Why do drains need vents

The drain gurgles, stinks, water comes up - that helps!

There is hardly anything more unpleasant in the household than a drain that stinks or even sends sewage back. This is often related to the fact that water accumulates in the pipe system at a more distant point. More often, however, the cause of the problem is the ventilation of the sewage system. We show here how you can get to the bottom of the problem and solve it.

Many users observe the problem, especially when a large amount of water is drained from the toilet or bathtub. Water from the tub comes up in the toilet or worse, the toilet water suddenly sloshes in the tub. It gurgles in the sewage system and the air that rises from it smells extremely bad. So that you can clarify whether it is a simple blockage in the siphon or a problem with the ventilation, we have put together the causes and solutions with the appropriate tips. The action runs without a pipe!

What you need:

  • Spindle large and small
  • Water pump pliers
  • soft cotton towels
  • bucket
  • mechanical pipe vent
  • possibly branch pipe
  • new siphon with ventilation nozzle


Check the causes

If there is a strong odor of sewage treatment plant when the sewage gurgles, there is usually something wrong with the ventilation of the system. But you should still rule out all simpler causes beforehand. In any case, it cannot do any harm if the siphon is cleaned from time to time. At least the siphon can be a source of the unpleasant smell.

  1. Check siphon

First of all, you should check whether the siphon or odor trap can work, or whether hair and dirt are blocking or restricting the way there. Before you start working with baking soda or other pipe cleaners for a long time, you should grab the water pump pliers immediately if the drain is gurgling. You can very easily open the siphon for a wash basin or kitchen sink.

  • Place a bucket under the drain
  • Loosen the screw connection of the siphon
  • Clean the siphon
  • possibly pierce the drain a little more with a fine spindle
  • Screw the siphon back on

Tip: Nowadays there are often very elegant decorative siphons under a washbasin, which are chrome-plated with a high-gloss finish. Be extremely careful with these specimens. In order not to scratch the surface with the water pump pliers, you should wrap soft old cloths around the jaws of the pliers.

  1. Check inspection openings

You should look at the inspection openings of the sewer pipe to see whether the sewage can flow freely. To do this, have someone else flush the toilet and see if the water comes past you in a gush. If it is just a trickle, there could be an obstacle in the pipe in front of the opening in question. Then you should try to open the pipe backwards with a long spindle. If you actually encounter an obstacle there, solve it by poking the spindle several times and then rinse thoroughly.

Tip: With a vertical inspection opening, you should of course only open the flap a small gap at the upper edge and shine a flashlight into it. Otherwise you are literally standing in the rain, but not in a pleasant one.

  1. Check ventilation on the roof

If it gurgles and stinks, however, this is a clear sign in the direction of the ventilation, because if this does not work, the sewer pipe sucks the odor trap empty due to insufficient counter air. The rising of water from another sanitary object is also an indication of this error. If the ventilation of the sewage pipe does not work or no longer works, this may either be due to a structural fault in the construction of the ventilation or to a closure. So first look to see whether birds have built a nest there by the vent plug, which will make it gurgling and smelly.

Venting the sewer pipe

The ventilation for a sewer pipe must be led upwards on every vertical pipe of the sewer pipe. It should protrude through the roof and not, as is often the case, end under the roof. A completely closed sewer pipe cannot inevitably work, but there are usually several odor traps and the water then looks for an alternative route by sucking them out.

Function of ventilation

You can roughly imagine what goes on in a vent if you pick up a clear hose. Fill it a little more than halfway with water. If you now change the height at one end of the hose or pour in water with a swing, it begins to gurgle. In the vertical part, the ventilation ensures that the air can escape. If it is missing or clogged, water and air slosh out at the other end. However, if water is poured into a trickle very slowly, no air bubbles are created.

So if a lot of water is poured in in one go, the vent for pressure equalization sucks the siphons in the whole house empty, the result is that it gurgles and smells. If you just wash your hands with a small trickle, usually nothing will happen. This is also the reason why we often first notice the problem when flushing the toilet. The problem often occurs in an old building because a new pipe may not have been connected to the ventilation after a structural change. Do-it-yourselfers often install an additional guest bathroom without even knowing about this problem.

Solution for lack of ventilation

Of course, you cannot remodel the whole house just because of the ventilation. Especially if the house has already been rebuilt several times and the top floor has now also become a living space, on the one hand nobody knows where the pipes are exactly and on the other hand, venting through the roof is so costly and time-consuming that many homeowners already rather accept that it gurgles and smells. But there is a much simpler solution today anyway.

