How to Fix a Kitchen Sink P-Trap That Keeps Losing Water: Causes & Easy Solutions
You smell something foul coming from your kitchen sink, or you hear gurgling sounds when you run the water. If the P-trap keeps losing its water seal, sewer gases can enter your home—and the problem is usually simple to diagnose.
TL;DR
A P-trap that keeps losing water is typically caused by siphonage from improper venting, evaporation from infrequent use, or a slow leak. The water in the P-trap is your home’s first defense against sewer gas—when it disappears, odors follow. This guide explains how to tell the difference between evaporation, siphoning, and leaks, and offers simple fixes you can try before calling a plumber.
Key Takeaways
- The P-trap holds water to block sewer gases—if it keeps going dry, something is breaking that seal .
- Siphonage is the most common culprit—water rushing down the pipe can suck the trap dry if the venting is wrong .
- Evaporation happens in unused sinks—if the kitchen sink isn’t used for weeks, the water can simply dry up .
- A slow leak can drain the trap without you noticing, especially if connections are loose .
- Improper installation, like an S-trap or flex pipe, can cause repeated siphonage .
The Real Reason Your P-Trap Keeps Losing Water
The P-trap under your kitchen sink isn’t just a pipe—it’s a water seal. The curve holds about one and a half to two inches of water that blocks sewer gases from traveling up the drain. When that water disappears, gases flow through, bringing unpleasant odors and potentially harmful fumes.
1. Siphonage (The Most Common Cause)
This is when the trap gets “sucked dry” by water rushing through the drain . When you drain a full sink of water, the falling water creates a vacuum effect in the pipe. If the system isn’t properly vented, that vacuum can pull the water right out of the P-trap along with the wastewater .
What poor venting looks like:
- The drain pipe connects to the wall at a weird angle
- The trap arm drops downward instead of going horizontal
- An S-trap setup (trap drops straight down) instead of a P-trap (trap goes horizontal to a vent)
- A flexible “accordion” pipe that disrupts proper flow
If the sink gurgles when water drains or the odor appears after doing dishes, siphonage is likely the issue .
2. Evaporation
Every P-trap loses water over time through evaporation . A kitchen sink used daily refills automatically. But if you’ve been away for a few weeks or the sink rarely gets used, the water can simply disappear. In dry climates, this can happen faster .
3. A Leak
Not all leaks are obvious. A slow drip from a loose slip-joint nut or a worn washer can drain the trap’s water seal over time . Even if the trap doesn’t leave a puddle on the cabinet floor, a slow leak can be enough to empty it.
4. Incorrect Installation
Sometimes the trap itself is the problem. The water seal has to be the right depth—typically 2 inches or 75mm . If the trap was installed with too shallow a curve, it may not hold enough water to maintain the seal. Also, misaligned pipes can create stress that leads to persistent small leaks .
Step-by-Step Fix Guide
Follow these steps in order. Most P-trap issues are easy to fix yourself.
Step 1: Refill the Trap and Test
Start with the simplest test.
- Run hot water down the sink for 1–2 minutes to refill the trap.
- Check the sink daily for a few days.
- If the smell returns within 24 hours: the issue is likely siphonage or a leak.
- If it takes a week or more: evaporation is probably the cause.
Step 2: Check for Leaks
Loose connections or worn washers are common and cheap to fix .
- Place a dry paper towel under the P-trap connections and leave it overnight.
- Check for moisture the next morning. If you see any wet spots, you have a leak.
- Tighten slip-joint nuts hand-tight, then snug them gently with pliers—don’t overtighten plastic pipes or they can crack .
- If the leak persists, replace the washers (nylon or rubber rings inside the connections) .
Safety reminder: Place a bucket under the trap before loosening any nuts—the trap will have water in it, and it’s rarely clean water.
Step 3: Check the Venting Configuration
If there’s no leak, siphonage is the most likely cause.
Look under the sink at the trap arm (the horizontal pipe leaving the trap toward the wall). If it slopes straight down into the wall instead of running horizontally, you may have an S-trap configuration . S-traps aren’t up to modern plumbing codes because they siphon themselves dry .
Flex pipes (accordion-style plastic hoses) can also create siphoning because they don’t maintain proper flow .
If the configuration looks wrong, the fix may involve re-plumbing with a proper P-trap that connects horizontally to a vented drain line. This is often a job for a plumber .
Step 4: Consider an Anti-Siphon Trap
If you can’t change the venting setup easily, an anti-siphon P-trap can help . These have a small valve that opens when negative pressure builds, letting air into the system and preventing the trap from being sucked dry .
Step 5: Restore a Dry Trap (Ongoing Prevention)
If the issue is just evaporation from infrequent use:
- Run water down the sink for a minute once a week to refill the trap .
- Add a small amount of mineral oil to the drain—this forms a thin layer on top of the water and slows evaporation .
P-Trap Water Loss Causes Comparison
| Cause | What It Feels Like | Diagnostic Test | DIY Fix |
|---|---|---|---|
| Siphonage (venting issue) | Odor appears after draining a full sink; gurgling sounds | Check if trap arm runs horizontal to a vent or drops down | Install anti-siphon trap; fix S-trap; call plumber for venting correction |
| Evaporation | Sink unused for weeks; odor returns slowly | Refill trap; smell returns after 7+ days | Run water weekly; add mineral oil to slow evaporation |
| Slow leak | Damp spots under the sink; moldy smell | Place paper towel under connections overnight | Tighten slip-joint nuts; replace worn washers |
| Incorrect trap | Persistent odors; trap installed with shallow curve or flex pipe | Inspect trap; check if it’s an S-trap or too shallow | Replace with proper P-trap; call plumber for wall connection issues |
Frequently Asked Questions (FAQ)
Why does my kitchen sink smell even though I keep it clean?
The most likely cause is a dry P-trap. The water seal has been lost, allowing sewer gases to escape into your kitchen .
How do I know if my P-trap is being siphoned dry?
If the smell returns after running a full sink of water, or if you hear gurgling sounds when water drains, siphoning is likely the issue .
Can a kinked or flexible pipe cause P-trap problems?
Yes. Accordion-style flex pipes can disrupt proper water flow and contribute to siphoning . They can also trap food debris, making clogs more likely.
Is an S-trap the same as a P-trap?
No. An S-trap drops straight down after the curve and can siphon itself dry. A P-trap runs horizontally to a vented connection, which protects the water seal. S-traps are no longer up to modern plumbing codes .
Why does my sink smell a week after I moved into a new place?
If the sink wasn’t used during the moving process, the trap may have dried out from evaporation. Run water for a minute to refill it .
When should I call a plumber?
If you’ve checked for leaks, refilled the trap, and the problem keeps returning, the issue may be improper venting or incorrect installation. A plumber can inspect and correct the configuration .
References
Expert Plumbing Advice & Homeowner Solutions for P-Trap Problems:
- HomeTips – How to Fix or Replace a Leaky Sink Trap
- Angi – What Is a P-Trap? Complete Guide
- Jaspector – P-Traps and S-Traps Explained
- Reliance Plumbing – P-Trap Guide
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