RO filter housing sizes: 10 inch, 20 inch, slim, big blue, and why the cartridge that looks right does not seal.

The four housing sizes

Domestic filter housings come in two diameters and two lengths, and the names are loose. What the trade calls a standard or slim housing takes a cartridge roughly two and a half inches across. What it calls big blue or jumbo takes one roughly four and a half inches across. Each comes in a 10-inch and a 20-inch length. So there are four sizes, and the number on the box only tells you one of the two dimensions.

• Slim 10-inch. The housing inside almost every domestic RO purifier in India, and the common wall-mounted pre-filter bowl. Every LUXGEN cartridge is this size. The Universal-Fit sediment cartridge is the plain example.
• Slim 20-inch. Same diameter, twice the length, for higher flow or longer intervals. Rare inside a purifier, occasionally used as a pre-filter. A 10-inch cartridge does not fit it, and two stacked do not seal.
• Big blue 10-inch and 20-inch. The fat housings on whole-house lines and softener inlets. Far more media, far more flow. Nothing on this site fits them; if your pre-filter is one of these, the cartridge you need is a different product.

The first check, before anything else on this page: measure the cartridge you took out, across and along. If it is about two and a half inches across and about ten long, keep reading. If not, this site's cartridges are the wrong size, however right they look.

The three threads that matter

A housing has three threaded joints, and each one causes its own returns.

The bowl-to-cap thread. Coarse, plastic, and the one you undo with the spanner. It is not a seal on its own; the seal is an O-ring in the cap that the thread compresses. A stripped or cross-threaded bowl will not hold pressure whatever cartridge is inside. When that happens, replace the housing rather than the cartridge: the housing bowl kit at ₹499 or the kit with two candles at ₹899.

The inlet and outlet ports. The two connections on the cap. On domestic purifiers these take quarter-inch push-fit tubing, which is what both LUXGEN kits ship with, and on some older bowls a threaded elbow instead. Check the tubing you have before buying a housing; a quarter-inch fitting on three-eighths tubing is the second most common mismatch.

The candle thread in the cap. The one most people do not know exists. Some 10-inch pre-filter caps have a female thread in the centre, and the cartridge screws into it. That housing takes a 9-inch threaded candle such as the LUX-YW-9, not a 10-inch cartridge. A plain cap, with no thread, takes the full-length 10-inch cartridge and holds it by pressing down. The fit guide has the pictures and the rule: check the cap, not the bowl.

Why a cartridge that looks right does not seal

Two cartridges can be the same length, the same diameter and the same media and still one seals and the other does not. The seal happens at the ends. A standard cartridge is open at both ends and seals on flat gaskets that the cap and the bowl press against. A cartridge cut for a brand housing seals on ends shaped to that housing, sometimes with a different gasket, sometimes with a spigot. Put a cartridge with the wrong ends in a housing and one of three things happens.

1. It is a few millimetres too long. The cap will not close, or it closes on the thread and strips it.
2. It is a few millimetres too short. The cap closes, nothing leaks, and water goes around the cartridge instead of through it. This is the worst case, because the purifier runs as if it were filtering. Silt lands on the carbon block and then on the membrane, and the ₹190 cartridge that never sealed costs a ₹650 to ₹900 membrane.
3. The ends do not match the seat. Right length, wrong gasket or a missing spigot. Same result as too short.

That is the whole reason LUXGEN sells its 10-inch sediment and carbon cartridges in two fits. Branded-Fit is cut for the housings the large purifier brands use. Universal-Fit is the standard 10-inch cartridge for everything else. Same 5 micron media, same 125 PSI rating, same rated life, different ends.

How to identify your housing in two minutes

1. Close the inlet tap and open the purifier's tap to release pressure. Unplug the machine.
2. Unscrew the bowl. Measure the old cartridge across and along. Two and a half by ten inches means this site's size.
3. Look inside the cap. A threaded socket in the centre means a 9-inch candle. A plain cap means a 10-inch cartridge.
4. Look at the ends of the old cartridge. Flat and open at both ends means Universal-Fit. Shaped, capped or spigoted ends, on a cartridge that came with a branded machine, means Branded-Fit.
5. Look at the port fittings. Push-fit collars on quarter-inch tube, or threaded elbows. Note which.

Write the answers on the housing with a marker. Every later order takes thirty seconds. If the bowl is cracked or the thread is gone, skip the cartridge and buy the housing; both LUXGEN kits are Universal-Fit with quarter-inch push-fit ports, one with a threaded cap and a 9-inch candle, one with a plain cap and two 10-inch candles. The installation checklist covers fitting either.

What fits what, in one table

The intervals for each cartridge are on the filter change schedule, and which sediment media suits your water is on the spun vs wound guide.