Water purifiers rarely fail dramatically. They decline slowly: the tank takes a little longer to fill, the water tastes a little different, and months go by before anyone notices. Knowing the signs helps you change filters when they actually need it, rather than when a service call reminds you.
The filters inside a typical purifier
Most Indian RO and UV purifiers have several stages. Each wears out differently:
| Stage | What it does | Typical life (rough guide) |
|---|---|---|
| Sediment filter | Traps sand, silt, rust and mud | 6–12 months, often less with borewell or monsoon water |
| Pre-carbon filter | Removes chlorine and odours, protects the membrane | 6–12 months |
| RO membrane | Removes most dissolved salts and heavy metals | 1–3 years, depending on input water |
| Post-carbon filter | Polishes taste after storage | 6–12 months |
| UV lamp | Disinfects bacteria and viruses | As per manufacturer’s schedule |
| UF membrane | Filters bacteria, cysts and fine particles | As per manufacturer’s schedule |
Those intervals are a starting point. Your manual, and your water, decide the real ones.
1. The flow is slower, or the tank takes longer to fill
This is the most common sign. Clogged sediment and carbon filters restrict water flow, and a scaled or fouled RO membrane produces less purified water for the same input. If the storage tank used to fill in a certain time and now takes noticeably longer, filters are the usual suspect.
Before blaming the filters, check that your inlet pressure hasn’t dropped. Low water in the overhead tank, a partly closed inlet valve or a kinked pipe can cause the same symptom.
2. The water tastes or smells different
A chlorine smell, a musty or earthy taste, or a general “off” flavour usually means the carbon filters are exhausted. Carbon removes chlorine and odour-causing compounds, and once it’s saturated it stops working.
Other causes to rule out: a storage tank that needs cleaning, a new tanker supplier, or monsoon runoff changing your raw water.
3. The output TDS is creeping up
If you have a TDS meter, test the purifier’s input and output now and then. Our TDS meter guide shows how. An RO purifier normally removes most dissolved solids, so the output should be much lower than the input.
If the output TDS rises steadily over weeks or months, the RO membrane may be wearing out. It could also be the TDS controller drifting, so ask the technician to check both before replacing the membrane.
4. The water looks cloudy or discoloured
Purified water should be clear. Cloudiness, a yellowish tint or fine particles at the bottom of a glass can mean:
- the sediment filter is overloaded and letting particles through,
- a filter housing or seal has failed, or
- the storage tank needs cleaning.
Tiny air bubbles that clear from the bottom up within a minute are normal and harmless. Cloudiness that stays is not.
5. The pump runs longer or sounds different
Listen to your purifier. If the booster pump runs much longer than it used to before the tank fills, or makes a straining or rattling sound, the system is working harder than it should. Clogged filters and membranes are a common cause.
Also check the drain pipe. If water keeps flowing to the drain long after the tank is full, the auto shut-off may be faulty, which wastes water. Our guide to reducing RO water wastage covers this in more detail.
6. You can see dirt, or there’s a leak
Many purifiers have a transparent sediment filter housing, either inside the unit or on the inlet line. A cartridge that has turned brown or orange is full of sediment or iron and needs changing.
Drips around filter housings or fittings, or damp patches under the unit, can mean a cartridge isn’t seated properly or a seal has worn. Leaks need attention promptly, both for the wall or cabinet and for hygiene.
7. The indicator is on, or the schedule says so
Many purifiers have a filter-change light or app alert. Most work on a timer or by counting litres, not by testing the water. So treat the alert as a reminder, not a diagnosis.
The UV lamp is a special case. It can keep glowing while its disinfecting strength fades, and you won’t notice any change in taste or flow. Replace it on the manufacturer’s schedule, even if the water seems fine.
If your purifier has no indicator, write the date of each filter change on a strip of tape on the unit, or set a phone reminder.
What shortens filter life
Expect to change filters more often if:
- your water comes from a borewell or tanker (see our borewell water purifier guide),
- your water is hard or high in TDS,
- the supply turns muddy during the monsoon, or
- your overhead tank or sump hasn’t been cleaned in a long time.
A separate sediment pre-filter on the inlet line can protect the purifier’s internal filters and is cheap to replace.
DIY, technician or AMC?
- DIY: fine for simple twist-off cartridges designed for home replacement. Follow the manual and flush new filters as instructed.
- Technician visit: usually needed for RO membranes and units with push-fit tubing inside.
- AMC: bundles filters and visits for a yearly fee. Check exactly which filters are covered (some plans exclude the membrane or UV lamp) and how many visits you get. Compare the total with buying filters separately.
Ask for genuine filters, and look at the old ones before the technician takes them away. A filter that looks almost new may not have needed changing yet.
Bottom line
Don’t wait for the water to taste terrible. Watch the flow, taste, output TDS and clarity, listen to the pump, and keep a note of when each filter was last changed. Change sediment and carbon filters roughly every 6–12 months and the RO membrane every 1–3 years, sooner if your water is hard, muddy or from a borewell.
Frequently asked questions
How often should water purifier filters be changed?
As a rough guide, sediment and carbon filters every 6 to 12 months, and the RO membrane every 1 to 3 years. Hard, muddy or high-TDS water shortens these intervals. Always follow your purifier's manual and indicator, and change filters sooner if you notice any of the warning signs.
Can I keep using a purifier after the filter-change light comes on?
Most indicators are timers or flow counters, so the light means a filter is due, not that the water has suddenly become unsafe. Still, don't put it off for long. An exhausted carbon filter stops improving taste and protecting the membrane, and the UV lamp gradually loses strength even while it still glows.
Why does my purifier water suddenly taste bad?
The most common cause is an exhausted carbon filter, which removes chlorine and odours. Other causes include a dirty storage tank, a fouled membrane, or a change in the incoming water, such as a new tanker source or monsoon runoff.
Can I change water purifier filters myself?
Some purifiers have simple twist-off cartridges designed for home replacement. Many RO units need a technician, because the membrane housing and fittings have to be sealed properly. Using non-genuine filters may affect your warranty, so check the manual first.
