Standing water after filter install

Quick Answer:

If you find water pooling under the sink right after installing a new filter, don’t ignore it. First confirm the filter’s drain line is routed to a proper waste connection (not into a floor drain), then check fittings, the trap, and any check valve or air gap per the manufacturer. Run the unit briefly while watching where the water goes so you can see the problem source.

Why This Happens

New filter systems add a drain line and a few extra fittings near plumbing cavities. Common causes for water collecting on the floor include a drain line routed incorrectly, loose or cross-threaded fittings that leak into cabinet or floor cavities, debris left in a trap that blocks flow, or a missing/misinstalled air gap or check valve that allows backflow. Packaging, cloth, or small parts pushed into the trap during installation can also hold water and cause overflow.

Step-by-Step What to Do

1. Shut off and dry the area

Turn off the filter feed or the cold-water supply to the unit.
Use towels to soak up standing water so you can see new leaks clearly.

2. Confirm the drain line routing

Follow the filter’s discharge hose from the unit to its termination point. It should go to a proper waste connection (a sink drain tailpiece, dedicated waste inlet, or an approved air gap) — not into a floor drain or open cavity.
If the line is shoved into a floor drain or left loose, reposition it to the correct waste connection per the manufacturer’s guidance.

3. Inspect fittings and hidden spaces

Check all visible fittings on supply and drain lines for hand-tight connections and any signs of water. Look behind the cabinet backer and in floor cavities if accessible.
Don’t assume tiny fittings can’t leak; even a slow drip into a cabinet floor or wall cavity will pool and cause immediate standing water in finished areas.

4. Run the filtered water while watching the path

Turn the water back on and briefly run the filter while observing the entire drain path and all fittings. Watch the trap and the cabinet bottom for new drips.
If water appears only when the filter runs, the drain tie-in or outlet connection is likely the source.

5. Check the trap for debris

Remove the trap and look inside for packaging scraps, cloth pieces, or other installation debris that could block flow and force water out of the wrong place.
Clear the trap, reassemble it hand-tight plus a slight wrench turn where needed, then retest.

6. Verify check valves and air gaps

Confirm any check valve or air gap supplied with the filter is installed exactly as the manufacturer instructs. A reversed or missing check valve can allow backflow; an improper air gap can let water escape or siphon back.
If instructions aren’t clear, stop and contact the manufacturer or a plumber rather than guessing.

What Not to Do

Do not route the filter discharge directly into a floor drain or open floor cavity.
Do not assume small fittings or plastic inserts can’t leak — check them carefully.
Do not ignore leaks that enter finished areas. If you find water running into cabinets, walls, or flooring, call a professional promptly rather than trying major repairs yourself.
Do not guess how a check valve or air gap should be oriented — if the discharge fittings are unclear, get professional help.

When to Call a Professional

If water is entering finished floors, walls, or cabinetry and you can’t stop it immediately.
If the discharge fitting or the required waste connection is unclear or inaccessible.
If there’s persistent standing water immediately after installation and you can’t find the source with the steps above.
If you suspect hidden damage, contaminated water, or need parts that require soldering or structural access.
If you want someone to inspect the work and prevent repeat problems—this is a good time to have a pro check fittings and code compliance.

For similar situations after other work, see Standing water after plumbing repair for comparison and next steps you can expect from a plumber.

Safety Notes

Turn off the water supply before disconnecting hoses or traps.
If the cabinet or floor is soaked, cut power to nearby outlets before working in the area to avoid electrical risk.
Wear gloves when handling debris in a trap — it may be contaminated.
Don’t open or alter electrical devices or wiring under the sink; call an electrician if wiring is wet or exposed.

Common Homeowner Questions

Q: How long should I run the filter while watching for leaks?
A: Run it for a minute or two — long enough to see discharge behavior and any drips form at connections.

Q: What if I find packaging stuck in the trap?
A: Remove the trap, clear the debris, and reassemble. Test with a short run. If the trap or drain looks damaged, call a plumber.

Q: Can I just redirect the drain hose into the floor drain to stop pooling?
A: No. Do not route the discharge into a floor drain. Always terminate the discharge to an approved waste connection or air gap per instructions.