Hot Spring 33521 Filter Cartridge
Hot Spring OEM number • 25 sq ft • 5″ OD × 13.9375″ long
33521 is Hot Spring's number for this cartridge, not a part you have to buy from Hot Spring. The filter itself is a 25 square foot cartridge measuring 5 inches across and 13.9375 inches long, and Pleatco, Unicel, and Filbur all build it. If you read 33521 off an old cartridge or found it in your manual, any of the numbers below drops into the same housing.
Fitment notes
2-1/8 inch top and bottom opening. Drop-in style. Used in Beachcomber 1994-present. Also the replacement for Pentair Rainbow Dynamic 25 in-line filters (Dynamic Series I RDC-25 through Series IV DFM/DFML), Hot Spring and Hot Spot skim filters (33521), Coleman Spas, and Waterway 25 sq ft top-load housings. For more capacity in the same housing see PRB37-IN or PRB50-IN.
What to buy instead
Replacements for the Hot Spring 33521
Each button searches Amazon for one of the numbers this cartridge is sold under. The OEM version is usually the most expensive of the set and the aftermarket versions are the same physical filter, so it is worth checking all of them before you order.
Every number for this filter
| Brand | Part Number | Type |
|---|---|---|
| Hot Spring | 33521 | The number you searched |
| Pleatco | PRB25-IN | Aftermarket equivalent |
| Unicel | C-4326 | Aftermarket equivalent |
| Filbur | FC-2375 | Aftermarket equivalent |
| Beachcomber | 3005845 | Other OEM number |
| Coleman/Maax | 100586 | Other OEM number |
| Coleman/Maax | 3301-2242 | Other OEM number |
| Custom Molded Products | 25392 | Other OEM number |
| Pentair | 17-2327 | Other OEM number |
| Pentair | R173429 | Other OEM number |
| Waterway | 817-2500 | Other OEM number |
Full specs, the cartridge diagram, and the complete compatibility list live on the PRB25-IN cartridge page. You can also see every 5″ × 13.9375″ cartridge if you are checking a measurement rather than a part number.
Equipment that uses this cartridge
Looking for a different model? Browse every Hot Spring model we cover.
Frequently Asked Questions
What filter replaces the Hot Spring 33521?
The PRB25-IN replaces it directly, and so does every cross-reference for it: C-4326, FC-2375. All of them are the same 25 sq ft cartridge at 5" outer diameter and 13.9375" long. Buy whichever is cheapest at the time with solid recent reviews. The same cartridge also shows up as Beachcomber 3005845, Coleman/Maax 100586, Coleman/Maax 3301-2242, Custom Molded Products 25392, Pentair 17-2327, Pentair R173429, Waterway 817-2500, so those are not different filters either.
Is the Hot Spring 33521 the same as the PRB25-IN?
Yes, physically the same filter. Hot Spring 33521 is an OEM number, meaning it is the code Hot Spring put on the cartridge that shipped with the equipment. Hot Spring does not manufacture cartridges. Pleatco, Unicel, and Filbur do, and they sell the identical part under their own numbers at a lower price. Match the dimensions against your old cartridge before ordering and you are covered.
Do I have to buy the Hot Spring version?
No. Nothing about the housing requires an OEM-branded cartridge, and the warranty on your equipment is not voided by using an aftermarket filter that meets the same spec. The reason to buy OEM is convenience, not performance. The reason not to is price, which is often a meaningful gap for an identical part. What matters is that outer diameter, length, and end-cap style all match.
What size is the Hot Spring 33521?
5 inches in outer diameter, 13.9375 inches long, with 25 square feet of filter media. Both measurements need to land within about an eighth of an inch, and the end caps have to match so the cartridge seats correctly when you close the lid. Measure your old one rather than trusting a number that has worn off the label.
Last verified August 2026