Master Spas Eco-Pur filters: outer cartridges, the inner element, and the stand pipe
Unicel splits Master Spas Eco-Pur filtration into two eras. C-7330 is the outer cartridge for 2002–2005 (7" × 7-1/4", 30 sq ft). C-8341 is the outer cartridge from 2006 on (8" × 6-3/4", 40 sq ft), and Unicel says two of those 40 sq ft cartridges are required[5]. The small C-3310AM element (3-7/8" × 7", antimicrobial media; see what AM means) is used with either outer cartridge. On the threaded Eco-Pur replacements, Unicel says to remove the black stand pipe[5].
One product this answer points to
8" Eco-Pur outer, 2006 on. Unicel says two are required.
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Eco-Pur is an assembly, not a single cartridge
Master Spas cartridges fall into two groups in the Unicel 2026 guide: ordinary threaded top-load cartridges, and the Eco-Pur system, which pairs an outer cartridge with a small inner element. Ordering “a Master Spas filter” does not say which half you need, or whether the spa holds one outer cartridge or two. The notes below are Unicel’s, from the Master Spas section of the 2026 guide and the spec table[5].
| Unicel | Role Unicel gives it | Size | Ends | Pleatco / Filbur |
|---|---|---|---|---|
| C-7330 | Eco-Pur outer, 2002–2005 | 7" × 7-1/4", 30 sq ft | 4-1/8" open both ends | PMA30-2002-R / FC-1003 |
| C-8341 | Eco-Pur outer, 2006 to present; two 40 sq ft cartridges required | 8" × 6-3/4", 40 sq ft | 4" open with a raised ring, both ends | PMA45-2004-R / FC-1007 |
| C-3310AM | Antimicrobial inner element, used with C-7330 or C-8341 | 3-7/8" × 7", 8 sq ft | 2-3/16" open both ends | PMA10; no Filbur number on our row |
Count what comes out of the housing. If you remove two large cartridges and one slim element, that is the C-8341 arrangement Unicel describes, and you replace both outers plus the inner on its own schedule. If you remove one outer and one inner from a 2002–2005 system, C-7330 is the outer Unicel names. The diameters differ (7" versus 8"), so a 2002 outer does not take the place of a 2006 outer. Pleatco’s numbers carry the year in the part itself: PMA30-2002-R and PMA45-2004-R[2]. Filbur lists FC-1003 and FC-1007[31].
C-3310AM is not a substitute for the outer. It is 3-7/8" across. Unicel calls out antimicrobial media and says it is used with a C-7330 or a C-8341[5]. Replacing only the inner leaves the outer media in place. Replacing only the outer leaves the inner element in place. They are different part numbers because they are different pieces.
The stand-pipe note on the threaded replacements
Other Master Spas rows are threaded cartridges rather than the open-end Eco-Pur outers, and Unicel attaches an installation note to them.
- 6CH-352, OEM PMA-R1. 6" × 8", semi-circular handle, 2" male MPT, 35 sq ft, Pleatco PAS40-F2M. Unicel says to remove the black stand pipe. Filbur cross-references 6CH-352 to FC-0312 but lists FC-0312 at 6" × 7", not 6" × 8"[5][31]. When the two catalogs disagree on length, measure the cartridge you removed. Do not average 7" and 8" into a length neither book printed.
- 7CH-322. 7" × 8", semi-circular handle, 2" male MPT, 32 sq ft on our row, Pleatco PAS35-F2M, Filbur FC-0420. Unicel says it is a replacement for 7" or 8" OD Eco-Pur systems, and to remove the black stand pipe for an 8" OD system[5].
- 4CH-935 and 4CH-949 are Unicel’s replacements for 5" OD Eco-Pur systems. 4CH-935 is 4-15/16" × 9-1/4". 4CH-949 is 4-15/16" × 13-1/2". Neither row has an OEM number in our table, and Unicel says to verify the overall length. “Verify OAL” means measure. A 9-1/4" cartridge and a 13-1/2" cartridge are both 4-15/16" in diameter and they are not the same part.
