The cartridge filter is the only mechanical barrier between you and everything that ends up in your hot tub water — skin cells, cosmetics, dust and fibres from swimwear. The short answer to the main question is this: rinse the filter every week, deep-clean it once a month in a dedicated cleaning solution, and replace it with a new one after roughly a year of use. This simple rhythm does more for water clarity, chemical consumption and pump lifespan than anything else.
What role does the filter play in a hot tub?
A hot tub works with a small volume of warm water — typically 800 to 1,500 litres at around 37 °C — shared intensively by several people. Unlike a pool, there is no dilution effect: everything released from skin and cosmetics stays in concentrated form. Sanitiser (chlorine or bromine) deals with microorganisms, but coarse and fine debris has to be physically captured by the filter. How filtration and chemistry work together is covered in our article Hot Tub Chemistry Step by Step.
A cartridge filter consists of pleated polyester paper (the pleats) wound around a plastic core. It is precisely the large pleat surface — 25 to 100 square feet on common cartridges, i.e. roughly 2.3 to 9.3 m² — that makes it possible to trap particles while still allowing enough flow to the pump and heater.
When the pleats clog up, three things happen at once. The water starts to turn cloudy, because debris bypasses the filter or is released back out of it — a typical scenario we break down in Cloudy Hot Tub Water. The pump works against resistance, gets noisy and wears out needlessly. And eventually the flow drops so far that the control unit reports an FLO error and shuts the heater down for safety — you will find the complete repair procedure in our guide Hot Tub Showing an FLO Error. A clogged filter is by far the most common cause of this error.
How often should you clean the filter? The care rhythm at a glance
Caring for a cartridge is not complicated, but it has to be regular. The proven rhythm is summed up in the table — treat the timings as a starting point for a hot tub used 2–4 times a week by two to four people. With more intensive use, shorten the intervals.
| Interval | Task | How to do it | Time |
|---|---|---|---|
| Weekly | Rinse with a garden hose | Remove the cartridge and rinse with a stream of water along the pleats, top to bottom, pleat by pleat | 5–10 minutes |
| Monthly | Deep clean | Soak overnight in a filter cleaner solution, rinse thoroughly in the morning and let it dry | 10 minutes of work + overnight soak |
| At every water change | Always deep-clean | Fresh water belongs in a hot tub only with a clean filter — otherwise it gets dirty within days | together with the water change |
| Yearly (as a guideline) | Replace with a new filter | After 12 months of use, the pleats lose their filtering ability even after cleaning | 2 minutes |
Deep cleaning at water-change time is a logical opportunity: you are draining the water anyway, the hot tub is out of service and the filter has time to dry properly. How often to change the water, and how to tell when it is time, is covered in detail in How Often to Change Hot Tub Water.
How to clean the filter properly, step by step
The weekly rinse
Switch off the filtration, remove the cartridge from the filter housing and rinse it with a garden hose at normal pressure. The technique is what matters: direct the stream of water along the pleats, from top to bottom, and rotate the cartridge gradually so the water reaches every gap. It is precisely in the folds of the pleats that hair, fibres and greasy film collect. Rinsing against the pleats only pushes the debris deeper.
The monthly deep clean
Rinsing with water removes coarse debris, but not the grease from cosmetics and sunscreens that gradually glues the pleats together. For that you need a dedicated cartridge filter cleaner — an alkaline or enzymatic product that dissolves the grease. Submerge the cartridge in the solution according to the instructions (typically a bucket of water with a measured dose of cleaner) and let it work overnight, i.e. 8–12 hours.
In the morning, rinse the cartridge very thoroughly — cleaner residue would cause foaming in the hot tub — and let it dry completely in the air, out of direct sunlight. Drying is not just cosmetic: drying out kills some of the microorganisms lodged deep in the pleats, and the paper “straightens” back into its original structure.
A tip from practice: get a second cartridge to rotate. While one goes through soaking and drying, the other serves in the hot tub. Filtration never stops, you never have to cut corners under time pressure, and both cartridges last longer because they regularly dry out completely.
What ruins a filter? The most common mistakes
Dish soap — never
The most widespread mistake of all. Dish soap does dissolve grease, but its surfactants never rinse completely out of the pleats. The result is a hot tub full of thick foam the first time the jets come on — and foam is very hard to remove from a small volume of warm water. If it has already happened, the rescue procedure is described in Foam and Smell in a Hot Tub. Use only products made for cartridge filters.
Sunscreens and cosmetics
Creams, body lotions, deodorants and hair products are the worst thing for the pleats: the greasy film glues the paper together, reduces flow and serves as a breeding ground for bacteria. The most effective defence is a short soap-free shower before every soak — it washes away most cosmetics and sweat and extends the filter’s life by months.
Hard water and limescale
In hard water, calcium carbonate settles not only on the shell and heating element but also in the filter pleats, where it forms a grey-white crust. Pleats clogged this way cannot be restored even by deep cleaning with a regular cleaner — you need a product with a descaling component. If you have hard water, read how to treat it right at fill time: Water Hardness and Limescale.
