Caldera filter bottoms: male slip, open end, and the 2014 Cantabria split

Last reviewed October 27, 2026

On Caldera’s 7" handle cartridges, the older parts have a 2" male slip bottom (C-7451, OEM 033016, and C-7479, OEM 033017) and the later parts have a 2-11/16" open bottom (C-7350, OEM 1019401, and C-7375, OEM 1019301 / 73531)^[5]. On the 8" × 16-1/2" pair, Leisure Time says Utopia Cantabria uses OEM 74817 through the second quarter of 2014 and OEM 73722 from the third quarter of 2014 on^[41][40]. Look at the bottom of the old cartridge either way.

Why Caldera has two filters that measure the same

Caldera and Hot Spring come from the same Watkins factory, but most Caldera collections do not use the Hot Spring open-end cartridge. Paradise, Utopia and Highland models in the cited listings use a 7"-diameter cartridge with a semi-circular handle on top. The length is either 10-3/4" (about 50 sq ft in the rows below) or 14-3/4" (about 75 sq ft). The detail that changed over time is the bottom.

Unicel’s note on C-7451 says C-7350 can also be used, and the note on C-7479 says C-7375 can also be used^[5]. Read that as Unicel’s interchange note, and still look at the cartridge you removed. A male slip is a stub that pushes into a fitting. An open bottom is a hole. If you still have the stub, the slip-bottom part is the one the spec row describes. If you have a hole about 2-11/16" across, the open-bottom part is the one the spec row describes. Pleatco marks the open-bottom numbers with an N (PCD50N, PCD75N) and the slip-bottom numbers without it (PCD50, PCD75)^[2].

The open-bottom 14-3/4" cartridge is the one with the long model list. C-7375 is cited for Utopia Geneva, Hawaiian, Niagara, Provence and Tahitian years, and for Paradise Geneva, Hawaiian, Makena, Moorea, Niagara, Reunion, Salina and Seychelles years, among others. Two of those lines carry an extra warning from Leisure Time: a 2004 Hawaiian HAA and a 2024 Makena need a serial-number check before you order^[5]. The short open-bottom cartridge, C-7350, is the one cited for later Utopia Ravello, several Paradise models including a 2024-only Makena note, and Highland Cascade, Cumberland, San Gabriel and Teton. The Caldera model-year lookup is the index of those rows. A model name without the year is how people order the slip bottom for a spa that left the factory with the open bottom.

The 8" Cantabria cartridges are a quarter-year problem

Two Caldera cartridges share an 8" diameter, a 16-1/2" length and a 100 sq ft rating, and they do not share end caps.

• C-8300, OEM 74817, Filbur FC-3966. Top and bottom are 3-1/8" open with a molded gasket. No Pleatco number is shown: the Leisure Time listing also names Pleatco PWK100, and PWK100 is not in the Pleatco 2023 spec tables, so we do not list it as an equivalent. The cited fitment is Utopia Cantabria 2009 through the second quarter of 2014. Leisure Time says to confirm the production quarter on a 2014 Cantabria, because third and fourth quarter production uses 73722^[41].
• C-8399, OEM 1039607 / 73722, Pleatco PCD100W, Filbur FC-3965. The top is a semi-circular handle and the bottom is 3-5/8" open with a lip seal. Leisure Time says this is the cartridge for Cantabria from the third quarter of 2014 onward, and that first and second quarter 2014 uses 74817^[40]. The same listing names later Utopia Geneva, Niagara, Tahitian, Florence and Elation years, plus Aquatic Melodies Aspire and Elation for 2005–2008.

A 2014 Cantabria owner who only knows the model name has a coin flip. The serial number or the production quarter is the tie-break, and the cartridge in the spa is the better one. Measure the bottom: a 3-1/8" opening with gaskets is the C-8300 description; a handle and a 3-5/8" lip-seal bottom is the C-8399 description.

Vacanza is the Caldera line that borrows Hot Spring sizes

Vacanza Marino, Palatino (2010–2026) and Vanto (2011–2026) are listed on Caldera C-8465 for OEM 76136: 8-1/2" × 10-1/2", open 3" both ends, 65 sq ft, Pleatco PWK65, Filbur FC-3960. That is the same Unicel body as Hot Spring’s 71827 cartridge, with a Caldera OEM number. Caldera C-6430 is the 6" × 10-1/2" open cartridge under OEM 31489, shared with Hot Spring Highlife. If your Caldera is a Vacanza, do not assume it takes the 7" handle cartridge the Paradise and Utopia listings use. Read the end cap, or start at the model-year table.

C-4335 (OEM 74148, Pleatco PRB35-IN) and C-4950 (OEM 302005, Pleatco PRB50-IN) are the smaller open-end Caldera cartridges, 4-15/16" in diameter, 9-1/4" and 13-5/16" long. They are a third family. The fitting guide open-end vs threaded puts the three families side by side.

How to order without guessing the year

1. Read the OEM number on the cartridge. 033016 and 033017 are the slip-bottom pair. 1019401, 73532, 1019301 and 73531 are the open-bottom pair. 74817 and 73722 are the 8" pair.
2. If the number is gone, measure diameter, length, and the bottom. Write “slip,” “open,” or “gasket,” and the size of the opening if you can take it.
3. For a 2014 Cantabria, get the production quarter or compare the ends with the two descriptions above^[40][41].
4. For a 2024 Makena or a 2004 Hawaiian, the cited listing says to check the serial number^[5].
5. Then confirm on the cartridge page, which cites the catalog page the numbers came from. How Filbur, Unicel and Pleatco numbers compare shows what those three columns mean.

Hot Spring’s chemical page is the one we cite for Watkins water ranges if you are balancing after a filter change: free chlorine 1–5 ppm, pH 7.2–7.8, alkalinity 40–120 ppm, calcium hardness 75–150 ppm on a standard system^[18]. The filter fitting and the water chemistry are different jobs. A correct bottom does not fix a low sanitizer reading, and a bottle of balancer does not seal a slip fitting.

Questions

Sources

2. Pleatco 2023 Catalog (Pentair)
4. Unicel: Maintenance & FAQ
5. Unicel 2026 Replacement Guide
18. Hot Spring: How do I choose the right spa chemicals for my hot tub?
31. Filbur Catalog & Reference Guide
40. Leisure Time Inc.: Caldera 73722 filter cartridge
41. Leisure Time Inc.: Caldera 74817 filter cartridge

This guide summarizes published manufacturer and public-health guidance; always follow your own spa’s owner’s manual. See our disclaimer.