In short
- Tubing is sized by its outside diameter, and a push-in fitting must match it. Two common sizes are 1/4 inch and 3/8 inch.
- A square tube cut and a tube pushed in all the way to its stop help prevent push-in leaks.
- A check valve has a direction arrow, and it must point the way the water flows.
- Match the flow restrictor to the system maker's instructions; do not swap one for another size.
- After any change, turn the water on slowly and check each joint with a dry tissue, then look again later.
A reverse osmosis system is not only filters and a membrane. Between them run lengths of tubing, joined by small fittings and controlled by a few small valves. They are easy to ignore, but they are where a drip would show up, and you handle them every time you change a filter set. This guide covers tube sizes, threads and thread tape, how push-in fittings work, what the common valves do and how to test your work for leaks. Your system's manual comes first; the steps here are general habits.
Tube sizes: measure the outside
Tubing is described by its outside diameter, the width of the tube measured across the outside, not the opening inside. A push-in fitting grips the outside of the tube, so the two sizes must match. Two common sizes are 1/4 inch and 3/8 inch. Metric tubing exists too, and its sizes are close to the inch sizes but not identical, so do not mix them. Many tubes have their size printed along the side, and the manual says which size each line of your system uses. If you are not sure, measure the outside of a tube with calipers before you order Tubing & Connectors.
A fitting has more than one end, and each end has its own size. A fitting for a 1/4 inch tube can have a thread that is a different size, so read both numbers on the product page.
Threads and thread tape
Where a fitting screws into a part, the thread matters. A common type in the United States is NPT, which stands for national pipe taper. The thread is tapered, so it tightens and seals as it goes in, and it is usually sealed with thread tape. The number in a thread size, such as 1/4 NPT, is a nominal size, so you cannot read it off a ruler. Look it up on the part or the product page.
- Use tape made for plumbing threads.
- Look at the end of the threaded part and wrap the tape clockwise, so that screwing the part in tightens the tape instead of unwrapping it. A few turns are usually enough.
- Keep the tape off the very first thread so that no scrap can end up inside the fitting.
- Some fittings seal with an O-ring or a flat gasket instead. Follow the instructions for those.
- Tighten as the instructions say. Plastic threads can crack if you force them.
How a push-in fitting holds
A quick-connect fitting has a ring of small teeth inside, often called a collet, and an O-ring. When you push a tube in, the O-ring seals around the outside of the tube and the collet grips it. The harder you pull the tube, the tighter the collet holds. To take a tube out, you first turn off the water and release the pressure. Then you press the collet ring in toward the fitting and pull the tube out while you hold it in. Some designs add a clip or a locking piece, so read the instructions that come with the fitting. See Quick-Connect Fittings for the range.
Cut it square and push it in all the way
The tube end is a common cause of push-in leaks. Use a tube cutter or a sharp blade and cut straight across at a right angle. Do not use scissors that crush the tube. Check that the end is round and free of burrs, and that the outside is not scratched, because the O-ring seals against that surface. Then push the tube in until it stops. This is farther than feels natural the first time. Give it a gentle tug to confirm that it is held. If you re-use a tube, cut off the end that was gripped before.
Valves, restrictors and gauges
Check valves. The arrow on the body shows the direction water is meant to travel. Fitted backward, the valve blocks the flow or does not do its job. Check the arrow against the manual before you push the tubes in.
Saddle (drain) valves. The waste line of an RO system has to go somewhere, and a drain saddle clamps around the sink's drain pipe so the line can connect to it. Many are fitted by making a small hole in the drain pipe, as the instructions describe. Local plumbing rules can apply to how a drain is connected, so ask a licensed plumber if you are unsure.
Shut-off valves. A shut-off valve stops the water so you can work. Many are ball valves that turn a quarter turn, usually with the handle along the pipe when open and across the pipe when closed. Check how yours behaves, and learn where it is before you need it. See Valves.
Flow restrictors. A flow restrictor is a small part in the drain line. It controls how fast water runs to drain while the system is working, and the system maker chooses its size to suit the design. Match it to the instructions. A different size changes how the system behaves, so do not swap it or remove it unless the maker says to.
Pressure gauges. A gauge lets you watch pressure at a point in the system. Note the reading when everything is new and the system is full, and glance at it from time to time. A change from your own baseline is a reason to look at the manual, not a diagnosis by itself, and a gauge shows pressure, not the quality of the water. The manual says which readings to expect.
Testing for drips after a change
- Before you start, turn off the water supply and release the pressure as the instructions say, and put a towel and a container under the work area.
- Turn the water back on slowly, not all at once.
- Dry every joint, then touch each one with a dry tissue. A damp spot shows a leak you cannot yet see.
- A storage tank takes time to fill, so look again once it has, and again some hours later or the next day.
- If a joint drips, turn off the water, release the pressure, and re-cut and reseat the tube, or re-do the tape on a threaded joint. Do not overtighten threaded parts.
Do not ignore a slow drip in a cabinet, because water damage can build up out of sight. If you are unsure about any step, ask a licensed plumber.
Before you order
Write down the tube size of each line, the thread size at each connection and where each valve goes, and compare them with each product page. The Installing & maintaining view brings the related parts together. If you also refill carbon or other loose media, see Activated carbon and other filter media: what they are and how to handle them. If your question is about your water rather than your plumbing, contact your local water utility (most publish an annual water quality report), a certified testing laboratory or a health professional.