Washing with a pressure washer
It is tempting — a pressure washer cleans a cartridge in a minute. But the pressure of hundreds of bar tears the fine filter paper, frays the edges of the pleats and creates micro-cracks in the media through which debris then passes uncaptured. A filter looks clean after a pressure wash, but it filters significantly worse. A garden hose at normal pressure is the maximum the pleats can take.
When is it time for a new filter?
No cleaning lasts forever — the filter paper ages mechanically and its pores clog irreversibly. Buy a new filter as soon as you notice any of these signals:
- Frayed or flattened pleats — the paper has lost its stiffness, the gaps stick together and the effective filter surface shrinks.
- Persistent greying — the cartridge stays grey or yellowish even after soaking in cleaner; dye and grease have penetrated deep into the media.
- Low flow even after cleaning — weaker jets, a noisier pump or a repeated FLO error shortly after a deep clean mean the pores are irreversibly blocked.
- Age over 12 months — even a visually decent cartridge filters noticeably worse after a year of use. For intensively used hot tubs, count on 6–9 months instead.
A new cartridge costs on the order of €20–60 depending on size — compared with the cost of the extra chemicals a clogged filter consumes, and the risk of pump damage, it is the cheapest part of the whole hot tub.
How to choose the right replacement filter
Cartridges are not universal — they have to fit your hot tub’s filter housing exactly. When choosing a replacement, verify four parameters:
- Dimensions: outer diameter, overall length and core opening diameter. Measure to the millimetre — manufacturers differ even by a few millimetres.
- Thread or mounting: some cartridges simply slide in, others have a bottom screw thread (typically a coarse plastic thread) or a top handle. The mounting type must match the original.
- Filter media area: stated in square feet (e.g. 25, 50 sq ft). A smaller area than the original means more frequent cleaning and a shorter lifespan.
- Manufacturer’s code: the cartridge code (e.g. formats like PWW50, C-4326) is usually printed on the top or bottom end cap of the cartridge or on its core. It is the easiest way to find a compatible replacement — photograph it before you throw the old filter away.
Replacement cartridges and other parts can be found in our structural and spare parts category.
Why is an old filter a hygiene risk?
The damp, warm, nutrient-rich environment inside the pleats is an ideal breeding ground for biofilm — a slimy layer of bacteria that shields microorganisms from chlorine and bromine alike. A filter that only ever gets rinsed and never dries out properly gradually turns from a debris trap into a debris source: the biofilm releases bacteria back into the water, the sanitiser loses effectiveness and chemical consumption rises.
With hot tubs this is a particularly sensitive topic because of Legionella bacteria, which thrive at exactly around 37 °C and in the aerosol from the jets. A regularly deep-cleaned and promptly replaced filter is, together with proper sanitising, the basic prevention — we cover the whole topic in Hot Tub Hygiene and Legionella. A simple rule applies: if you are hesitating between cleaning the filter once more or replacing it, from a hygiene standpoint the right answer is to replace it.
Frequently asked questions
Can I wash a cartridge filter in the washing machine or dishwasher?
No. The mechanical stress in a washing machine deforms the pleats, and residues of detergents or dishwasher tablets cause heavy foaming in the hot tub. The only correct procedure is a hose rinse and a soak in a cartridge filter cleaner.
How long should the filter dry after a deep clean?
Ideally 24 hours in the air, out of direct sunlight, standing upright so water drains from the core. Drying out completely restores the pleat structure and limits the survival of microorganisms. That is exactly why a second cartridge to rotate pays off — the hot tub runs on it in the meantime.
Will more frequent cleaning help if the water stays cloudy?
Only partly. A clean filter is a necessary condition, but cloudiness often has chemical causes too — low sanitiser levels, wrong pH or old water. Work through the procedure in Cloudy Hot Tub Water; if the cartridge is more than a year old, start by replacing it.
Does the filter have to be a brand-name one, or is a compatible replacement enough?
What matters is dimensions, mounting type and media area, not the logo. A quality compatible cartridge with identical parameters works just as well as the original. Just avoid noticeably cheaper units with a smaller media area — they clog faster and you save nothing overall.
Will the hot tub run for a while without a filter?
Technically yes, but do not do it. Without the barrier, debris goes straight into the pump and heater, where it can damage the impeller or the flow sensors. On some models, a filter housing without a cartridge also draws in air. Removing the filter is fine only for a few minutes while replacing it.
In conclusion: ten minutes a week that decide everything
Caring for the cartridge filter is the cheapest and most effective maintenance you can do for your hot tub: a weekly rinse, a monthly soak in cleaner and a new cartridge once a year. The reward is crystal-clear water, lower chemical consumption, a quiet pump and no error messages. Replacement cartridges, filter cleaners and other parts can be found in our structural and spare parts category — and if you are still choosing a hot tub that will be easy to maintain, browse our range of hot tubs.
