A slow leak under a kitchen sink can run for weeks before anyone notices. The cabinet stays closed and dark, and a small drip can soak the base board, swell the particle board, and leave a musty smell. Many St. Louis homes have older cabinets, so a drip that starts at a fitting can reach the floor faster than you expect. A dry paper towel test is a simple way to find moisture that you cannot see.

Why a Paper Towel Test Works

Paper towels show moisture quickly, even when the wetness is only a few drops. Because the test does not depend on a pressure gauge or special equipment, any homeowner can run it. The goal is to find out whether any connection under the sink is wet when the system is idle.

What to Gather Before You Start

  • A stack of dry paper towels
  • A flashlight to see into the back corners of the cabinet
  • A dry, empty space under the sink so you can see any new wet spots
  • A pencil or painter's tape to mark any spot you find

Step by Step

  1. Dry the cabinet floor and wipe each fitting, tube, and valve body with a dry towel.
  2. Lay dry towels under and around the feed line, the storage tank line, the drain line, and the faucet line.
  3. Leave the system idle overnight. Do not run water during the test, and do not change filters while it is running.
  4. In the morning, check the towels for damp spots, color changes, or wrinkled edges.
  5. Inspect each fitting with the flashlight, looking for beads of water, dried mineral rings, or wet tubing ends.

Common Spots to Check

  • Compression nuts at the filter housings, especially right after a filter change
  • The storage tank valve and the tank fitting
  • Push-fit tubing ends where the tube was cut or pushed in again
  • The drain connection where the line meets the drain pipe
  • The faucet base connection at the sink deck

Signs That Point to a Leak

Watch for staining on the cabinet wood, a faint musty smell, a soft or warped drawer bottom, or a white mineral crust around a fitting. Any of these can point to a slow leak even if the towels look dry in one spot. Check the back wall of the cabinet as well, since water often runs along a tube before it drips.

What to Do When You Find Moisture

  1. Turn off the supply valve to the system and let the area dry.
  2. Mark the wet spot so you can find it again after the cabinet dries.
  3. If the fitting is a compression nut, snug it gently with the correct wrench. Do not overtighten, since that can crack the fitting or damage the threads.
  4. If the tubing is cracked, cut back to clean tube and reconnect it, or replace the section with the matching part.
  5. Turn the supply back on, then repeat the overnight towel test with fresh towels.

When to Stop and Get Help

Stop if the moisture comes from the storage tank itself, if you cannot identify which fitting is leaking, or if the leak returns after you tighten or reconnect the part. A licensed plumber in the St. Louis area can look at the connections and tell you whether a part needs replacing. Keep the towels and your notes so you can describe what you saw.

Keeping the Check Routine

Running this test once a month, or after any filter change, takes only a few minutes and makes it easier to catch a problem early. Write the date and the result in your maintenance notes. Over time, a dry cabinet floor and clean towels are the simplest sign that the connections under your sink are holding.