When to change a water filter cartridge: volume, not the calendar

Most filter systems tell you to change a cartridge "every six months". Yours has a number printed on it that is far more useful: a volume. This article explains why the volume is the number that matters, and how to work out what a year of filtered water costs.

The three cartridges, and what each is rated for

Cartridge	Rated volume	Price	Job
PAC composite	1,000 L	£29	Sediment, chlorine, taste
200G RO membrane	2,000 L	£39	Dissolved salts, microplastics, bacteria
CF-Strontium	1,000 L	£29	Final carbon pass
Complete set	1,000 L of water	£82	All three together

Prices are this store's prices including VAT; delivery is free in the UK. The volumes are the rated lives published on each product page.

Why volume beats the calendar

A cartridge is spent when it has processed the water it was designed to process — not when a date on the box arrives. Two households can share a postcode and a machine and use wildly different amounts of water: one fills a kettle twice a day, another fills bottles for four people and a dog. A calendar schedule over-serves one and under-serves the other.

The rated volume also tells you when a cartridge is doing its job rather than failing. A PAC cartridge that has processed 1,000 litres is not broken; it is finished. That distinction matters when you are deciding whether something is a fault or simply maintenance.

The arithmetic of a year of water

Take the complete set at £82. It covers the shortest-lived cartridges in the train — the PAC composite and the CF-Strontium, both rated to 1,000 litres — so 1,000 litres is the volume a set buys you. That is:

• 8.2 pence per litre of filtered water, before electricity (82.00 / 1,000).
• At a household use of 8 litres a day, 1,000 litres lasts about 125 days — roughly four months, so three sets a year, £246.
• At 4 litres a day, 1,000 litres lasts about 250 days — two sets a year is nearer, £164.

Those are illustrations, not predictions: the only figure that matters is your own daily use, which you can read off a measuring jug for a week. The point is that the cost per litre is a division, not a marketing claim — and it moves with how much you drink, not with how long you have owned the machine.

The RO membrane is the exception: rated to 2,000 litres, it lasts two sets. Buy it on its own when it is due rather than replacing it with every set, unless the machine's own indicator says otherwise.

What the machine actually tracks

The KW-01 and KW-02 both monitor filter life and indicate when a cartridge is due, based on the water the machine has produced. That indicator is the primary schedule; the rated volumes are what sits behind it. When the two disagree — a machine used hard for a year versus a quiet one — trust the indicator, because it is counting the water, not the days.

Signs a cartridge is past its volume

• Flow slows noticeably at the spout — the membrane is the usual cause.
• Water starts to taste or smell of chlorine again — the carbon is spent.
• The machine's filter indicator has moved into its end-of-life state.
• Limescale returns to the kettle faster than it used to.

If any of those appear well before the rated volume, that is worth a message rather than a purchase: it can mean an unusual water supply, a cartridge fitted the wrong way round, or a genuine fault.

What not to do

• Do not stretch a cartridge to save £29. The cost of a cartridge is 8.2p a litre. The cost of drinking water through a spent one is that you no longer know what you are filtering.
• Do not replace the RO membrane with every set unless its own 2,000 litres or the indicator says so.
• Do not buy a cartridge for the wrong model. The cartridges in this store are made for the KW-01 and KW-02. If you are unsure, ask before ordering — it is confirmed in writing.

Sources

1. This store's product pages for the PAC composite (£29, 1,000 L), 200G RO membrane (£39, 2,000 L), CF-Strontium (£29, 1,000 L) and complete set (£82), read 11 November 2026.
2. Cost per litre is the set price divided by the set's rated volume: £82.00 / 1,000 L = £0.082 per litre.
3. Daily-use figures are stated assumptions for illustration, not survey data.