P-traps are an important part of any plumbing system. Without P-traps, your plumbing would become clogged much more easily and your drains would always smell like sewage. To understand why this is, here is a guide to plumbing P-traps and how they work.

What Are P-Traps?

A P-trap is a special type of pipe found underneath sinks and other drains. On one end of the trap is a U-shaped bend where it connects to the drain. The other end has a long, straight leg where the trap connects to the drain line that carries wastewater into the building’s main vent-waste stack. P-traps are named as such because the bend and leg on the trap somewhat resemble the letter P turned on its side.

The Uniform Plumbing Code states that P-traps are required for every drain line that carries wastewater into the plumbing vent-waste drain system. This means that every sink, shower and floor drain should have a P-trap that connects the drain to the drain line.

However, some older homes don’t have any drain traps or they may use S-traps instead. In either case, it is highly recommended that you hire a plumber to install P-traps to ensure your plumbing system works properly. This is especially true if you have S-traps as the shape of this type of trap prevents it from working as it should.

How P-Traps Help to Prevent Clogged Drain Lines

P-traps are an important part of plumbing systems that serve two main purposes. The first is to trap any solids that go down the drain to prevent them from clogging the drain line or the main vent-waste stack. Any food particles or other solids that go down the drain should get stuck inside the bend on the P-trap instead of flowing out into the drain line. This happens because the force of the water usually isn’t strong enough to carry any solids up through the bend and into the drain line.

Since any solids will remain inside the bend on the trap, the P-trap itself can easily become clogged. If your sink drains slowly or not at all, most likely it means the P-trap is clogged. This is actually a good thing as it is easy to clear a clogged P-trap. It is far more difficult to clear the clog if it occurs further along in the drain system.

If you accidentally lose your wedding ring down the drain, the P-trap ensures that you can easily retrieve it as it will get stuck inside the low bend on the pipe. All you need to do is loosen the fittings at each end to disconnect the P-trap from the drain and drain line. If no P-trap was present, the ring would simply get flushed down through the drain system and most likely be lost forever. Similarly, if you accidentally washed any other large solid particles down the drain, you can take apart the P-trap to remove them so they don’t potentially clog your drain line.

How P-Traps Prevent Sewage Odors

The other primary purpose of a P-trap is to block sewer gases in the drain line from being able to rise back up through the drain. Noxious sewer gases are always present inside all of your drain lines. The vent-waste stack works to exhaust these gases out through your roof. However, if there were no P-traps in the plumbing system, these gases would be constantly vented out through all of your drains instead. This would result not only in nasty odors wafting up from your drains, but could also lead to health issues as some sewer gases are harmful.

The shape of a P-trap is what allows it to block sewer gases inside the drain pipe. Whenever you run your sink or use your drain, some water will always remain stuck inside the bend in the P-trap. This water creates an airtight seal that blocks off the pipe and traps sewer gases inside the drain line. If no water was present in the P-trap bend, the sewer gases would naturally flow up out of the drain and into your kitchen or bathroom.

What Can Cause a P-Trap to Dry Out?

P-traps can sometimes go dry, and this will allow sewer gases to escape out of the drain line. This issue is most common in drier climates and especially in winter when your heating system is running. In areas like Idaho that experience extremely dry winter weather, the water inside your P-traps can evaporate quite quickly if you don’t continually replenish it.

If no water goes down the drain for a week or two, the trap can dry out and stop working properly. The only way to prevent this from happening is by making sure to turn on your sinks, showers and other plumbing fixtures and let them run for a few seconds at least once every week or so. This also works to flush out the stale water inside of the bend. If you don’t flush the water periodically, mildew and bacteria can grow and cause the water to become quite smelly.

While evaporation is the most common reason that P-traps go dry, this issue can also occur for various other reasons. For instance, strong winds can sometimes cause the trap to go dry. Wind gusts can travel down through the vent stack in your roof and into the rest of the plumbing system. These winds can disturb the water inside the P-trap, causing an oscillation effect that causes water to splash out and flow down the drain line.

There are also times when you may notice that all of your P-traps are drying out quickly and your drains are constantly smelly. When this happens, it usually means that the vent pipe in your roof is clogged or partially obstructed. The vent pipe works by allowing sewer gases to escape while also bringing air into your plumbing system. If air can’t get into the system, it causes the pressure to change and can form a vacuum inside the plumbing system. If a vacuum is present, the water won’t be able to drain and will eventually begin to back up inside your home.

When you flush water down the drain, the force of the water pushes the sewer gases down through the drain line and into the main vent-waste stack. All of the wastewater and solids travel down through the stack and into your main sewer line while all of the gases rise up and flow out through the pipe in your roof. At the same time, the pressure of the water flowing through the system also draws air in through the vent pipe.

If the vent pipe is clogged, the only way for air to get inside the plumbing system is through all of your drains. As a result, every time you flush the toilet, the pressure will draw air in through the drain. This results in the water in the P-trap getting sucked out and going down the drain line. This problem means that some or all of your P-traps could potentially go dry every single time you flush the toilet. The only way to prevent this from happening is to have a plumber clean out the vent pipe and remove whatever is clogging it.

Idaho’s Plumbing and HVAC Experts

If you need drain cleaning or any other professional plumbing services in the Caldwell, Nampa or Boise areas, Express Plumbing Heating & Air is here to help. We specialize in a full range of plumbing services including sewer repairs, leak detection, septic pumping, plumbing inspections and water line services. Our team also works on water heaters, faucets, toilets, bathtubs, garbage disposals and other plumbing fixtures. We also specialize in installing, servicing and repairing furnaces, air conditioners and other HVAC units. To schedule any plumbing or HVAC service, give us a call today.

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