How often should you change your water filter cartridge?

A water filter is only as good as its cartridge. It is easy to forget about it: the water still comes out, it still looks clear, and a new cartridge costs money. But a cartridge has a limited capacity, and once it is used up it stops doing its job. Worse, an overdue cartridge can affect the quality of the water that passes through it.

Why an old cartridge is a problem

The Drinking Water Inspectorate, which regulates drinking water in England and Wales, is clear about this. Its guidance says that filters must be maintained according to the manufacturer's instructions, and that failing to do so will cause a build-up of deposits, create a breeding ground for bacteria and allow removed material to leach back into the drinking water.

In other words, a cartridge that has been in use for too long can release some of what it has collected. Changing it on time is not just about taste — it is about hygiene.

Typical replacement intervals

Always follow the instructions for your specific product, because cartridge capacity varies between brands and models. As a general guide:

• Filter jug cartridges: most manufacturers recommend replacing them about every four weeks, or after a set number of litres, whichever comes first. In very hard water areas, the limescale-reducing part of the cartridge may be used up sooner.

• Tap-mounted filters: usually every two to three months, or after the number of litres stated by the manufacturer.

• Under-sink filters: often every six to twelve months, depending on the system and how much water you use.

• Reverse osmosis systems: the pre-filters, membrane and post-filters each have their own schedule — the membrane typically lasts longest. Check your manual or service plan.

Signs your cartridge needs changing

Do not wait for these signs if the replacement date has passed, but they are a useful warning:

• The water starts tasting or smelling of chlorine again.

• Limescale returns to the kettle more quickly than usual.

• Water flows through the filter much more slowly — or much faster — than when the cartridge was new.

• You notice cloudiness, particles or an unusual taste.

Many jugs have an indicator on the lid that counts days or refills. Use it, or set a monthly reminder on your phone.

How to change a jug cartridge properly

1. Wash your hands. Empty the jug and remove the old cartridge.

2. Wash the jug, lid and reservoir in warm, soapy water and rinse well. Most jugs are not dishwasher-safe, so check the instructions.

3. Prepare the new cartridge as instructed — many need soaking or flushing before first use.

4. Insert the cartridge firmly, so water cannot bypass it.

5. Discard the first one or two jugs of filtered water if the manufacturer recommends it.

6. Reset the indicator on the lid.

Everyday hygiene between changes

Filtered water has less chlorine than tap water, and chlorine is what keeps tap water free of bacteria on its way to your home. That means filtered water is best used fresh.

• Keep the jug in the fridge, especially in warm weather, and out of direct sunlight.

• Use filtered water within a day. The Drinking Water Inspectorate's guidance on chlorine suggests discarding water kept in a jug in the fridge after 24 hours.

• Clean the jug every time you change the cartridge, and more often if you see a film or residue.

• Do not leave a cartridge dry for long periods. If you go away, follow the manufacturer's advice — some recommend changing the cartridge on your return.

What about plumbed-in filters?

Under-sink and reverse osmosis systems need maintenance too, and it is easier to forget about a filter you never see. The Drinking Water Inspectorate recommends that plumbed-in devices are installed by a qualified plumber and maintained according to the manufacturer's instructions. If you are not confident changing cartridges yourself, consider a service agreement with the supplier.

Count the cost before you buy

Replacement cartridges are the real price of a water filter. Before choosing a model, check how much cartridges cost, how many litres each one filters and how easy they are to buy. Divide the cartridge price by its capacity and you get the cost per litre — the fairest way to compare filters. We explain the other points worth comparing in our guide Jug, tap or under-sink: which water filter suits your home?

And if your main reason for filtering is limescale, it is worth understanding how hard your water is first: see Hard water and limescale: what UK households need to know.