PWK35 / C-6433 / FC-3920 / Watkins 31250
35 sq ft replacement filter cartridge • 6″ OD × 9″ long
Compare on Amazon
PWK35 replacements on Amazon
The buttons below pull up Amazon results for each cross-reference part number. Same physical cartridge, different stickers, often different prices. Cross-shop and pick the cheapest option that has solid recent reviews.
Cross-Reference & Interchangeable Part Numbers
| Brand | Part Number | Type |
|---|---|---|
| Pleatco | PWK35 | Primary |
| Unicel | C-6433 | Equivalent |
| Filbur | FC-3920 | Equivalent |
| OEM | Watkins 31250 | Original Equipment |
Always verify dimensions and end-cap type before purchasing a replacement.
Frequently Asked Questions
What are the dimensions of the PWK35?
The PWK35 is 6 inches in outer diameter, 9 inches long, with an open top and an open bottom. Total filtration area is 35 square feet. Before ordering, pull your existing cartridge and measure it yourself, or check the label inside the housing. OD and length both have to match within about an eighth of an inch, and the end-cap style needs to match exactly so the cartridge seats properly when the lid closes.
What other part numbers are interchangeable with the PWK35?
PWK35 is the same physical cartridge as C-6433, FC-3920, plus OEM versions like Watkins 31250. Same dimensions (6" × 9"), same open top, same open bottom. The Pleatco, Unicel, and Filbur boxes ship the same media inside, just with different stickers. The only thing to watch is the end-cap style: if a cross-reference has a slightly different cap design, it may not seat right in your housing.
Is the Watkins 31250 the same filter as the PWK35?
Yes. Watkins 31250 is an OEM number, the code the tub or pool builder stamps on the cartridge they ship. The filter itself is made by Pleatco, Unicel, or Filbur, so Watkins 31250 and PWK35, C-6433, FC-3920 all point to one interchangeable cartridge. Buy whichever is cheapest with solid recent reviews, and just check that the dimensions match your old cartridge before you order.
How often should I replace the PWK35?
Most filter cartridges last between 12 and 24 months. Pull the PWK35 every few months for a hose rinse, soak it in cleaner quarterly, and replace it when the pleats are torn, channeling, or won't come clean. Bather load, water chemistry, and how often you actually clean it all matter more than the calendar.
How do I clean the PWK35?
The PWK35 has open ends on both top and bottom, so water flows straight through. Pull the cartridge from the housing and hose it down. Hit the pleats from above at about a 45-degree angle so water rinses debris out instead of packing it deeper, and work top to bottom while rotating the cartridge. Skip the pressure washer. The high pressure tears or collapses pleats, and once that happens you've lost media that doesn't come back. Every few months (or at every water change, whichever comes first), do a deep clean: overnight soak in a hot tub filter cleaner, then a thorough rinse. If your cartridge has a closed end cap, let it dry before reinstalling so you don't trap moisture inside the cartridge core.
Which hot tubs and spas use the PWK35?
Pull your existing cartridge and read the part number off the end cap, or check the label inside the filter compartment. That's the most reliable way to confirm compatibility.
Dimensions
Specifications
| Outer Diameter | 6″ |
|---|---|
| Length | 9″ |
| Top Type | Open (no end cap) |
| Bottom Type | Open (no end cap) |
| Filtration Area | 35 sq ft |
| Primary Brand | Pleatco |
| Part Number | PWK35 |
Fitment notes
1-15/16 inch top and bottom openings. Watkins and Unicel rate this cartridge at 33 sq ft while Pleatco rates the same part at 35 sq ft. Unicel has discontinued the C-6433, so the Pleatco and Filbur numbers are the ones still in production. Not the same cartridge as the PWK35B, which is 14 inches long and fits Limelight models. Replaces Watkins OEM 31250.