Guides · Buying
Buying a Used RO System
What to inspect, what to replace on day one, and when a used system costs more than a new one.
When used makes sense
RO hardware is mostly a housing, a manifold and a membrane. A used unit with an intact manifold and a clean tank can be a genuine bargain, because the parts that wear out are the ones you replace anyway.
It makes the most sense when you can see it running, or when it is a model whose cartridges are still sold. It makes the least sense when the model is discontinued and the cartridges are unobtainable.
Inspect before you pay
Manifold and fittings: look for cracks, cross-threading, and white mineral crust around joints. Cracks in a manifold housing usually mean the unit is done.
Pressure tank: tap it. A tank that feels waterlogged or makes no hollow sound when empty has a failed bladder and must be replaced.
Tubing and connectors: stiff, cloudy or kinked tubing is cheap to replace; push-fit connectors that have been repeatedly reused are not trustworthy.
Pump and transformer, on tankless or boosted units: ask to see it run, and check that the pump is quiet and the power supply is the original.
Replace on day one
Every cartridge: sediment, carbon pre-filter, post-carbon. They are the cheapest part of the system and the only way to know the history.
The membrane, unless the seller can show a recent TDS reading that matches a healthy rejection rate. A membrane of unknown age is a membrane of unknown performance.
All O-rings and the tank bladder if there is any doubt. Both are inexpensive and both cause the leaks people blame on the housing.
Sanitise the tank and lines per the manufacturer's procedure if the unit sat unused.
The arithmetic
Add the purchase price to the cost of a full cartridge set, a membrane, and any tank or fitting replacement. Compare that total to a new unit of the same class, including a warranty.
If the used price plus parts lands within about a third of new, buy new — you get a warranty, known history and current cartridge availability.
If the model is discontinued, factor in the risk that you cannot buy cartridges next year at all.
Installation cautions
Test every joint with a dry tissue for a full ten minutes after installation, and again after a day.
Confirm the drain connection has a proper air gap. A used unit often arrives without the drain fittings the original had.
Check that the feed adaptor matches your shut-off valve before you cut or connect anything.
FAQ
Quick answers
+Can I reuse the old membrane if it still works?
You can, if the seller can demonstrate a healthy rejection ratio with a TDS meter. Without that evidence, assume it needs replacing.
+Are used RO systems worth it?
Sometimes, especially for a current model with cheap cartridges. Run the arithmetic including a full cartridge set and a membrane before deciding.
+What is the biggest risk?
A discontinued model with no cartridges available, followed by a cracked manifold. Both are hard to see in a photo, so inspect in person or buy new.