Which filter does my Strong spa take?
Strong’s own FAQ gives the hot tub filter area, not a part number. Vienna 60, Madrid 60, every Embark, every G-2, and Summit or Ascent SL and S models built in 2023 or later take a 50 sq ft filter. Summit and Ascent SL and S models built in 2022 or earlier take a 75 sq ft filter. None of Unicel, Pleatco or Filbur names those series. The Strong cartridge all three catalogs do list is a different spa filter: a 20 sq ft cartridge with a 1-1/2" SAE thread for the Antigua / Futura (Unicel 4CH-925, Pleatco PSANT20P3, Filbur FC-0126), and the three catalogs do not agree on its length.
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The cartridges, with equivalents
| Factory part | What it is | What the catalogs list | Rated area | Our page |
|---|---|---|---|---|
| No Strong part number published | FAQ: 50 sq ft for Vienna 60, Madrid 60, Embark, G-2, and 2023-or-later Summit and Ascent SL/S | Not identified. Unicel lists a model-less 50 sq ft C-4950 under Strong Industries; that is not a model assignment. | Not printed by Strong | not listed |
| No Strong part number published | FAQ: 75 sq ft for Summit and Ascent SL/S built in 2022 or earlier | None found in Unicel 2026, Pleatco 2023 or Filbur | Not printed by Strong | not listed |
| Catalogs only (Antigua / Futura) | 20 sq ft, handle, 1-1/2" SAE. Not in Strong’s FAQ. | Unicel 4CH-925; Pleatco PSANT20P3; Filbur FC-0126 | Unicel 4-5/8" × 7-1/4"; Pleatco 4-3/4" × 7-1/16"; Filbur 4-3/4" × 8" | 4CH-925 |
The series table below is Strong’s FAQ, quoted for area only. Strong prints no diameter, thread or part number there. The catalog row is a different cartridge. Do not order 4CH-925 for a Vienna, Summit, Ascent, Embark or G-2 unless the old cartridge measures as that 20 sq ft part.
What Strong publishes
Strong’s FAQ, “What size filter does my spa use?”, says to use the owner’s manual or contact Strong if the model is not on the list. The list it does print is area only. “SL & S models” is Strong’s wording; the FAQ does not define SL or S. The year split is also Strong’s wording: “Built in/after 2023” for the 50 sq ft Summit and Ascent, and “Built in/before 2022” for the 75. A 2022 spa falls in the earlier group. A 2023 spa falls in the later group.
| Series | Models and years Strong names | Area | Part number |
|---|---|---|---|
| Premium | Vienna 60 and Madrid 60 | 50 sq ft | Not stated |
| Summit, SL and S models | Built in 2023 or later | 50 sq ft | Not stated |
| Summit, SL and S models | Built in 2022 or earlier | 75 sq ft | Not stated |
| Ascent, SL and S models | Built in 2023 or later | 50 sq ft | Not stated |
| Ascent, SL and S models | Built in 2022 or earlier | 75 sq ft | Not stated |
| Embark | All models | 50 sq ft | Not stated |
| G-2 | All models | 50 sq ft | Not stated |
Strong also says to deep-clean the filter each month, or to swap in a spare and clean the dirty one, and to replace the cartridge once the pleats are frayed. It does not give a replacement interval in months.
What the catalogs list for Strong, which is not that chart
Unicel’s Strong Industries section has two rows and no model names: a 20 sq ft cartridge, 4-5/8" × 7-1/4", 1-1/2" SAE thread, 190 pleats, 3 oz, part 4CH-925; and a 50 sq ft open cartridge, 4-15/16" × 13-5/16", 3 oz, part C-4950, with no OEM number. Filbur’s Strong Industries section lists only FC-0126. It does not list a 50 sq ft cartridge and it does not list a 75. So Unicel’s C-4950 line is not confirmed by Filbur, and it is not Strong’s statement that a Vienna takes C-4950. The C-4950 lookup is the same Unicel number as used by other brands. Measure before treating it as the FAQ’s 50 sq ft filter.
