When to Change Your RO Filter in Delhi (2026)

The short answer: your water decides, not the calendar

Three things set how fast anything in your RO wears out, and none of them is the date on a sticker:

• Your input TDS — how much dissolved salt is in the water going in. The higher it is, the harder the membrane works and the faster it fouls. This is the single biggest factor, and it is the one the calendar completely ignores.
• Your usage — a family of six drinking and cooking off the machine puts several times more water through it than a couple. More litres through the same filter means a shorter life, obviously and unavoidably.
• Your water source and season — piped municipal, borewell, or tanker; clear or monsoon-muddy. A dirty, turbid input chokes a sediment filter in weeks that would otherwise have lasted months.

Put those together and you get why a calendar is the wrong tool. Two identical machines, bought the same day, will need their parts changed on wildly different timelines if one is on Delhi supply and the other on Noida borewell with twice the people drinking from it. The manual cannot know any of this. A TDS meter can — and it is one of the cheapest pieces of equipment you can buy for a home. That is the whole shift this guide is asking you to make: stop counting months, start taking readings.

Every stage is different — the real lifespans

The most expensive misunderstanding about an RO is that "changing the filter" is one job on one schedule. It is not. Your machine is a chain of parts with completely different lifespans and completely different jobs, and treating them as one bundle is exactly what an upsell relies on.

The one intuition to keep: the cheap parts exist to protect the expensive one. Sediment and carbon cost a fraction of a membrane, and their entire purpose is to keep grit and chlorine away from it. Skipping them to save money is the surest way to pay for a membrane early. If you remember nothing else, remember that the cheapest filters are the ones you must never neglect.

The UV lamp is the one part that is on the calendar

Everything above is driven by your water and your usage. The UV lamp is the exception, and it is worth understanding why, because it is the one place a calendar is the right tool.

A UV lamp does not clog and it does not foul. It ages. The lamp's ability to actually kill bacteria fades steadily with hours of use whether or not much water has passed through it — and, critically, it dims long before it goes dark. A UV lamp can be glowing away, looking perfectly healthy behind its little window, while its germicidal output has dropped well below what actually disinfects. "It still lights up" is not evidence it is working. That is the single most misunderstood thing about UV, and it is why UV is the one component where a time-based replacement — typically once a year — is genuinely the honest schedule.

So the rule splits cleanly. Filters and the membrane: change on evidence, on a reading, on your water. The UV lamp: change on time, because you cannot see it fail. Do not let a technician invert that on you — a membrane condemned "because it has been a year" is a calendar claim about a part that does not obey the calendar, while a UV lamp waved off "because it still glows" is ignoring the one part that genuinely does.

The signs a change is genuinely overdue

You do not need a manual to know your RO needs attention. The machine tells you — you just have to know which signals mean which stage:

• The flow has slowed to a trickle. The tank takes far longer to fill than it used to. Most often a choked sediment or pre-carbon filter — the cheap parts — strangling the flow before the membrane even sees the water.
• The taste or smell has changed. Chlorine coming back, or a flat, "off" taste. Usually an exhausted carbon filter that has stopped adsorbing. Cheap to fix, and worth fixing promptly because a dead pre-carbon stops protecting the membrane.
• Your output TDS is creeping up over months, with the input unchanged. This is the honest, unambiguous signal that the membrane is fouling and nearing the end. It is a reading, not a feeling — which is exactly why it is trustworthy.
• Output TDS is now close to input. The membrane is spent or bypassed and a replacement is genuinely due. Confirm with a reading before authorising it.
• The reject-to-pure water ratio has changed, or the machine runs far more often. A struggling membrane and a tired pump make the unit work harder for the same glass of water.

Take two readings, always: input and output. Input is the raw water feeding the machine; output is what comes out of the tap. That pair, tracked over a few months, is the entire diagnosis. Output far below input and holding steady means the membrane is fine — say no to a replacement. Output climbing while input holds means it is fouling. Nobody took a reading at all means nobody has diagnosed anything, and you should not be authorising a part.

AMC or pay-as-you-go? Match it to your water

Once you accept that the interval is set by your water, the AMC-versus-pay-as-you-go question answers itself — it is not about which is cheaper in the abstract, but about which fits your consumption. There is no rupee figure needed to reason about it; if you want the actual ranges to run the sum, our RO service cost guide for Delhi NCR lays them out and does the arithmetic properly.

• Low-consumption home, moderate municipal TDS, filters looked after — pay-as-you-go usually fits. You simply will not get through enough filters in a year to earn back a filter-inclusive contract. A service when it is due and a filter when the flow drops is genuinely less than a bundled annual fee. Anyone telling you an AMC is always the smart choice is selling one.
• High-TDS borewell or tanker water, heavy usage, large family — an AMC often fits. You are burning through sediment and carbon filters, the membrane is under real load, and multiple changes a year plus visits add up. An AMC bundles them and caps a bad year.
• You know you will never remember to book — an AMC wins on behaviour, not price. A slightly worse deal that actually gets your sediment filter changed beats a better deal you never act on, because that neglected cheap filter is what kills the expensive membrane.

Whichever way you lean, before signing any AMC ask exactly which parts are named as included ("filters" almost always means sediment and carbon, rarely the membrane, and maybe not the UV lamp), how many visits you get, and whether the number of filter changes is capped. If you are the high-TDS household an AMC is meant to protect, that cap is the whole deal.

Avoiding both traps: under-servicing and the over-servicing scam

Changing filters by the calendar fails in two directions, and it is worth naming both, because most advice only warns you about one.

Under-servicing is the familiar one: leaving a spent sediment or carbon filter in for far too long, so grit and chlorine reach the membrane and shorten its life. On hard NCR water this is a real and expensive risk, and it is why the flow and taste signals above matter.

Over-servicing is the quieter trap, and the RO trade is unusually prone to it for one structural reason: you cannot see any of it. The parts sit inside a sealed box, they all look identical, none of them fails visibly, and you have no way to verify a single claim. So the honest technician and the one padding the bill say almost the same words. The difference is whether they show you a number. Watch for:

• No TDS reading, before or after. The master red flag. A technician who condemns a membrane without measuring your output TDS has not diagnosed anything — they have guessed, or they have decided. Ask for the input and output readings, and ask to see the meter. An honest one is glad to show you; it is the evidence for their own recommendation.
• "The membrane has to be changed every year." On moderate municipal supply, with the pre-filters changed on time, a membrane very often lasts considerably longer. An annual membrane change stated as a fixed rule — rather than concluded from your rising output TDS — is a sales script, not a diagnosis. (On genuinely high-TDS borewell water a short membrane life can be real. It still has to be shown with a reading, not asserted.)
• Everything is due at once. The stages have different lifespans, as the table above sets out. A visit that finds the sediment, the carbon, the membrane and the lamp all need replacing today is describing an unlikely coincidence.
• The part is condemned before the machine is even opened. Diagnosis follows inspection, not a phone call.
• One lump-sum price, no itemisation. Labour and each part should be separate lines. Fusing them is how a filter you did not need hides inside a bill you cannot audit.

None of this means most technicians are dishonest — most are not, and a good RO technician is worth keeping for years. It means the trade has no natural check on the bad ones, so you have to supply it. One handheld meter and one question — "what is my output TDS?" — supplies almost all of it, and it is the same tool that stops you under-servicing too.