Plumbing traps keep your home safe by blocking sewer gases from entering your living spaces. However, these parts can lose water over time, leading to unpleasant smells and potential issues. This Old House plumbing and heating expert Richard Trethewey shares common reasons why a trap may lose water and offers solutions using a mock-up of a plumbing system in the video above.
Understanding Plumbing Traps
Plumbing traps are designed to create a water seal that blocks sewer gases from entering a building. These U-shaped pipes are found beneath sinks, toilets, and other plumbing fixtures. There are several types of plumbing traps commonly used in residential settings:
- Bell traps: Found in floor drains and some outdoor applications
- Drum traps: Used in older homes, particularly for bathtubs
- P-traps: The most common type, found under sinks and other fixtures
- S-traps: An older design that is no longer allowed in most states due to potential siphoning issues
The primary function of a plumbing trap is to maintain a water seal that prevents sewer gases from entering the home. This seal is created by a small amount of water that remains in the curved portion of the trap after each use of the fixture.
Common Causes of Water Loss in Plumbing Traps
Several factors can contribute to the loss of water in plumbing traps, potentially compromising their effectiveness.
Improper Venting
One of the most common causes of water loss in plumbing traps is improper venting. Trethewey explains how a lack of proper venting can cause water to be pulled from a toilet bowl when a nearby washing machine drains.
Venting systems allow air to enter the plumbing system, preventing the creation of a vacuum that can siphon water from traps. Without adequate venting, the rapid flow of water from one fixture can create negative pressure and pull water from nearby traps.
Oscillation
Wind blowing across vent pipes on the roof can create air pressure changes within the plumbing system, causing water in traps to bounce. Over time, this oscillation can lead to water loss.
Siphonage
Siphonage is a big problem with S-traps, which are now illegal in most jurisdictions. The long leg of an S-trap can create enough suction to pull water completely out of the trap, leaving it dry and ineffective.
Capillary Action
Capillary action can occur when foreign objects, such as strings or other fibrous materials, become lodged in the trap. These materials can wick water out of the trap over time, eventually leaving it dry.
Evaporation
In dry environments or in fixtures that are rarely used, water in the trap can simply evaporate. This is particularly common in vacation homes or seldom-used guest bathrooms.
Identifying Water Loss in Plumbing Traps
Be aware of these indicators that your plumbing traps may be losing water:
- Gurgling sounds when water drains from nearby fixtures
- Slow draining or bubbling in sinks or toilets
- Unpleasant sewer odors near drains or fixtures
- Visible lack of water in the toilet bowl
Homeowners can perform simple inspections to check for water loss in traps. Listen for gurgling sounds when running water in nearby fixtures. Pour water into seldom-used drains and observe if it remains visible. Remove the cover from floor drains and check for visible water.
For persistent issues or if you’re unsure about the state of your plumbing traps, consider hiring a professional plumber. They can perform a thorough inspection and recommend appropriate solutions.
Solutions for Water Loss in Plumbing Traps
Addressing water loss in plumbing traps involves identifying the root cause and then implementing appropriate solutions.
Improving Ventilation
Proper ventilation is important for maintaining water seals in plumbing traps. Solutions may include the following:
- Clearing blocked vents: Over time, vents can become clogged with debris like leaves, dirt, or nests, which can restrict airflow. Clearing these blockages ensures air circulation within the plumbing system, maintaining the necessary pressure balance to keep water seals intact in traps.
- Installing additional vent pipes: In homes with extensive or complex plumbing systems, adding more vent pipes can help improve overall airflow. This enhances the system’s ability to balance pressure and maintain water levels in traps, especially in areas far from the main vent.
- Adding air admittance valves in areas where traditional venting is difficult: An air admittance valve (AAV) allows air to enter the plumbing system without needing a full vent pipe to the roof. It opens when negative pressure occurs in the pipes, preventing water from siphoning out of traps while keeping sewer gases from escaping. This is a good solution for spaces where installing traditional vent pipes would be challenging or costly.
Trethewey shows how an air admittance valve can allow air into the system without letting sewer gases escape, solving venting issues in problematic areas.
Replacing S-traps
If your home has S-traps, replacing them with properly vented P-traps can eliminate siphonage issues. This may require the services of a licensed plumber to ensure proper installation and compliance with local codes.
Regular Maintenance
Regular maintenance can prevent many issues that lead to water loss in traps. Inspect and clean vent pipes for proper airflow, and periodically run the water in seldom-used fixtures to replenish trap seals. You should also remove and clean traps to clear any debris that might cause capillary action.
Addressing Evaporation
For fixtures that are rarely used, consider these solutions:
- Install trap primers that automatically add small amounts of water to maintain the seal.
- Regularly add water to the traps.
- Use non-toxic antifreeze in traps of seasonal properties during winter months.