| Source | Part | Size it prints | Thread | Model name it uses |
|---|---|---|---|---|
| Unicel spec table and Strong section | 4CH-925 | 4-5/8" × 7-1/4", 190 pleats, 3 oz, 20 sq ft | 1-1/2" SAE | Strong Industries; no model |
| Pleatco spec table | PSANT20P3 | 4-3/4" × 7-1/16", 3 oz, 20 sq ft | Handle top, 1-1/2" SAE | Futura Spa (Strong Industries) |
| Pleatco application index | PSANT20P3 | See the spec row | 1.50" SAE | Futura Spa, Strong Antigua |
| Pleatco spec table (a different bottom) | PSANT20P4 | 4-3/4" × 7-1/16", 20 sq ft | 1-1/2" MPT | Futura Spa (Strong Industries) |
| Filbur Strong section | FC-0126 | 4-3/4" × 8", 20 sq ft | Handle, 1-1/2" SAE | Antiqua (Filbur’s spelling). Unicel cell on this row is N/A; Pleatco PSANT20 / PSANT20P3 |
| Filbur size table | FC-0126 | 4-3/4" × 8", 20 sq ft | 1-1/2" SAE | Strong Spas - Antiqua, crossed to 4CH-925 and PSANT20P3 |
| Filbur size table | FC-0208 | 4-3/4" × 7-1/16", 20 sq ft | 1-1/2" MPT | Futura Spa (Strong Industries), Pleatco PSANT20P4 |
The SAE cartridge and the MPT cartridge are not interchangeable. Unicel’s Strong section lists only the SAE 4CH-925. Pleatco and Filbur also sell an MPT version (PSANT20P4, FC-0208) at Pleatco’s shorter length. Read the bottom of the old cartridge. Open-end versus threaded shows what SAE and MPT look like, and how to find your spa filter size is the measuring order.
Where the sources disagree
- Length of the 20 sq ft SAE cartridge. Unicel 7-1/4". Pleatco 7-1/16". Filbur FC-0126 8". Those are not a rounding difference. Order from the cartridge you measured, not from the brand name.
- Outside diameter. Unicel 4-5/8". Pleatco and Filbur 4-3/4".
- The name. Pleatco says Antigua. Filbur says Antiqua. Strong’s FAQ says neither.
- Filbur’s Unicel cell. The Strong Industries section prints N/A in the Unicel column for FC-0126. A different Filbur table crosses the same Filbur number to 4CH-925.
- The 75 sq ft filter. Strong’s FAQ is the only source here that mentions it, and it gives no dimensions and no part number. No matching row in the three catalogs.
- The FAQ’s 50 sq ft filter. Strong does not say it is C-4950. Unicel lists a 50 sq ft C-4950 under Strong with no model. Filbur’s Strong section has no 50 sq ft row.
How to confirm yours
- If the spa is a Vienna 60, Madrid 60, Summit, Ascent, Embark or G-2, start from Strong’s area and the year, then measure the old cartridge. The FAQ does not finish the identification.
- A 20 sq ft cartridge about 7" long with a 1-1/2" SAE thread is the Antigua / Futura part, 4CH-925 / PSANT20P3 / FC-0126. Compare the length you measure with all three catalog figures on the 4CH-925 page.
- An MPT bottom is the other Pleatco/Filbur number (PSANT20P4 / FC-0208), not 4CH-925.
- If the model is not on Strong’s list, Strong says to use the owner’s manual. The Strong Spas table on this site has only the cartridge the catalogs document.
Why one cartridge size can carry two different square-foot ratings: our explainer with Unicel’s pleat-count table.
Sources
- Strong Spas: FAQ, “What size filter does my spa use?” (Vienna, Madrid, Summit, Ascent, Embark, G-2; 50 or 75 sq ft; no part number)
- Unicel 2026 Replacement Guide: Strong Industries section, PDF p.58 (printed p.54), 4CH-925 and C-4950; 4CH-925 spec table, PDF p.7 (printed p.3), 190 pleats
- Pleatco 2023 Catalog: PSANT20P3 and PSANT20P4 spec table, PDF p.31; application index PDF p.27 (Futura Spa, Strong Antigua, 1.50" SAE and 1.50" MPT)
- Filbur Catalog: Strong Industries section, PDF p.26 (printed p.51), FC-0126 at 4-3/4" × 8" with Unicel N/A; size table PDF p.9 crosses FC-0126 to 4CH-925; FC-0208 is the MPT Futura cartridge
This page summarizes published catalogs and dealer fitment tables; always follow your own spa’s owner’s manual. See our disclaimer and methodology.