Express Water WH300SCKS Replacement Filters: Which Cartridges Fit, and Cross-Fit Alternatives

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Short answer

The Express Water WH300SCKS takes three 4.5 x 20 in cartridges, one per housing. The Replacements Table in Express Water’s 3 Stage Whole House installation manual lists them by stage: FLTWH2045S1, a 5 micron layered polypropylene sediment cartridge, in stage 1; FLTWH2045K, the Kinetic Degradation Fluxion (KDF) cartridge of catalytic carbon with copper and zinc, in stage 2; and FLTWH2045C1, a 5 micron coconut carbon block, in stage 3. The same table sells all three as the 1 Year Heavy Metal Set, SKU FLTWH2045CKS1, and rates each cartridge at 6 to 12 months or 100,000 gallons. Any 4.5 x 20 in cartridge fits the housings: the Pentek DGD-5005-20 and iSpring FP25B replace the sediment stage, and the Waterdrop WF20PG set covers the sediment and carbon stages, but none of the three is KDF media. The one spec that must match is the 4.5 x 20 in size; micron rating, media and certification change with each cartridge.

What fits

All three stages: the OEM 1 Year Heavy Metal Set
FLTWH2045CKS1 set
FLTWH2045S1 sediment, FLTWH2045K KDF and FLTWH2045C1 carbon block, 4.5 x 20 in; 6 to 12 months or 100,000 gallons
Check price on Amazon Why this one
Cross-fit for stage 1, sediment
Pentek DGD-5005-20
4.5 x 20 in, 5 micron nominal dual-gradient polypropylene; rated to 145 F; NSF/ANSI 42 material
Check price on Amazon Why this one
Cross-fit for stage 1, bought in bulk
iSpring FP25B 10-pack
4.5 x 20 in, 5 micron polypropylene; up to 100,000 gallons; up to 15 GPM; ten per pack
Check price on Amazon Why this one
Cross-fit for stages 1 and 3, not the KDF stage
Waterdrop WF20PG set
Sediment, GAC and carbon set, 4.5 x 20 in; 15 GPM; NSF/ANSI 372 lead-free materials, no reduction claim
Check price on Amazon Why this one

The part number you would guess from the system name, FLTWH2045SCKS, appears in neither Express Water’s manual nor its Amazon listings: the set is FLTWH2045CKS1 , and the singles are FLTWH2045S1, FLTWH2045K and FLTWH2045C1.

Which cartridge goes in which stage?

The Replacements Table on page 4 of Express Water’s installation manual is the only document that lists all four SKUs together.

KDF, Kinetic Degradation Fluxion, is the manual’s name for the copper and zinc media in stage 2.

Stage	SKU	Media (manual)	Micron	Size	Link
1, sediment	FLTWH2045S1	Layered pure polypropylene	5 micron	4.5 x 20 in	In the set below; no single listing opened
2, KDF	FLTWH2045K	Catalytic carbon and high purity copper and zinc	None published; listed as “Heavy Metals”	4.5 x 20 in	In the set below; no single listing opened
3, carbon block	FLTWH2045C1	Polypropylene shell and coconut carbon	5 micron	4.5 x 20 in	In the set below; no single listing opened
All three	FLTWH2045CKS1	1 Year Heavy Metal Set (SED, KDF, ACB)	5 micron on stages 1 and 3	4.5 x 4.5 x 20 in (listing)	FLTWH2045CKS1 set, 3 cartridges

The housing’s working window stays the same whichever cartridge is fitted. The manual’s Conditions page sets feed pressure at 20 to 80 psi and feed temperature at 40 to 100 F, with 1 in NPT in and out. Express Water’s product page rates flow at up to 0.25 gallons per second at 40 to 80 psi.

How often do I change them, and what is the warning sign?

The manual’s table rates each cartridge at 6 to 12 months or 100,000 gallons. Express Water’s separate filter-change guide says approximately 6 months or up to 100,000 gallons, and adds that the time and water depend on the quality, temperature and pressure of the supply.

The pressure trigger is where Express Water’s own two PDFs disagree. The filter-change guide says to replace once pressure has dropped 10 to 20 psi from the initial reading; the installation manual’s Filter Change section on page 12 says 10 to 12 psi. The 10 to 12 psi figure is the more conservative of the two.

What else fits the housings?

The one spec that must match is the 4.5 x 20 in housing size. Each cross-fit listing below states 4.5 x 4.5 x 20 in, the same dimensions the FLTWH2045CKS1 listing gives. Every rating in the table is listing copy, not a spec sheet; Pentair’s DGD spec sheet is cited on the iSpring WGB32B replacement filters page.

