A leak under a treatment system rarely starts as a puddle. It starts as a damp spot the size of a coin, a cabinet floor that feels soft when you press it, or a faint white line on a fitting that you might take for dust. The cabinet under the sink is a poor place for a slow leak to hide, because water soaks into particleboard and wood long before anyone opens the door. This guide covers where leaks usually start, how to find them while they are still small, and what to ask before anyone tells you the answer is more equipment.

Where the water usually gets out

Most leaks come from a short list of places. Check each one before you assume the unit itself has failed.

Filter housing bowls. Many cartridge housings seal with a round O-ring where the bowl meets the head. If that O-ring is dry, nicked, or out of its groove, water weeps at the seam, often only while the line is pressurized. Grit caught on the seal when the bowl was closed can cause the same problem.

Push-fit and compression fittings. Tubing that is not fully seated in a quick-connect collet will work loose over time. A compression nut that was tightened once and never rechecked can drip slowly at the ferrule. Tubing that was cut at an angle or kinked near the fitting is a common cause of drips that start after someone moves the cabinet contents.

Storage tank and tank valve. Tanks sit low and get bumped. The tank valve, the tank connection, and the tubing that runs from the tank each have their own seal.

Drain lines. Reverse osmosis and softener drain lines discharge water on every cycle. A drain tube that slipped off its fitting, or a clamp that loosened, will spill into the cabinet while the rest of the system appears normal.

Supply and bypass connections. The lines feeding the unit and any bypass valve assembly are plumbing joints and deserve the same inspection as everything else.

Early signs you can see or smell

Look for a dark ring or damp patch on the cabinet floor or the bottom of the back panel. Check for a white or light crust on a fitting or housing edge. That crust is mineral left behind as drips evaporate, so it tells you water has been escaping at that spot for some time. Notice any musty or wet-wood smell, a cabinet floor or panel that has swollen, flaked, or feels soft, a drip pan or towel that is wet when you check it, water beaded on tubing near a joint, and a cabinet door that sticks or a floor edge that has lifted.

Find the leak in about ten minutes

First, empty the cabinet floor and dry it completely with a towel. Lay strips of paper towel under each housing, fitting, tank connection, and drain line. Second, mark the edge of any wet area with a pencil line or a strip of painter's tape and write the time beside it, so you can tell later whether the area is growing. Third, run the system through normal use for a set period, then check the towels and the floor. Some leaks appear only under pressure, so close the faucet and check again after the pressure drops. Fourth, if the unit has a drain cycle, watch the drain line through a full cycle. Finally, photograph the wet spots from above and from the front so you can compare them at the next check.

Tell a seal leak from a fitting leak

A housing seal leak usually appears at the seam between the bowl and the head and stops when the water supply is closed. A fitting leak appears at a joint, often at the collet or nut, and may drip even when the system is idle if the line is still under pressure. To test tubing, push it in until it bottoms, then pull gently. If it moves, the collet is not gripping. Cut back a short length square with a sharp tool, check for kinks, and reinsert it fully.

Tighten housings until they are snug and stop. Plastic bowls crack under force, and a cracked bowl is a bigger problem than a worn O-ring. Unless the manufacturer's instructions say otherwise, do not use more force than a firm hand grip.

Shut off before you open anything

Close the inlet valve before you open any housing or disconnect any tubing. Then open a nearby faucet to bleed off the pressure. Expect some residual water to come out of a housing even after that step, and keep towels under the work area.

What to ask before you accept a bigger fix

When a provider proposes replacing the whole system because of a drip, ask which specific part failed and whether that part is the leak point. Ask whether the leak was checked at closeout after the last service visit. Ask who owns the warranty on fittings and seals, and whether that warranty covers water damage to the cabinet. Those are often separate questions from whether the part itself is covered. Ask who will check the same connection at the next visit, and whether that check is written into the service record.

When to stop and call a plumber

If the leak is at the main supply valve, at a joint you cannot isolate with the inlet closed, or somewhere that water keeps coming back after you have reseated and dried everything, stop and bring in a plumber. Stop as well if the cabinet floor is soft underfoot, the damage has reached the wall or the subfloor, or you see water near an electrical outlet or cord. Keep your paper-towel notes and photographs with the service records, since they show when the problem first appeared and where it was.