Inline Under Sink Water Filter Replacement Cartridge for the iUS4-25K, NSA 100S & NSA 100SX

$199.00 $179.00

Under sink water filter replacement cartridge for the following models: iUS4-25K, NSA 100S, NSA 100SX and H2O-US4-13.

The iUS4-25K drastically improves your water’s taste while eliminating its odor and color for 25,000 gallons. Its 5-stage filtration process includes bacteriostatic media (KDF + GAC) which prevents bacteria growth while eliminating chlorine and a variety of chemicals and organic contaminants.

  • 25,000-gallon capacity
  • Replacement for NSA models 100S and 100SX
  • Bacteriostatic media prevents bacteria growth (algae, fungi)
  • 2 gpm flow enables filtration for multiple appliances
  • Available with quick-connect or compression fittings
  • Includes 50-psi pressure regulator
  • Includes 1-year pro-rata warranty

Note: This replacement cartridge model does not include tubing or fittings because it’s intended only to replace the filter cartridge on an existing setup. If you don’t already have one of the models that are listed above and you would like an inline under sink water filter, the US4-25K is what you’re looking for.

Compression Fittings (optional)

(this unit comes standard with quick connect fittings. Check the box to receive compression fittings on your unit.)

SKU: iUS4-25K-CART Category:


The iUS4-25K-CART is our under sink water filter replacement cartridge for the iUS4-25K, NSA 100S, NSA 100SX and H2O-US4-13 models. Whereas the iUS4-25K includes a filter cartridge, tubing and fittings for a brand new installation, the iUS4-25K-CART includes only the filter cartridge. After receiving the new cartridge, simply swap it out with the expired cartridge and use the tubing and fittings you already have. If you don’t have these parts or wish to replace these parts, then you should order a new iUS4-25K.

The iUS4-25K is an inline unit which filters water between the cold water valve and the sink’s main faucet. It can be configured to feed water to a separate faucet. However, if that’s what you’d like to do, we would recommend either the US4-35K or the US3-25K. Those units already include a faucet and specifically serve that purpose.

Under Sink Water Filter Replacement Cartridge

The iUS4-25K filters 100% of the cold water that runs through the main faucet. For this reason, it reaches its capacity relatively (25,000 gallons) quicker than other undersink models. It includes a 1-year pro-rata warranty. It also comes with a 50-psi pressure regulator which protects your cartridge from spikes in water pressure. A proper installation of the pressure regulator with your unit is a required condition of the warranty.

NSA 100S and NSA 100SX Replacement Cartridge

National Safety Associates (NSA) discontinued its water filter production in 2007. At that time, however, thousands of their 100S and 100SX models were still in the hands of dealers who had not yet sold them. Even today, original NSA water filters are available on Amazon, eBay and elsewhere. In spite of NSA’s questionable business model, the filters they manufactured were quite effective and popular.

Since the discontinuation of NSA-branded models, various distributors have made a market of selling NSA-style replacement cartridges to those who liked the products. Several improvements in the design and strength of the replacement models have been made over the years.

The iUS4-25K and its replacement cartridge feature a 5-stage filtration process including KDF media and GAC (granular activated carbon), plus three 5-micron filter pads. Its bacteriostatic media prevents bacteria growth and improves the taste of your water while eliminating its odor and color. A standard unit includes 3/8″ quick-connect fittings at the inlet and outlet. Compression fittings are also available for this unit. After flowing upward through the filter cartridge, the filtered water then continues to the main faucet of the sink, providing great-tasting, chlorine-free water.

Not only does the iUS4-25K work great under the sink in residences, but it also works great on water coolers, appliances, boats, RVs or in any other inline arrangement. With a maximum flow of 2.0 gallons per minute, it’s capable of feeding filtered water to several appliances at the same time.

If you have any technical questions, send us a message on our contact page.

Maximum Capacity 25,000 gallons (94,635 liters)
Maximum Flow 2.0 gpm (7.57 lpm)
Unit Dimensions 16.5″ height x 5.0″ diameter (41.9 x 12.7 cm)
Box Dimensions 18″ x 7″ x 6″ (44.5 x 16.5 x 14.6 cm)
Unit Shipping Weight 10.4 lbs. (4.72 kg)



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