Alternative	Same size?	Rating (listing copy)	Replaces	What does not carry over
Pentek DGD-5005-20, pack of 1	4.5 x 4.5 x 20 in, yes	5 micron nominal, dual-gradient spun polypropylene; rated to 145 F; replace about every 6 months	Stage 1 sediment only	NSF/ANSI 42 for material requirements, not a reduction claim. The 145 F rating exceeds the system’s 100 F window, which stays the limit
iSpring FP25B, 10-pack	4.5 x 4.5 x 20 in, yes	5 micron polypropylene; up to 100,000 gallons; up to 15 GPM	Stage 1 sediment only	Listing names REACH as its testing certification, not an NSF/ANSI standard. Sold as ten cartridges
Waterdrop WF20PG set	4.5 x 4.5 x 20 in, yes	Sediment, GAC and carbon media; 15 GPM; NSF/ANSI 372	Stages 1 and 3; its GAC stage is not KDF	NSF/ANSI 372 is a lead-free materials standard, not a reduction claim. Listing names the WH300SCKS as compatible

NSF’s standards page defines NSF/ANSI 42 as the aesthetic-effects standard, with claims for chlorine, taste and odor, chloramine, particulate, iron, manganese, zinc and total dissolved solids, plus material safety per NSF/ANSI/CAN 372. Express Water’s set listing gives “WQA, NSF, Ro” as its external testing certification with no standard number, and no certificate was looked up for this page.

Does a cross-fit replace the KDF stage too?

No. Express Water’s manual gives the FLTWH2045K media as catalytic carbon and high purity copper and zinc, and the set listing calls it KDF85. The Pentek and iSpring cartridges are polypropylene sediment, and the Waterdrop listing gives its media as sediment, GAC and carbon.

If stage 2 is the reason you bought the WH300SCKS, the cross-fits cover stages 1 and 3 and you still buy FLTWH2045K, either on its own or inside the FLTWH2045CKS1 set. If the complaint is orange staining, this is the wrong page: none of these is an iron cartridge, and the iron in well water guide carries those limits.

Which set to buy

The certifications do not swap like for like. Express Water’s set lists “WQA, NSF, Ro” with no standard number. The Pentek prints NSF/ANSI 42 for material requirements, the Waterdrop prints NSF/ANSI 372 for lead-free materials, and the iSpring FP25B lists REACH.

For all three stages at once, the manual’s own FLTWH2045CKS1 set is the only option that includes the KDF stage.

For a sediment change between full sets, a single Pentek DGD-5005-20 matches the 5 micron rating.

Skip the iSpring FP25B 10-pack unless you will use ten, and skip every cross-fit if you are replacing stage 2.

Frequently asked questions

Which part numbers fit each stage of the Express Water WH300SCKS?

The Replacements Table in Express Water's 3 Stage Whole House installation manual lists FLTWH2045S1 (5 micron layered polypropylene sediment) for stage 1, FLTWH2045K (catalytic carbon with copper and zinc, the KDF stage) for stage 2, and FLTWH2045C1 (5 micron coconut carbon block) for stage 3. All three are sold together as the 1 Year Heavy Metal Set, SKU FLTWH2045CKS1, each 4.5 x 20 in.

Is FLTWH2045SCKS a real Express Water part number?

No such SKU appears in Express Water's installation manual or on the Amazon listings opened for this page. The three-cartridge set is FLTWH2045CKS1, and the manual's single-stage numbers are FLTWH2045S1, FLTWH2045K and FLTWH2045C1. The Amazon listing for the WH300SCKS system itself tells buyers to search Amazon for FLTWH2045CKS1 to find the compatible replacement set.

How often does Express Water say to change the WH300SCKS filters?

The manual's Replacements Table rates each cartridge at 6 to 12 months or 100,000 gallons; the separate filter-change guide says approximately 6 months or up to 100,000 gallons, depending on the quality, temperature and pressure of the supply. On the pressure trigger the two PDFs disagree: the filter-change guide says a 10 to 20 psi drop from initial pressure, the manual's Filter Change section says 10 to 12 psi.

Will Pentek or iSpring 20 x 4.5 in cartridges fit the WH300SCKS housings?

The housing size is the only fit spec Express Water publishes, 4.5 x 20 in, and the Amazon listings for the Pentek DGD-5005-20 and the iSpring FP25B both state 4.5 x 4.5 x 20 in. Both are 5 micron polypropylene sediment cartridges, so they substitute for stage 1 only. The Pentek listing names NSF/ANSI 42 for material requirements; the iSpring listing names REACH, not an NSF/ANSI standard.

Does the Waterdrop WF20PG replace the WH300SCKS KDF stage?

No. The Waterdrop WF20PG listing names the WH300SCKS as a compatible system and states 4.5 x 4.5 x 20 in, but gives its media as sediment, GAC and carbon, and its certification as NSF/ANSI 372 for lead-free materials. Express Water's manual gives the stage 2 media as catalytic carbon with high purity copper and zinc, which none of the cross-fits on this page carries.

Express Water WH300SCKS Replacement Filters: Which Cartridges Fit, and Cross-Fit Alternatives

Short answer

What fits

FLTWH2045CKS1 set

Pentek DGD-5005-20

iSpring FP25B 10-pack

Waterdrop WF20PG set