Spas that use more than one filter cartridge at the same time

Last reviewed November 9, 2026

Some housings are built for more than one cartridge. Leisure Time’s listing for Hot Spring OEM 71825 says many of those models take three of the 6" cartridges^[38]. Unicel says Master Spas Eco-Pur systems from 2006 on require two C-8341 cartridges, and a separate inner element is used with them^[5]. Sundance’s 680 and 780 manuals describe a pleated filter plus a disposable MicroClean insert, on two different replacement clocks^[11][12]. A second set you rotate while one soaks is a spare, which Unicel, Pleatco and Jacuzzi recommend, and it is not the same as a housing that holds two^[4][1][10].

Three different meanings of “two filters”

Owners get sold a two-pack and then are not sure whether both cartridges go in the spa today. The documents use “two” in three ways, and only one of them means the housing is full when a single cartridge is installed.

1. The housing holds more than one at the same time. You replace every cartridge that was in the well, because each one is part of the flow path.
2. One pleated cartridge and one smaller insert occupy the same well and are not the same part. They can have different lifetimes and different cleaning rules.
3. A spare set waits while the set in the spa gets dirty. The spare is not installed on top of a full housing.

A retail “2-pack” is usually the third meaning, or simply how the manufacturer boxes the part. Intex 29001E is a two-pack of Type S1 cartridges. Intex’s pages do not say a PureSpa holds an S1 and an S2 together. They say which set numbers take which type. That split is in Intex and Bestway filters.

Unicel’s Waterway list is another housing count^[5]. It prints 817-0050 once as a single front-access cartridge and again with the note that two are used side by side, and it prints 817-0011 stacked on 817-0012. How the catalogs rate those bodies is on the Waterway filter lookup.

Hot Spring wells that hold three

The 6" × 10-1/2" open cartridge C-6430 (OEM 31489 / 71825 / 71826) is the Highlife size. The Leisure Time listing for 71825 says many of these models take three cartridges, and that some models of the same footprint use a Tri-X ceramic filter instead of pleated media. Use the pleated cartridge only where the spa specifies 71825 or a listed equivalent^[38].

If you open the well and see three pleated cartridges, order three of the part that matches, not one. If you see a ceramic block, C-6430 is the wrong object even though the footprint is the one the listing mentions. The collection-by-collection sizes, including the 8-1/2" cartridge and the 2018 Limelight cartridge that is not this 6" part, are in Hot Spring filter sizes by collection. Hot Spot models that the dealer assigns to this cartridge are on the Hot Spot page; count the filters in that housing too, because several Hot Spot tubs are listed with more than one.

Master Spas: two outers, plus an inner

Unicel’s note on C-8341 is direct: from 2006 onward this Eco-Pur outer is a 40 sq ft cartridge and two of them are required. Each one is 8" × 6-3/4" with 4" open ends and a raised ring. The 2002–2005 outer is the different part C-7330, 7" × 7-1/4", 30 sq ft. The inner element C-3310AM is 3-7/8" × 7" and is used with either outer. Unicel calls that inner piece antimicrobial media^[5].

Buying one C-8341 leaves the housing half serviced if both outers were due. Buying a C-8341 for a 2004 spa ignores the 7" versus 8" change. The stand-pipe note on the threaded Master Spas replacements is a separate installation step and is explained in Master Spas Eco-Pur filters.

Sundance’s outer cartridge and disposable insert

Sundance’s 2024 manuals describe a two-stage arrangement rather than two copies of one cartridge. On the 680 Series, the pleated MicroClean Mini filter is on a 12-month replacement and the insert is on a 4-month replacement. On the 780 Series, the small disposable MicroClean cartridge is replaced every 3 months, or earlier^[11][12]. Those are not suggestions from a retailer. They are the intervals in the owner’s manuals.

The cleaning rules differ inside the same well. Sundance says not to use filter cleaners on the inner inserts. The 680 manual calls for a high-pressure hose nozzle on the outer pleated filter and a low-pressure nozzle with no chemicals on the insert^[11]. Unicel’s one-hour degreaser soak is written for a pleated cartridge you intend to reuse^[4]. It is the wrong soak for the insert Sundance says not to chemically clean. Replace the insert on its short interval. Clean or replace the outer on its longer one. Our general cleaning steps are in how to clean a hot tub filter, with that Sundance exception spelled out.

A cage is not a second cartridge

Bullfrog’s coreless filters are a pleated element plus a plastic cage and cap. Leisure Time says the cage and cap must be installed with OEM 10-00281 and 10-00282, and it names a cage assembly for specific years^[43][42]. You do not stack two coreless socks to make up for a missing cage. You install the cage the listing calls for, or you confirm the spa has moved to the Simplicity flat filter that the 10-00282 listing assigns to 2023-and-newer A Series models. The thread-versus-coreless split is in Bullfrog threaded and coreless filters.

The spare set, which is the recommendation you can apply to any of these

Unicel and Pleatco tell owners to keep two sets and alternate them. The set that comes out gets a hose rinse and a degreasing soak of at least an hour, overnight if you can, and then it dries. The other set goes into the spa so you are not running without filtration^[4][1]. Jacuzzi gives the same advice and adds that a cartridge may be reinstalled damp, while a full dry extends its life^[10]. For a three-cartridge Hot Spring well, a “set” is three cartridges, not one. For a two-cartridge Eco-Pur well, a set is two outers, and the inner element is its own part. For a Sundance two-stage well, the spare you rotate is the pleated outer; the insert is a disposable on a shorter clock, not a piece you soak and put back.

Running while a set soaks, with nothing in the well, is the case Jacuzzi and Hot Spring reject^[10][9]. The consequences, weak jets, high-limit trips, flow codes, are in what a clogged or missing filter does. Putting the spare in without overtightening the retainer is in how to install a hot tub filter.

How to count yours

1. Power off, then remove every cartridge and lay them in a row^[9][10].
2. If they are the same part, count them. That count is what you replace.
3. If one is much smaller, or is a thin insert inside a pleated cartridge, treat it as a different part number and a different interval.
4. Read the OEM number off one of the large cartridges and search it here. The page’s fitment notes are where “three required” or “two required” is written when a source says it.
5. Order that many, and order a second set only if you want the rotation. Do not install the spare set in a housing that is already full.

Questions

Sources

1. Pleatco Catalog & Cross Reference Guide: filter care FAQ
4. Unicel: Maintenance & FAQ
5. Unicel 2026 Replacement Guide
9. Hot Spring Hot Spot Collection Owner’s Manual (2026)
10. Jacuzzi: Hot Tub Filter Cleaning Made Easy
11. Sundance Spas 680 Series Owner’s Manual (2024)
12. Sundance Spas 780 Series Owner’s Manual (2024)
38. Leisure Time Inc.: Hot Spring Highlife 71825 filter cartridge
42. Leisure Time Inc.: Bullfrog A/R/X series 10-00282 filter cartridges
43. Leisure Time Inc.: Bullfrog STIL series 10-00281 filter cartridges

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