Maintenance
How to change reverse osmosis filters
Quick answer
Reverse osmosis prefilters are typically changed annually, while the membrane lasts considerably longer since the prefilters exist to protect it. Depressurize by closing the tank valve and running the faucet dry, sanitize the housings during the change, flush new prefilters before reconnecting the membrane, and confirm performance with a TDS meter before and after.
A reverse osmosis system has two very different service schedules living inside one cabinet. The prefilters, typically a sediment stage and one or two carbon stages, are consumables that load up and get replaced on a routine, usually annual, schedule. The membrane behind them is the expensive part of the system and is built to last far longer, on the order of two to three years under normal conditions, precisely because the prefilters are standing in front of it removing the particles and chlorine that would otherwise damage it.
Skipping prefilter changes is the single most common way to shorten membrane life. Chlorine attacks the thin film membrane directly, and sediment fouls its surface, so a prefilter left in service well past its interval stops protecting the membrane long before it visibly clogs, and the reader gets no warning except a membrane that fails earlier than its rating suggests.
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Two schedules, one cabinet
Confusing the prefilter schedule with the membrane schedule is the most expensive mistake in reverse osmosis maintenance, since changing prefilters on time is what makes the far more expensive membrane last its full service life.
Prefilters change roughly annually; the membrane is rated for a manufacturer published two to three years and lasts closer to that figure only when prefilters were changed on schedule.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Component | Typical change interval | Why |
|---|---|---|
| Sediment prefilter | Annually | Protects the membrane from particulate fouling |
| Carbon prefilter | Annually | Protects the membrane from chlorine attack |
| Membrane | Manufacturer rated, commonly 2 to 3 years | The expensive part the prefilters exist to protect |
| Post carbon (polish) filter | Annually | Final taste polish after the tank |
Manufacturer intervals vary by model and by incoming water quality. Check the specific system's manual for its published figure rather than assuming this table applies to every unit.
Depressurize before opening anything
Close the small ball valve at the top of the storage tank first, which isolates the tank's stored pressure from the rest of the system. Then open the RO faucet and let it run until it stops entirely, which confirms the housings downstream of the tank valve are no longer under pressure. Skipping this step is how a housing wrench slips or a sump lets go with several ounces of pressurized water still behind it, which is a mess rather than a serious hazard but an entirely avoidable one.
Sanitize the housings during the change
With the housings open and empty, a filter change is the easiest opportunity all year to sanitize the system: a small amount of unscented household bleach or a dedicated sanitizing solution in the empty housings, following the specific system's manual for concentration and contact time, addresses any biofilm that has built up inside the cabinet over the previous service interval. This step is easy to skip because it adds no visible difference to the moment of reassembly, and skipping it repeatedly is how a system develops a persistent musty taste that a filter change alone never resolves.
Flush new prefilters before reconnecting the membrane
New carbon and sediment cartridges release manufacturing fines and carbon dust for the first several minutes of flow, and that debris is exactly what the membrane exists to reject at massive cost to its service life if it arrives concentrated. Reconnect the prefilters first, run the feed water through them to drain rather than through the membrane housing, and only reconnect the membrane once that initial flush runs clear. This single step is the difference between a prefilter change that protects the membrane and one that dumps a slug of fines directly onto it.
A TDS meter is the only real test that the membrane still works
The membrane's whole purpose is total dissolved solids rejection, and a TDS reading taken on the feed water and again on the product water, sold as a simple handheld meter, is the only genuinely direct evidence that it is still rejecting properly after a filter change or at any other point in its service life. A properly functioning membrane typically rejects somewhere around ninety percent or more of total dissolved solids, and a manufacturer's specific published rejection figure for that model is the number to compare a fresh reading against. A reading that has crept up over time, even gradually, points toward a membrane nearing the end of its service life rather than toward a prefilter problem.
A TDS reading says how much is dissolved in the water, not what, and it is not a safety measurement for any specific contaminant. It is the correct tool for confirming rejection performance and nothing beyond that.
Changing the filters, in order
- Close the tank valve. Turn the small ball valve at the top of the storage tank to the closed position, isolating the tank's stored pressure from the housings.
- Run the faucet dry. Open the RO faucet and let it run until flow stops completely, confirming the system downstream of the tank valve is depressurized.
- Take a baseline TDS reading if changing the membrane. Measure product water TDS before opening anything, for comparison against the reading taken after the new membrane is flushed in.
- Open the housings and remove old cartridges. Use the housing wrench to loosen each sump, discard the spent prefilters, and set the membrane aside separately if it is also due.
- Sanitize the empty housings. Add a small measured amount of unscented bleach or a dedicated sanitizer to the empty housings per the system's manual, following the specified contact time.
