Water from the shower does not drain properly, ventilation error?

Hello forum,

we recently renovated a house and I am stuck as a hobby handyman.

The following case:

We renovated the guest bathroom (shower / toilet / sink) and installed new tiles and bathroom fixtures. The pipes have remained mostly the same.

Before the renovation, there were no problems with the shower drainage.

Now we installed a shower drain (Camargue 80cm) from Bauhaus. Previously, there was a raised tub with a normal siphon underneath.

In the attachment you can see an approximate drawing of the drainage system.

When the shower is turned on now, the water does not drain with the integrated siphon. If I remove the siphon, air rises and the water drains. Then I wait until no more air rises while the water is running, put the siphon back in, and the water continues to drain without problems. If I turn the water off, it still drains and suddenly bubbles... somehow air is getting back into the system. If I turn the water on again, it does not drain, only when I remove the siphon again and wait until the air escapes from the drain.

Furthermore, I noticed that when the cistern empties completely, the water in the toilet collects a bit towards the end and then drains delayed; afterwards, bubbling can also be heard in the toilet. It seems as if something has clogged somewhere in the drain.

My question is how it can be that the water drains when the siphon is removed, as I described above?

Does it have to do with the ventilation of the drainage system? Could something have clogged the ventilation and the air is being drawn from somewhere else, causing this phenomenon?

I would be grateful for your advice and ideas.

Regards Alex

The pipe ventilation serves to prevent a rigid air column in the soil pipe.

The amount of water from the cistern initially follows gravity and inertia. As the volume increases, the ratio of the wavelengths between the water column in the drain and the pipe diameter becomes less favorable; a slight pulsation is normal. If there are narrowing sections in the pipe, this reduces the pipe cross-section on the one hand and secondly makes it less round, which affects the screw-like movement of the water column. The fundamental effect then occurs at a lower flow rate.

Regarding the shower, I gather from your description that the slope of the drain to the connection into the soil pipe has decreased compared to before. This, of course, influences the flow behavior, up to pulsating backflow. With a freer "breathing volume" of the drain, the phenomenon is expected to ease.

The reduced slope is a design flaw, and concerning the shower probably the main culprit. Toilet drains must be cleaned as needed and it is advisable not to subject them to foreign disposal tasks (food residues, candy wrappers, and similar sins). And: occasionally using the large flush button even for small tasks helps to dislodge deposits.

Hello 11ant, first of all thank you for the quick answer to my question. I only got around to responding today. I had to read the post a few times to understand it halfway as a layman.

Does the pulsating backing up then have something to do with the air in the drain pipe?

A design flaw would of course be unfortunate and possibly only fixable by reopening the tiles... or do you see another possibility here?

In the shower, the pipe remained connected up to the vertical pipe, only at the end the first slope from the drain to the drain pipe was reduced by the new shower.

Is that enough to change the drainage behavior so that the water backs up?

I would now try to clear the pipe of any constrictions and see if anything changes with the shower drainage afterwards.

I just don’t quite understand why air is pushed up when I turn off the shower. The air then empties the water from the integrated siphon of the shower channel. How can I explain that air suddenly rises through the drain pipe, where water previously flowed completely through...

Regards Alex

I would now try to clear the pipe of any constrictions and see if anything changes in the shower drain afterwards.

Constructions in the toilet drain will not affect the shower drain at all.

How can I explain that air suddenly rises through the drain pipe, where before water was flowing through completely...

Not at all – these are mistaken assumptions. The acceleration of the fall causes the water to flow down the pipe wall in the drain pipe; the drain pipe is not completely filled in cross-section. Air does not really rise; it is simply lighter than water.

[NARROWINGS IN THE TOILET DRAIN DO NOT AFFECT THE SHOWER DRAIN AT ALL.


not at all - those are mistaken assumptions. The acceleration due to gravity causes the water to rush down the pipe wall in the soil pipe; the soil pipe is not completely full across its cross-section. Air does not really rise in the strict sense; it is simply lighter than water.]


Ok, that sounds plausible.

Then asked differently. Why does the integrated siphon of the shower channel not work, but the water in it disappears over time?

Would you have a proposed solution for the problem?

Regards