistry may be more demanding to restore.

Backwash the pool filter step by step

Start by checking that the pool’s water level is sufficiently high. Since backwashing drains water, the level may drop, especially if the process needs to be repeated. Also ensure that the hose or drain line for wastewater is correctly positioned and that local discharge regulations are followed.

1. Turn off the pool pump

Never change the position of the multiport valve while the pump is running. This can damage the valve, the seals, and in some cases create unnecessary strain on the system. Switch the pump off completely before turning the handle.

2. Select the Backwash mode

Press down the valve handle and turn it to Backwash. Then start the pump. In the valve's sight glass, you can often see the water first become dirty and then clearer. If there is no sight glass, follow the recommended time in the system's manual and assess the need based on the filter pressure.

Backwash usually for about two to three minutes, or until the water in the sight glass is clear. For a very dirty filter, it may take a little longer, but avoid letting the process continue unnecessarily. Backwashing consumes both water and heat energy.

3. Turn off the pump and select Rinse

When backwashing is complete, switch off the pump again. Then turn the valve to Rinse, sometimes called Skölj. This step should not be skipped. The rinse setting packs the filter bed again and flushes away any remaining dirt from the valve and pipes before the water returns to the pool.

Start the pump and let it run in rinse mode for approximately 20-30 seconds. Then switch off the pump once again.

4. Return to Filter and refill with water

Set the valve to the normal Filter position, start the pump, and check the pressure gauge. The pressure should now be close to the clean baseline value you noted earlier. If necessary, top up with fresh water to the correct level and then check The pool's pH level, as the water change may affect the balance somewhat.

Common mistakes that impair filtering

The most common mistake is turning the multiport valve while the pump is running. It may feel like a small shortcut, but the risk of wear and leakage is unnecessarily high. Always disconnect the pump from the power supply between each valve change.

Another mistake is switching directly from Backwash to Filter. Without rinsing, dirt and finer particles can end up back in the pool. The water may then look cloudy even though the filter has just been cleaned.

Many people also backwash too often. Over time, the filter media collects very fine particles and can filter more effectively once it has been working for a while. If the pressure is normal and the water is clear, there is rarely any benefit to backwashing simply out of habit.

At the end, it is important not to try to solve all water problems with backwashing. If the water is green, milky, or has visible buildup, the cause may be incorrect pH, insufficient disinfection, too short a circulation time, or a combination of several factors. The filter is central, but it works best as part of a well-dimensioned pool system.

Sand or filter glass - does it affect the routine?

Both filter sand and filter glass can be backwashed, but they have different properties. Filter sand is a proven solution with good filtration capacity and predictable maintenance. Filter glass has a rougher surface and can often capture smaller particles, while in many systems it requires less water for backwashing.

Which material is best suited depends on the size of the filter tank, how the pool is used, and how you want to prioritize operation, maintenance, and investment. Regardless of your choice, the filter media needs to be checked regularly. Sand may need to be replaced after several seasons, while filter glass often has a longer service life. The exact interval depends on water quality, usage, and how well backwashing is carried out.

If you notice that the pressure does not drop after backwashing, that the water remains cloudy, or that the filter media is being carried back into the pool, it is wise to have a qualified technician inspect the valve, filter bed, and circulation system. In a premium installation, every component should work together to ensure low operating costs and safe bathing comfort over a long period.

Make backwashing part of the pool routine

A simple note about the date, filter pressure before and after backwashing, and any water values makes maintenance significantly easier. After a few weeks, you will quickly see what is normal for your particular pool and can detect deviations before they become bigger problems.

Pooltime helps homeowners with pool equipment, service, and solutions adapted to Swedish conditions. When filtration, circulation, pool enclosure, and water care are properly dimensioned, the pool not only becomes easier to maintain - it also becomes a more reliable place for the family's swimming and relaxation throughout the season.

A clean filter is not a minor detail in pool maintenance. It is the foundation of the clear water quality and everyday convenience that make the pool get used more, and cause less concern.