PWK35B / C-6350 / FC-3921
35 sq ft replacement filter cartridge • 6.375″ OD × 14″ long
The PWK35B fits Hot Spring's newer Limelight tubs, the Pulse, Flair, Flash, and Beam from 2018 on, and it also turns up in some Highlife NXT models built after 2014. It's a 35-square-foot element, a bit fatter than the older Hot Spring cartridges at 6-3/8 inches across, with open ends. Watkins part numbers include 78161, 78286, and 1642301-1. Unicel calls it C-6350, Filbur FC-3921. Don't confuse it with the smaller PWK30 that fits the older Highlife tubs. The two aren't interchangeable, and the easiest way to be sure is to measure the diameter of your old cartridge or read the number printed on its end cap.
Compare on Amazon
PWK35B 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 | PWK35B | Primary |
| Unicel | C-6350 | Equivalent |
| Filbur | FC-3921 | Equivalent |
| OEM | Watkins 78161 | Original Equipment |
| OEM | Watkins 78286 | Original Equipment |
| OEM | Watkins 1642301-1 | Original Equipment |
Always verify dimensions and end-cap type before purchasing a replacement.
Compatible Hot Tubs & Spas
The PWK35B (or any of its cross-references) fits these spa models. Cartridge sizing occasionally shifted between model years, so it's worth confirming against your owner's manual or the cartridge label.
| Brand | Model |
|---|---|
| Hot Spring | Limelight Pulse |
| Hot Spring | Limelight Flair |
| Hot Spring | Limelight Flash |
| Hot Spring | Limelight Beam |
Frequently Asked Questions
What are the dimensions of the PWK35B?
The PWK35B is 6.375 inches in outer diameter, 14 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 PWK35B?
PWK35B is the same physical cartridge as C-6350, FC-3921, plus OEM versions like Watkins 78161, Watkins 78286, Watkins 1642301-1. Same dimensions (6.375" × 14"), 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 78161 the same filter as the PWK35B?
Yes. Watkins 78161 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 78161 (along with Watkins 78286, Watkins 1642301-1) and PWK35B, C-6350, FC-3921 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 PWK35B?
Mid-size spa cartridges like the PWK35B (35 sq ft) usually last 12 to 24 months. It shows up most often in Hot Spring tubs. The real driver is use: a tub that gets daily soaks for a couple of people burns through cartridges about twice as fast as one used a few times a month. A good rhythm is a monthly hose rinse, a deep soak in cleaner at every water change (every three to four months), and replacement once the pleats tear, the seams stain brown, or a soak no longer brings post-clean pressure back to baseline.
How do I clean the PWK35B?
The PWK35B 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 PWK35B?
The PWK35B is compatible with: Hot Spring Limelight Pulse, Hot Spring Limelight Flair, Hot Spring Limelight Flash, Hot Spring Limelight Beam. Confirm against your owner's manual or the label inside the filter compartment, since some brands changed cartridge sizing between model years.
Dimensions
Specifications
| Outer Diameter | 6.375″ |
|---|---|
| Length | 14″ |
| Top Type | Open (no end cap) |
| Bottom Type | Open (no end cap) |
| Filtration Area | 35 sq ft |
| Primary Brand | Pleatco |
| Part Number | PWK35B |
Fitment notes
Hot Spring Limelight series 2018+: Pulse, Flair, Flash, Beam. Also some Highlife NXT 2014+ models.