Almost all of the plumbing in a reverse osmosis system is small diameter plastic tubing and push fittings, which is why the work is within reach of a homeowner and why the failures are so consistent. Nearly every leak in these systems is a tubing issue rather than a component one, and nearly all of them are avoidable with three habits.
Sizes and the colour convention
Domestic systems use a small number of outside diameters, and mixing them is the single most common cause of a joint that leaks a day after the work. Measure rather than judging by eye, since two of the common sizes are close enough to look alike. Colour is a convention rather than a standard: many installers use one colour for feed, another for product and another for the drain line, which makes a cabinet far easier to understand years later, but do not rely on colour to tell you the size.
Cut it square
A push fitting seals on the outside of the tube against an o-ring, and a cut that is angled or pinched will not seal evenly however far it is pushed in. Use a proper tubing cutter rather than scissors or a knife, which crush the tube and leave a lip. Trim back past any kink rather than trying to straighten it, since a kinked section holds its deformation. And do not sand or chamfer the end, which removes the smooth surface the o-ring needs.
Seat the fitting fully
Push the tube in until it stops, which is further than it feels, then pull back gently: a properly seated tube holds. A tube that has been pushed in only until it grips will hold at low pressure and release later, which is precisely the leak found the next morning. Where a fitting has a collet lock or a clip, fit it, because it prevents the collet being nudged and the tube backing out. Support long runs so nothing hangs on a fitting.
Routing and rework
Keep runs short and direct with gentle bends, since tight bends restrict flow and stress fittings, and leave slack where an appliance has to be pulled out. When removing a tube, press the collet in evenly while pulling the tube out rather than levering it, and inspect the end before reusing it: a tube that has been gripped once has marks on it and is better trimmed back a short length before refitting. Our RO installation guide covers the layout as a whole.
Questions people ask about ro tubing
Does tubing size matter?
Yes. A few common outside diameters are in use and they look alike. Mixing them causes the leak that appears a day later.
Why does my joint leak after a day?
Usually a tube not pushed fully home, or a cut that was angled or crushed so the o-ring cannot seal evenly.
What should I cut tubing with?
A proper tubing cutter. Scissors and knives crush the tube and leave a lip that prevents a clean seal.
Can I reuse a length of tubing?
Usually, after trimming back a short length past the marks left where the collet gripped it.