Plumbing traps are a critical component of any drain system, designed to prevent sewer gases from entering occupied spaces while allowing wastewater to drain away. However, there are several ways that traps can malfunction or become ineffective over time. Understanding the purpose of traps and what causes them to lose their water seal is key for homeowners trying to diagnose and fix drain-related issues. This article provides an in-depth look at how and why plumbing traps fail.
What is a Plumbing Trap?
A plumbing trap is a curved section of drain pipe designed to hold water and create a seal between the home’s plumbing system and the municipal sewer or septic system. Traps have a drain curve that causes a small amount of water to collect and block gases from rising up through the pipes.
Traps are typically found installed under sinks, tubs, showers, washing machines, floor drains, and other openings that connect to the wastewater drainage system. They may also be installed in internal drainage pipes to prevent sewer gas odors from wafting through vents.
The key purposes of a plumbing trap are:
- To prevent odor, gases, vermin, and other undesirable material from entering the occupied space through the plumbing drainage system.
- To provide proper waste drainage while maintaining the water seal that blocks gases and odors.
- To allow air circulation in the drainage system while blocking gases. Traps have vents after the curve to allow air movement.
- To serve as a collection point and barrier for debris, preventing clogs and jams down the line.
When functioning properly, traps constantly hold water in their curve to form a liquid gas seal. Typical traps hold 2-4 inches of water, providing an effective block for sewer gases. The most common trap types in residential plumbing are P-traps and S-traps.
How Traps Lose Their Water Seal
Plumbing traps depend on having water constantly held in the curved portion to keep sewer gases from passing. If the water seal is lost, it compromises the trap’s effectiveness and allows gases, smells, and vermin to enter living spaces. There are several common ways this water barrier can be breached:
Evaporation
The water held in a trap will eventually evaporate over time if the trap is not used and refilled regularly. Evaporation happens faster in dry environments and seasons. In rarely used drains, the water seal could evaporate within weeks, allowing sewer gases to enter the property.
Siphonage
When a large volume of water flows through the drain, it can siphon off or suck out the water held in the trap curve. This may occur when a toilet flushes or a washing machine pumps out water, temporarily removing the trap seal.
Backpressure
Sudden surges of pressure from the sewer side, like a clog getting cleared, can push past the trap water and allow gases to enter the plumbing system.
Clogs
A significant clog in the drain or vent pipe system can cause an imbalance of air pressure that draws water out of the trap. Tree roots, grease, and other debris are common culprits.
Improper Venting
Inadequate venting after the trap curve will not allow proper air circulation, causing the water to be siphoned off. Venting issues may involve blocked pipes or improper installation.
Material Failure
Over time, the metals or seals on traps can corrode and develop leaks that drain the water seal. Older types of traps and infrequently used drains are more prone to corrosion issues.
Groundwater Infiltration
Groundwater seeping into the drainage system can push past the trap water and allow gases and vermin to enter from the sewer pipes. This usually indicates a problem with cracked drain pipes underground.
Trap Design Issues
Some trap designs, like an S-trap, are prone to losing their water seal more readily than other types. These traps may siphon out with only minor pressure changes.
Signs of a Compromised Trap
Several signs will indicate that a plumbing trap has lost its water barrier and is allowing sewer gases to enter the property:
- Sewer gas smell – Foul odors from the drains like rotten eggs or methane gas are a clear sign sewer gases are entering through dry traps. The smells will be strongest near affected drains.
- Gurgling sounds – Draining water may make bubbling or gurgling noises if gases are escaping through an improperly sealed trap.
- Rodents or pests – A dry trap can allow rats, cockroaches, and other vermin to crawl up through drains and enter the property.
- Overflowing trap – Water dripping or overflowing from a full trap indicates a clogged vent system drawing water out of the trap. This compromises the water seal.
- Fixture drainage issues – Poorly draining sinks, tubs, or toilets may signal venting issues that are breaking the trap seal and causing drainage problems.
- Visible trap water loss – The water level visibly lowering in a toilet or transparent trap is an obvious sign the trap seal has leaked out.
Plumbing vent or drain clogs are often misdiagnosed when an improper trap seal is the real issue. Inspecting and refilling trap water levels can fix many mysterious drainage problems.