  1. Mechanical pipe aerator

The mechanical pipe aerator works automatically when a negative pressure arises. The small device opens and lets so much air into the sewage pipes that the siphons in the house are no longer emptied. Once the negative pressure has been eliminated in this way, the pipe aerator closes again automatically.

Tips: If the pipe aerator is to be installed in an adjoining room, such as a laundry room, which may not be frost-proof, then buy a special frost-proof device. However, many of these larger aerators are specially insulated anyway. The prices range from a little under 20 euros to around 60 euros for special tube aerators in the toilet area.

  1. Installation of a pipe aerator

You first need to find the highest point in the sewer line. The aerator is then installed there. Most manufacturers now even offer pipe aerators of different sizes for sewer pipes. These not only fit the different pipe sizes, but also offer different air volume flows. While the large pipe aerator at most manufacturers introduces around 30 liters of air per second into the system, the small aerator creates around eight liters of air per second.

  • large pipe aerator for pipes DN 70, DN 90 and DN 100
  • small pipe aerator for pipes DN 40, DN 50 and the special size 11/2

Attach the pipe aerator to the toilet

  • Dismantle the toilet bowl and separate it from the cistern
  • Pull the drain pipe out of the floor or wall at the back
  • Insert a new drainage pipe with an additional stopper / Y-pipe
  • place a large pipe aerator on the second stopper and seal it
  • Reinstall the toilet and screw it to the floor
  • Make sure that the toilet is properly sealed against the pipe
  • Reconnect the cistern and also check the seal

Tip: When buying the pipe aerator, pay attention to the rubber lip on the connection. This rubber lip ensures that no more odors get into the rooms and adapts to the different pipe sizes. When buying, you should also make sure that insect screens have been installed in the mechanical pipe aerator. Otherwise, annoying insects will get into your toilet or bathtub this way.

Mini solution - inexpensive and fast

There are not countless meters of sewer pipes in every household, and even if it gurgles and smells, there are always gradations in the size of the problem. Here, too, the manufacturers of sanitary accessories have developed ingenious solutions. A nice practical example is that Pipe extension for the siphon, which has built in a small vent. These are particularly easy to install and the costs are also very manageable.

  • Dismantle the siphon
  • Replace the vertical siphon pipe with a new ventilated pipe
  • Check seals and rubbers
  • Reattach the siphon

Depending on whether they are made of plastic or chrome-plated metal, these extensions adapt well to the existing system. These tube extensions are also available with a 90-degree angle in both versions. You can get this mini-solution from around 20 euros. The second simple solution consists of a complete siphon system in which a pipe ventilator is also integrated. These systems are available from around 50 euros for both the sink and the vanity.

Whether this simple solution really does feed enough air into the system when the toilet is flushed must be determined by a practical test at your home. Success will certainly also depend on how far the toilet is from the vent in question, but it should at least reduce the problem significantly.

Prices for the different solutions:

  • Pipe vent, large, insulated, with insect protection, white - between 30 and 70 euros
  • Pipe vent, large, uninsulated, without insect protection, gray - between 20 and 45 euros
  • Pipe vent, small, uninsulated, with insect protection, white - between 25 and 40 euros
  • Pipe vent, small, uninsulated, without insect protection, gray - between 10 and 30 euros

Tip: Remember that you will also need a new branch pipe for the above variants. It doesn't cost much, but it must both find space and be sealed. The effort is therefore significantly greater than with the simple solutions that are attached to the siphon.

  • Siphon pipe extension with integrated vent, white plastic - between 18 and 30 euros
  • Pipe extension siphon with integrated vent, chrome-plated metal - between 22 and 40 euros
  • Siphon pipe extension, integrated vent and 90-degree angle, plastic white - between 18 and 30 euros
  • Pipe extension siphon, integrated vent and 90-degree angle, chrome-plated metal - between 25 and 45 euros
  • Siphon complete with vent - sink with device connection - from around 45 euros
  • Siphon complete with air vent - sink - from around 40 euros

Tips for fast readers

  • Investigate problem for gurgling drain
  • Clean the siphon on the sink or vanity
  • Spindle the sewer pipe
  • Check inspection opening of the line
  • Check the venting of the sewer line
  • Remove bird nests and clean the exhaust air socket
  • Add an additional branch pipe to the toilet / wash basin
  • Install mechanical pipe aerator
  • Attach the integrated pipe aerator to the sink or vanity
  • Install new siphon complete with pipe aerator