Unicel C-4301 Replacement Spa Filter Cartridge – 27.5 Sq Ft (Open Top & Bottom, 2-1/8″ ID)
The Unicel C-4301 is a premium pleated replacement spa filter cartridge designed for select Martec and Sonfarrel-style filter canisters that use a compact cartridge with open ends. It captures fine debris to help keep water clear while maintaining strong circulation when rinsed and cleaned on a regular schedule.
Key Features
- 27.5 sq ft filtration area for effective debris capture and steady spa flow
- Open top & open bottom end caps for compatible standpipe-style housings
- Durable pleated media and rigid inner core built for repeated cleaning cycles
- Direct replacement when matched by part number and measurements
Specifications
- Part Number: C-4301
- Filtration Area: 27.5 sq ft
- Outside Diameter: 4-15/16″ (4.9375″)
- Length: 14-7/8″ (14.875″)
- Top End Cap: Open
- Top Inside Diameter: 2-1/8″ (2.125″)
- Bottom End Cap: Open
- Bottom Inside Diameter: 2-1/8″ (2.125″)
- Pleat Count (typical): 106
- Filter Media: 3 oz pleated media
Replaces / Cross References
- Pleatco: PMT27.5
- Filbur: FC-1610 (some catalogs also reference FC-1616)
- Common OEM / System References: Martec / Sonfarrel 25-220122 (verify by measurements)
When to Replace
- Pleats are torn, frayed, or separating from end caps
- Filter stays restrictive even after a thorough rinse and deep clean
- Water stays cloudy despite balanced chemistry and proper sanitizer levels
- End caps are cracked/warped or the inner core is weakening
Related Products
- Unicel C-4301 (Shop / Options)
- Pleatco PMT27.5 (Alternate)
- Filbur FC-1610 (Alternate)
- Spa Cartridge Filter Cleaner
- Cartridge Cleaning Wand
- Spa Water Test Strips
Maintenance Tips
- Rinse the cartridge regularly (often weekly with heavy spa use) to prevent restriction and keep jets performing well.
- Deep clean every 3–6 weeks with a cartridge cleaner to remove oils; rinse thoroughly before reinstalling.
- If scale is present, degrease first, then treat for scale, and rinse completely before returning to service.
- Rotating between two cartridges (one in use, one drying after cleaning) can extend overall filter life and improve performance.