How to Fix and Maintain Trap Water Seals
Reestablishing and maintaining the water seal in traps is the key to preventing unpleasant sewer gases from entering living spaces. Here are tips for fixing and keeping functional trap water barriers:
- Refill evaporated traps – Periodically pour water down unused drains to refill evaporated trap water. Add vegetable oil afterward to slow evaporation.
- Fix siphonage issues – Adjust drainage slope, increase pipe sizes, or replace siphoning trap types to prevent water being sucked out.
- Clear vent and drain clogs – Remove debris obstructions so air and water flow freely through the drainage system.
- Check for piping leaks – Inspect for cracks that allow groundwater infiltration past the trap seal. Repipe or seal leaks.
- Update corroded traps – Swap out old corroded galvanized or iron traps with new PVC or ABS plastic models.
- Install trap primers – Automated priming valves can regularly refill seldom-used floor drains to maintain water levels.
- Use trap seal protector valves – For problematic siphoning drains, a special anti-siphon valve can help retain water.
- Replace S-traps – Avoid this inherently faulty design that easily loses its seal, even with minor pressure changes.
- Update venting – Ensure drain vents are properly routed and sized to allow adequate air circulation after the trap.
Regular inspection and maintenance are critical for traps to remain effective and avoid the inconvenience and health hazards of sewer gases entering the home. Calling a professional plumber is recommended if DIY efforts cannot resolve a compromised trap.
FAQs About Lost Trap Seals:
What is the easiest way to tell if my trap seal has leaked out?
The easiest way to check a trap’s water seal is to simply look at the visible water level in sinks and floor drains. If you don’t see a few inches of water held in the curved portion, the trap seal has likely evaporated or leaked out. You may also pour water into a drain and listen for gurgles or bubbles, indicating gases coming up from a dry trap.
How often should trap water be replenished?
In frequently used fixtures like kitchen sinks or showers, the trap water seal gets refilled with regular usage. But old drains or rarely used bathroom sinks may need water added once a month or more. If sewer gas odors return quickly after refilling a trap, a deeper issue like cracked pipes, improper venting, or a faulty trap design may need to be addressed.
Why does my floor drain trap keep losing its water?
Floor drains often run dry because they are used infrequently. Small evaporation and air pressure imbalances can siphon out the trap seal over time. Installing a trap primer valve to automatically refill the trap, or adding vegetable oil to slow evaporation can help floor drain traps retain water longer.
There is a lot of gurgling coming from my bathtub drain. Does this mean the trap has run dry?
Loud gurgling or bubbling from a draining bathtub is often caused by a dry trap allowing sewer gases to escape up through the water flow. Refilling the trap and watching to see if the water level drops quickly again can confirm if a leak in the trap or venting issues are causing the noise and need to be repaired.
Can a small amount of water still allow gases to escape a trap?
Yes, if the water level is too low it may still allow sewer gases to bubble up through the water, even if it hasn’t emptied completely. Each trap type and size requires maintaining 2-4 inches of water to create an effective gas seal. Visually checking water levels and adding more if needed can resolve a weak trap seal.
Why does my trap keep losing its seal even though nothing is wrong with the piping?
Some trap designs are inherently prone to siphoning out and losing water, especially S-traps. Replacing problem traps with better P-trap or bottle trap designs can permanently fix traps that seem to lose their seal through no other identifiable causes.
Conclusion
Plumbing traps provide vital protection by sealing off dangerous and unpleasant sewer gases from entering living spaces. But a myriad of factors from evaporation to clogs can compromise trap water seals over time. Homeowners should inspect trap water levels periodically, watching for any gurgling, odors, or pests that indicate a breach. Refilling evaporated trap seals and making any necessary venting, piping, or trap design improvements can quickly resolve issues of sewer gas odors. If persistent problems continue affecting the trap seal, a professional plumber may be needed to diagnose and repair any deeper drainage system faults allowing the trap water to be siphoned out or leakage past the water barrier. With some vigilance and preventative maintenance, homeowners can ensure their plumbing traps are effectively isolating the home from the sewer’s contaminants.