How Does a Bell Trap Drain Work?

If you’ve ever wondered why your pipes twist and bend near a drain, then you’re wondering about plumbing traps! Drain pipes bend to trap hazardous gases that might otherwise travel up through the pipes and into your home.

In this article, we’ll examine popular types of plumbing traps – including the bell trap drain – and let you know which traps are more or less effective.

man repairing sink trap adjustable pipe

A bell trap drain is designed primarily as a drainage unit to be placed on the floor of a workshop, garage, or anywhere that needs floor drainage. A bell trap drain utilizes a water reservoir to stop gases from entering through the piping and out the drain.

Essentially, the bell trap drain has a bell-shaped reservoir surrounding the vertical drainage pipe, with a trap seal that closes over the entire unit. The water that drains through the top of the bell trap drain fills the reservoir, with excess water spilling down the pipe.

The water that fills the reservoir also blocks gases from traveling upward and out of the drain. Essentially, the water forms a water seal that prevents anything from escaping up the tub drain.

Are Bell Trap Drains Effective?

Bell trap drains are effective when used properly. The problem is that they are also easy to forget about, and many homeowners won’t know how to maintain them. As a result, the water reservoir can dry up quickly, eliminating the seal on the bell trap drain and allows hazardous gas to travel up and out of the bell trap floor drain cover. We don’t have to tell you that this is dangerous!

Most US building codes ban bell traps. Simply put, they’re not a great idea and can get you into trouble in many areas.

Bell trap drains also contain still water, making them susceptible to mold and hazardous material build-up (especially if you’re working with dangerous chemicals in your shop). Dangerous materials can collect in the reservoir, and no drain cleaner in a cleanout plug can change that. Once again, not a great utility for the bell trap drain!

So, that said, which fixture traps are effective?

Plumbing Traps

A plumbing trap, as mentioned above, is a fixture meant to trap poisonous or hazardous gas, similar to a grease trap, and won’t allow it to travel up through a pipe and into your home.

Because much of this gas rises, it would exit through your fixtures if not for these traps (bottle trap in sinks, plumbing fixtures, shower drains, septic tank, water heater, and basement floor drains, etc.). For this reason, plumbing traps are beneath these fixtures, just below where the drain opening. That curvy pipe located underneath your bathroom sink is actually a plumbing trap.

Plumbing traps are in different forms and designs, but the common theme is that they trap water inside their mechanism. This water is what blocks the gas from traveling up through the pipe. Check out the different types of traps:

Related: What is an S-Trap and Is it Illegal?

pvc pipe fittings selection

Let’s take a look at different types of traps. We’ll start with the current standard for most areas of the world that is generally considered the most effective plumbing trap: the P-trap.


P-Traps are considered the gold standard in modern plumbing (and also required by plumbing regulations in many nations). The P-Trap ensures that water always remains in place to block any gases from traveling up the pipe.

The P-Trap got its name because it resembles a P lying on its side (see picture below). The P-Trap drains down from the fixture, then curves back up before transitioning into a horizontal section of pipe.

The reason the P-Trap curves back upward is to ensure that water is kept in the trap, even when the draining is finished. The water sits in this portion of the trap at all times, preventing gas from traveling through the pipe.

After this curved trap portion, a P-Trap continues into a horizontal section, which allows the pipes to connect to a venting system (more on this below). The vent prevents the trap from being siphoned dry. It is important that the horizontal section of a P-Trap is not perfectly horizontal and will allow water to flow through the pipe.

crosssection explodeddiagram jbend ptrap

Does Every P Trap Need a Vent?

P-Traps require venting to function properly at all times. A vent on a P-Trap prevents air from being sucked or siphoned out of the trap portion, which would ruin the trap’s effectiveness.

By placing a vent at some point after the trap, it cannot be suctioned or siphoned dry. This vent must be connected to the outside of the building as it vents dangerous gas most of the time.

What is the difference between P trap and S trap?

An S-trap is now considered an outdated version of a plumbing trap. An S-trap resembles a P-trap and works in almost the same way. However, instead of extending into a horizontal section after the trap, an S-trap will simply continue vertically to drain the water.

As a result, there is no proper ventilation in an S-trap, and it can be siphoned dry on occasion. For this reason, an S-Trap is considered an outdated fixture and is disallowed in many nations.

Can P trap be installed higher than drain entry?

Technically, no. You should install the P-trap directly to the old drain in the laundry sink or shower base. If it’s higher, gravity cannot do its job, and the water does not flow down naturally. It might still work, but it is not ideal, and in most places, it’s illegal and not up to plumbing code.

How long does it take for a drain trap to dry out?

Some sink drain traps can dry out in as little as a month to dry out, and it usually happens in the winter.

What is a bell trap used for?

A bell tap is used to trap sewer gas and is used in floor drainage.

Bell traps are used as a building trap in commercial structures and are no longer permitted in residential structures, though you may see them in garages, patios, under a basement floor drain cover, in a basement drain, and other outdoor areas.

Drum Traps

Another form of sink trap that is considered outdated is the drum trap. A drum trap functions in a similar way to the Bell Trap, which we featured above. Unfortunately, it shares some common disadvantages with the bell drain.

Drum traps use a higher quantity of water than a bell trap to prevent drying. They initially gained popularity because they were able to effectively capture lost items if they were to fall down the existing drain.

However, much like the bell trap, a drum trap is considered outdated and is prone to clogging. It is not considered an effective running trap when compared to the modern P-Trap design.

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