If your reverse osmosis unit in St. Louis has slowed down, makes less water than it used to, or starts spitting air at the faucet, the cause is often something simple under the sink: a kinked or pinched length of plastic tubing. This guide walks through how to check the tubing yourself, what to look for, and when to call a plumber. The check is visual and hands-on, and it requires no parts to start.

What the tubing does in your setup

A typical under-sink reverse osmosis system uses several thin plastic lines. Water comes in from the cold supply, moves through the filters and storage tank, and exits to the faucet. A separate drain line carries rejected water away. Each line is a flexible tube pushed onto a fitting with a collar that grips it. Because the tubing is thin and sits in a crowded cabinet, it can bend sharply when the cabinet door closes, when a trash bin is moved, or when someone reaches in to change a filter.

Signs a tube may be kinked

  • The faucet fills slowly even though the storage tank seems full.
  • The faucet sputters or runs air for a short time after you open it.
  • The storage tank takes a long time to refill after a few glasses of water.
  • The unit makes gurgling or hissing sounds that change when you move the cabinet door.
  • Water beads or drips near a fitting after a tube has been moved.

None of these signs proves a kink by itself. Filter clogs, low supply pressure, and a worn membrane can cause similar symptoms. A tubing check is a good first step because it is quick and does not involve replacing anything.

Before you start

Turn off the cold water supply valve under the sink first. Then open the faucet to relieve pressure. Place a towel and a shallow pan below the fittings, since a little water will drain out when you disconnect anything. Use a flashlight, because many older homes in St. Louis have dim, crowded cabinets. If your unit has a pressure tank, do not press on its valve stem while you work.

A step-by-step tubing check

  1. Pull items forward in the cabinet so you can see every tube from the supply valve to the unit, and from the unit to the faucet.
  2. Follow each tube with your hand, feeling for flat spots, sharp folds, or places where the tube crosses a metal edge.
  3. Look at each spot where a tube enters a fitting. A tube pushed in crooked, or pulled partway out, can restrict flow.
  4. Check the drain line for sags. A drain tube that dips below the unit and then rises again can trap water, so keep the path as smooth and downward as the cabinet allows.
  5. Gently straighten any bends by hand. Do not use pliers, a knife, or heat on the tubing.
  6. Re-route tubes away from sharp cabinet edges, hinges, and the back panel where the door can hit them. Use the clips or cable ties the unit already has, and keep ties snug without pinching the tube.
  7. Reconnect the water supply slowly, then watch each fitting for drips for a short while.

Tubing you should not force

Plastic tubing can crack when it is cold or old. Cold St. Louis winters can stiffen a tube, and a cabinet on an exterior wall may feel colder than one on an interior wall. If a tube cracks, splits, or feels brittle when you move it, do not try to patch it with tape. Shut the supply valve and replace the tube with the same size and type the unit uses. Check the unit's manual or the marking on the fitting for the correct size, and bring the old piece with you if you are unsure.

When to call a plumber

Call a licensed plumber if a fitting still leaks after you reseat it, if a collar will not hold a tube, or if the supply valve is stiff or drips. A plumber can also check for a restriction further upstream in the cold supply line, which a cabinet check cannot see. Ask for a plain description of what was found and what was changed.

Keeping tubing clear over time

Once the tubing is straight, keep it that way. Avoid stacking cleaning supplies or bins against the lines. When you change filters, lift the tubing gently rather than pulling on it. Write the date of your check in a notebook or on the cabinet wall so you can compare after future filter changes. Looking over the tubes during every filter change helps catch small problems early.

A quick St. Louis cabinet note

Many St. Louis kitchens have tight under-sink spaces with pipes running through the same cabinet. When you finish, close the door gently and listen to the unit for a minute. If the sound changes as the door swings, the tubing may still be touching a cabinet edge, and a small adjustment will usually fix it.