What to Do If the Siphon Box is Leaking (by Capitan Farloc)

Noticed a bad smell or water leak coming from the bathroom floor? The floor drain box (siphon trap) might be the culprit — especially if the cap isn't sealing properly anymore. In this step-by-step guide, I’ll show you how to fix the problem easily, using just a few basic tools. With clear photos and simple instructions, even beginners can take care of it without calling a plumber!

Introduction

First things first, let’s clarify what we’re talking about. Despite its rather grand name, the siphon box is nothing more than that small floor drain commonly found in the bathroom.
Usually made of chrome-plated metal, it’s round in shape and has a screw in the centre. Occasionally, you might notice it starting to leak water—typically when certain events occur, such as the washing machine draining or the bathtub emptying.
A leaking siphon box can be a frustrating issue, but don’t worry—it’s often something that can be resolved with a few simple steps. In this guide, we’ll walk you through the possible causes and solutions so you can fix the problem quickly and effectively.

What is the Siphon Box and What is it For?

The siphon box is a floor drain installed in the bathroom, where wastewater from sinks, bidets, and showers—known as "grey water"—flows before reaching the main drainage pipe. The small plate you see fixed to the floor is simply a cover, hiding the cap that seals the drain below.
As you can see in the diagram, the inlet pipes are positioned lower than the outlet pipe, and this design serves two key purposes:

• The water at the bottom of the siphon box acts as a barrier, preventing unpleasant odours from the sewage system from rising back into the house.
• If a small object accidentally falls into the drain, unless it’s light enough to float, there’s a good chance it will settle at the bottom of the box—making it possible to retrieve it.

Why is the Siphon Box Leaking?

If water is seeping from the siphon box, the most likely culprit is a loose cap. Simply tightening the screw should compress the rubber gasket at the centre of the cap, allowing it to expand and properly seal the drain.
If this doesn’t fix the issue—and especially if the cap still wobbles even after tightening the screw—don’t worry. Some siphon box models have an internal locking mechanism, and the visible screw is only there to secure the metal plate. In this case, unscrew it, remove the plate, and try turning the hidden “knob” inside.

What If the Problem Persists?

If the previous steps don’t solve the issue, you’ll need to remove the cap and replace it with a new one.
In theory, if the cap is designed as shown in the diagram, loosening the screw slightly should release the pressure seal, allowing you to slide the cap out. However, experience has shown that the gasket often gets stuck, so the best approach is to fully unscrew the bolt and gently pry the cap loose with a screwdriver.

Possible Issues During Removal

During the removal process, the lower part of the cap might detach and fall to the bottom of the drain. If this happens, don’t worry—it can be easily retrieved using a screwdriver or a pair of pliers.
If you need to replace the expandable cap of the siphon box, make sure to purchase one with the same diameter. Before heading to buy a new piece, take the necessary measurements of the old one to ensure a proper fit.

That’s the End of the Article—I Hope You Found It Useful!

Thank you all!
Luciano (Capitan Farloc)