The best cold plunge filters

Two jobs, routinely confused. A filter removes what you can see — skin cells, hair, leaves, grit. Sanitation deals with what you cannot: bacteria and biofilm. Filtering alone gives you water that looks clean and is not, which is the most common mistake in this category.

Both are cheap. Neither is optional if water sits in a tub for more than a day or two, which for anyone running a chiller is the entire point.

How we picked

Standard fittings first, because a filter that takes a common cartridge costs a few dollars a change and one that takes a proprietary one does not. What is actually in the box second. Whether the product does the job it appears to do third — which is where the venturi injector earns its place, because an ozone generator without one mostly makes bubbles.

Why the standard housing matters

A 10-inch filter housing on 3/4-inch ports is the size the entire pool, spa and point-of-use water industry is built around. Cartridges for it are available everywhere, from many manufacturers, for a few dollars. A proprietary housing means paying whatever the original seller asks, for as long as they keep making them.

For a consumable you will replace several times a year, that is the difference between a trivial running cost and an annoying one. Buy the standard size.

Micron ratings, briefly

The venturi is the part people leave out

This is the most useful thing on this page. Ozone bubbled into a body of standing water mostly rises to the surface and escapes into the air, doing very little to the water on the way. Ozone drawn through a venturi injector into a moving line dissolves into the water under pressure and actually does its job.

A venturi is an inexpensive PVC fitting with a barbed inlet and outlet, a small ozone port and usually a check valve to stop water backflowing into the generator. It goes in your circulation line. Without it, an ozone generator is a device that hums.

Ozone safety, plainly

• Never run an ozone generator while anyone is in the water. Ozone is a respiratory irritant. Run it on a timer, between sessions, and give the water time before use.
• Ventilate the space. In an enclosed room, ozone accumulates in the air above the tub as well as dissolving into it.
• Ozone output is a consumable. Generator plates degrade over time and output falls. Nobody publishes a service interval and almost nobody publishes an output figure in mg/h either, which makes sizing it to your volume guesswork.
• Check whether your chiller already includes it. Several do. Running two ozone systems buys you nothing.

A working filtration setup

1. 1Pump — often the one built into your chiller. If the chiller has no pump, you need one.
2. 2Filter housing — 10-inch, 3/4-inch ports, 50 micron cartridge. Fit a condensation sleeve, because a cold housing drips continuously without one.
3. 3Venturi injector in the return line, if you are running ozone.
4. 4Ozone generator on a timer, sized to your volume as best you can given the missing output figures.
5. 5Back to the tub. Keep the return below the waterline so it does not aerate and warm the water.

What none of this replaces

Changing the water. Filtration and ozone extend how long a fill stays usable; they do not make it indefinite. Dissolved matter accumulates regardless, and no cartridge removes it.

No manufacturer publishes a change interval and it depends on how many people use the tub and how often, so we are not going to invent one. The practical guidance is to watch the water: cloudiness, smell or slipperiness on the walls mean it is past time, whatever the calendar says. The full picture is in the filtration guide.

Every pick on this page comes from published specifications, not from hardware we own. Read how we pick — including what spec-led ranking cannot tell you.