Reverse osmosis systems are assembled from largely standard components, which is good news for anyone replacing one, and the standards have enough variants to catch people out. Identifying what you own before ordering is a ten minute job that avoids the common experience of a part arriving that is almost right.
Membranes and housings
Domestic membranes are described by a size code and a production rating, and the size determines whether the element fits the housing at all. The rating describes production under reference conditions and is not a fit constraint, so a higher rated element of the same size will physically fit but will change the system's balance, particularly the flow restrictor which is matched to production. If you change the membrane rating, the restrictor usually has to change too, or the system will send the wrong proportion to drain.
The storage tank
Tanks are described by nominal capacity, which is larger than the usable volume they actually deliver, and by their connection size. They also carry a factory air charge that has to be checked when empty and set to the specification, since a tank at the wrong pressure delivers poorly and is routinely mistaken for a failing membrane. Physical size matters in a cabinet, and a larger tank needs more production to keep it filled, so upsizing a tank without considering the membrane is a common disappointment.
Valves, restrictors and the small parts
Four small components do most of the work and all of them are system specific. The automatic shut off valve stops production when the tank is full. The check valve prevents backflow from the tank towards the membrane. The flow restrictor sets the reject rate and is matched to the membrane's production. And the feed adapter connects to your particular shut off valve arrangement. Take the old part with you or photograph the markings, since these look similar across ratings that behave differently.
Faucets and tubing
Faucets differ in whether they carry an air gap, which changes the deck hole size and the number of tubes, and in the tubing size they accept. Tubing itself comes in a small number of outside diameters and mixing them is the most common cause of a leak that appears a day later. Measure the tubing rather than judging by eye. Where a system is old and parts are hard to source, standard components will usually replace proprietary ones. Our whole house RO guide covers the same components at larger scale.
Questions people ask about reverse osmosis parts
Can I fit a higher rated membrane?
Physically yes, if the size code matches, but the flow restrictor is matched to production and usually has to change with it.
Why does my new tank deliver so little?
Check its air charge when empty against the specification. Nominal capacity is also larger than the usable volume a tank actually delivers.
Which small parts are system specific?
The shut off valve, check valve, flow restrictor and feed adapter. They look similar across ratings that behave differently, so match the markings.
Does tubing size matter?
Yes. A small number of outside diameters are in use and mixing them causes leaks that appear a day after the work, not immediately.