Darlly 42521 Spa Filter Cartridge
Replaces Unicel C-4625 · Pleatco PRB25-IN-4 (now ULTRA-S5) · Filbur FC-2370
Darlly 42521 is a 25 sq ft spa filter cartridge, 4-15/16" x 13-5/16". It replaces Unicel C-4625, Pleatco PRB25-IN-4 (now ULTRA-S5) and Filbur FC-2370.
Specifications
| Darlly part number | 42521 |
|---|---|
| Outside diameter | 4-15/16" |
| Length | 13-5/16" |
| Top end cap | 2-1/8" Open |
| Bottom end cap | 2-1/8" Open |
| Filter area | 25 sq ft |
| Type | Spa |
| Darlly description | Rainbow, Waterway Plastics, Custom Molded Products |
Cross-reference
| Brand | Part numbers Darlly 42521 replaces |
|---|---|
| Unicel | C-4625 |
| Pleatco | PRB25-IN-4 (now ULTRA-S5), ULTRA-S5 |
| Filbur | FC-2370 |
Other Darlly cartridges the same size
Same diameter and length. Check the end caps and area before swapping.
Questions
- What is the Darlly replacement for Unicel C-4625?
- Darlly 42521 replaces Unicel C-4625. Both are 4-15/16" in diameter and about 13-5/16" long.
- What Darlly cartridge replaces Pleatco PRB25-IN-4?
- Darlly 42521 replaces Pleatco PRB25-IN-4, which Pleatco now sells as ULTRA-S5.
- What size is Darlly 42521?
- Darlly 42521 is 4-15/16" in outside diameter and 13-5/16" long, with 25 sq ft of filter area.
Cross-reference checked October 21, 2026 against Unicel's 2026 Replacement Guide, Pleatco's catalog (updated September 2025), filbur.com and darllyfilter.com. A competitor number is listed only when that brand's own data pairs it with the same cartridge size. Always compare the size and end caps of your old cartridge before ordering.
Brand names and part numbers are trademarks of their respective owners and are shown only to identify compatible replacements. Darlly Filtration is not affiliated with or endorsed by them. Cross-reference by The Grit Game, Darlly's sales agency.