Hot Tub Filters

The filter is the least glamorous part of a hot tub and the one that determines most of what you notice. Water clarity, how much sanitizer you get through, how often the tub needs shocking and how long a fill lasts before it starts fighting you all trace back to whether the cartridge is passing water properly. A neglected filter makes every other problem on this site more likely.

It is also the part most people buy wrong, for a specific and avoidable reason: cartridge fit is dictated by your tub, not by brand. Pleatco, Unicel and Filbur all make equivalents of the same physical part, cross-referenced to each other by number. Choosing between them is a price and availability decision. Choosing the wrong size is the mistake that costs you, because a cartridge that is nearly right sits in the housing looking fine while water bypasses the media entirely.

Start by measuring the one you have

Four measurements identify almost any spa cartridge: outer diameter across the widest point of the end cap, length of the media body excluding any handle, and what the top and bottom fittings are. That last one matters as much as the size. A cartridge can be the right diameter and length and still be useless because yours threads into the skimmer and the replacement is a drop-in with an open hole, or the top has a molded handle where yours has none.

Our filter finder works from those measurements to published cross-references, with the source we read each row from listed on the page. Where we have not verified a dimension against a manufacturer datasheet, the table says so rather than filling the cell with a plausible number.

Cleaning is what determines the lifespan

A cartridge fails from oil, not from dirt. Body oils, lotions and sunscreen coat the pleats in a film that water cannot shift, and once the film is established the cartridge restricts flow even though it looks clean. This is why rinsing weekly and never soaking produces a filter that dies in six months, while rinsing weekly and soaking monthly gets you a year or more out of the same part.

The rinse and the soak are different jobs with different tools. A rinse with a fan-jet wand removes debris from between the pleats. A chemical soak dissolves or digests the oil. Neither substitutes for the other, and the single most cost-effective habit in this whole category is owning two cartridges and rotating them, so one is always soaking while the other is in the tub.

What not to do

Do not put a pleated cartridge in the dishwasher. Do not soak one in bleach. Both circulate as advice and both damage the media in ways that are not visible until the filter stops working. Vinegar is a more interesting case, because it does have a real if narrow use, and we work through all three honestly on the home remedies page.

Knowing when it is finished

Cartridges do not last forever regardless of care. The pleats eventually compress and stop springing back, the media loses its ability to release trapped material, and the end caps deform after enough hot cleaning cycles. A cartridge that comes out of a proper soak looking clean but produces weak jets and cloudy water is done, and no further cleaning will change that.

As a planning number, most owners on a rinse-and-soak routine replace a cartridge somewhere around the twelve-month mark, and sooner if the tub sees heavy use. Our page on when to replace a hot tub filter covers the specific signs that mean now rather than later.