The stand pipe is a plastic tube in the filter well. Unicel’s instruction is to remove it when you install these replacements. That instruction is on the cartridge, not a general rule for every Master Spas model. If your housing does not have a black stand pipe, there is nothing to remove. If it does, and you are installing 6CH-352 or an 8" application of 7CH-322, the note says it comes out[5]. Forcing a threaded cartridge down over a stand pipe the note says to remove is how a new filter fails to seat.
How this differs from “buy a spare”
A second set of cartridges, which Unicel, Pleatco and Jacuzzi recommend so one set can soak while the other is in the spa, is a spare[4][1][10]. Two C-8341 cartridges in the housing at the same time are not a spare. They are the system Unicel says the 2006-and-later Eco-Pur filter uses. Buy two outers because the housing holds two, and keep a further spare only if you want one soaking. The guide to that distinction, including Hot Spring’s three-cartridge wells, is spas that use more than one filter.
What to check before you order
- Count the large cartridges and look for a slim inner element.
- Measure the outer diameter: about 7" points at C-7330, about 8" at C-8341, about 6" at 6CH-352, about 4-15/16" at the 4CH Eco-Pur replacements.
- Read any OEM number, especially PMA-R1.
- If the spa is around 2005 or 2006, use the year Unicel prints rather than a photo of “an Eco-Pur filter.”
- Read the stand-pipe note on the page for the part you are about to buy.
- Use the Master Spas filter by series and year lookup, then the Master Spas brand table for the rows that have no year.
Threaded versus open ends are covered in open-end vs threaded hot tub filters. Length disagreements between catalogs are the same problem as the square-foot disagreements in why same-size filters differ: the outside dimensions in the two books are not always identical, so the cartridge in your hand is the measurement that counts.
Search Master Spas Eco-Pur numbers on Amazon
Part-number searches
A search may return one cartridge when the housing holds two. Read Unicel’s “two required” note before you buy a single C-8341.
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Unicel C-8341 8" Eco-Pur outer, 2006 on. Unicel says two are required.
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Unicel C-7330 7" Eco-Pur outer, 2002–2005.
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Unicel C-3310AM Inner antimicrobial element used with the outer cartridge.
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Master Spas PMA-R1 OEM number Unicel lists for 6CH-352.
Questions
How many filters does a Master Spas Eco-Pur system use?
Unicel says the 2006-and-later outer cartridge, C-8341, is used two at a time (40 sq ft each). The inner element C-3310AM is used with that outer, or with the 2002–2005 outer C-7330. Count the pieces in your housing.
What is the Master Spas black stand pipe note?
On 6CH-352, Unicel says to remove the black stand pipe. On 7CH-322, Unicel says to remove it for an 8" OD Eco-Pur system. The note is about those replacements, not a rule we found for the open-end C-7330 and C-8341 cartridges.
Why does Filbur list a different length for 6CH-352?
Unicel’s row for 6CH-352 is 6" × 8". Filbur cross-references it to FC-0312 and lists FC-0312 at 6" × 7". Measure the old cartridge instead of picking one catalog’s length.
Is C-3310AM a full-size replacement filter?
No. Unicel lists it as an antimicrobial inner element, 3-7/8" × 7", used together with outer cartridge C-7330 or C-8341. It does not replace the outer.
Sources
- Pleatco Catalog & Cross Reference Guide: filter care FAQ
- Pleatco 2023 Catalog (Pentair)
- Unicel: Maintenance & FAQ
- Unicel 2026 Replacement Guide
- Jacuzzi: Hot Tub Filter Cleaning Made Easy
- Filbur Catalog & Reference Guide
This guide summarizes published manufacturer and public-health guidance; always follow your own spa’s owner’s manual. See our disclaimer.