- Install new prefilters and flush them alone. Seat the new sediment and carbon cartridges, reconnect only the prefilter stage to the drain or a bucket, and run feed water through it until it runs clear, without the membrane housing connected yet.
- Install the membrane and reconnect the system. Seat the new or existing membrane, reconnect the full system, and open the feed water supply.
- Refill the tank and take a final TDS reading. Let the tank fill fully, run the faucet, and take a fresh TDS reading on the product water once flow has stabilized, comparing it to the manufacturer's rejection specification.
Parts for a routine service
iSpring F5-75 Reverse Osmosis Filter Replacement Pack, 5 Stage
$55.09A full annual prefilter and membrane set for a five stage system, the maintenance most reverse osmosis owners forget until output drops.
Best for: The yearly service on a five stage system
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FilmTec BW60-1812-75 Reverse Osmosis Membrane, 75 GPD
$31.99A standalone replacement membrane in the widely used 1812 size, useful when only the membrane, not the full set, is due.
Best for: A standalone membrane replacement
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HM Digital TDS-3 Handheld TDS Meter, 0 to 9990 ppm
$14.50The standard handheld meter for confirming membrane rejection before and after a service, at a low enough price that most owners keep one on hand.
Check before buying: A TDS reading tells you how much is dissolved, not what. A low reading is not a safety result and a high reading is not automatically a problem.
Best for: Checking rejection performance at any point in the membrane's life
Check price on AmazonHow we chose
We did not test this equipment in person and we never claim to. Picks are researched from manufacturer specification sheets, published standards and regulatory limits, and the recurring themes in verified owner reviews. Every pick is matched to a real grain capacity, a real micron rating, a real certified claim or a real flow rating rather than to a price bracket, and it is placed in the tier where its published capability actually belongs. We rejected the unbranded flood that dominates several of these search results, because a softener is a pressure vessel on the main line and a filter is the only thing between a contaminant and a glass of water, and neither is a place for a product with no parts channel and no service path. Where a figure is trade convention rather than a published standard we label it that way, and where a manufacturer does not publish a figure we leave it out instead of estimating it. Above all, we do not recommend equipment for a problem nobody has measured.
Frequently asked questions
How often should reverse osmosis filters be changed?
Prefilters, typically a sediment stage and one or two carbon stages, are commonly changed annually. The membrane behind them is built to last considerably longer, commonly a manufacturer rated two to three years, and reaches closer to that figure specifically because the prefilters have been protecting it on schedule rather than left in service too long.
Do I need to depressurize the system before changing filters?
Yes. Close the small ball valve at the top of the storage tank first, then run the RO faucet until flow stops completely. This confirms the housings are no longer under pressure before a wrench touches the sump, which avoids a housing separating unexpectedly with pressurized water still behind it.
Why does my new reverse osmosis water taste odd right after a filter change?
New carbon and sediment cartridges release manufacturing fines and carbon dust for the first several minutes of flow. Flushing the prefilters to drain before reconnecting the membrane, and running the tank through one full fill and drain cycle before drinking, clears this and is a normal, expected part of any filter change.
How do I know if my reverse osmosis membrane still works?
A TDS meter reading on the feed water and again on the product water is the only direct test. A properly functioning membrane rejects a large majority of dissolved solids, and the manufacturer publishes a specific rejection figure for each model. A rejection percentage that has crept down over time points toward a membrane nearing the end of its service life.
What happens if I skip changing the prefilters?
Chlorine attacks the membrane's thin film directly and sediment fouls its surface, and a spent prefilter stops protecting against both long before it visibly clogs. Skipping prefilter changes is the most common reason an expensive membrane fails well before its rated service life, and the reader gets no warning because the system keeps running water the whole time.
Should I sanitize the housings every time I change filters?
Yes, since the housings are already open and empty, which makes it the easiest point in the whole year to do it. A small measured amount of unscented bleach or a dedicated sanitizing solution, following the system manual for concentration and contact time, addresses biofilm buildup that a filter change alone does not remove.
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Researched, not professional advice. This page is compiled from published standards, regulatory limits, manufacturer specifications and owner-review consensus, not hands-on testing. Nothing here diagnoses a water problem or says any product makes water safe to drink. Figures described as a rule of thumb are water treatment trade convention rather than published standards, and they are labeled that way wherever they appear. Test your water before you buy equipment: on city water start with the Consumer Confidence Report your utility publishes each year, and on a private well only a certified laboratory test tells you what you have. Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, they are invisible and tasteless, and they need a laboratory and usually a licensed professional rather than a product. Remember that a certification covers a specific claim under a specific standard, so "NSF certified" with no number attached tells you very little.