The Telltale Signs of Poorly Vented Plumbing Drain lines


The signs of poorly vented plumbing drain lines are slow draining fixtures, gurgling drains, weak flushing toilets, sewer odors in the house and empty toilet bowls. A toilet that bubbles when flushed or when another fixture drains is also a good sign.

In summary, these are the signs of poorly vented drain lines:

  • Slow Drainage: One of the most common signs is slow drainage from sinks, tubs, and toilets. Poor venting can cause airlock, hindering the smooth flow of water.
  • Gurgling Sounds: You may hear gurgling or bubbling sounds in drains when water is running. This is caused by air trying to escape through the water in the traps due to inadequate venting.
  • Odors: Poor venting can trap sewer gases within your plumbing system. As a result, foul odors, often smelling like rotten eggs, can emanate from your drains.
  • Toilet Flushing Issues: Insufficient venting can cause toilets to flush poorly or not at all. You may notice that flushing results in a slow and incomplete bowl evacuation.
  • Multiple Drain Problems: If several drains in your home are affected simultaneously, it could be a sign of a common venting issue in your plumbing system.
  • Water Backing Up: Poorly vented drain lines can cause water to back up into other fixtures when one fixture is in use. For example, flushing a toilet might cause water to rise in a nearby sink or tub.
  • Sewage Smells: Sewer gas odors escaping through drains or fixtures are a clear indication of venting problems. These gases can be harmful and should be addressed promptly.
  • Bubbling Toilets: When running water in a sink or tub, you might observe the water level in a nearby toilet bowl rising or bubbling, indicating improper venting.
  • Wet Venting Issues: In some cases, you may notice water draining from one fixture while using another, indicating problems with wet venting, a type of drain venting configuration.

Signs of Poorly Vented Plumbing Drains Lines

Poorly vented lines are really easy to tell. You just need to flush a toilet or try to drain a bathtub and you will see how the drains behave.

As I have already mentioned, you need to check if the vent is clogged before concluding that the drain lines are poorly vented. If for instance the fixtures in your house used to drain properly in the past and only deteriorated in the last few days, you most likely have a clogged vent stack.

On the other hand, if the vent stack is new but your drains are problematic, you most likely are dealing with poorly vented drain lines.

The following are the signs of poorly vented plumbing drain lines:

1. Slow Drains

One of the most important functions of a plumbing vent is to introduce air into the drainage system. That way, the pressure inside the drain lines and outside is equalized.

Think about it; what do you do if you are draining the water heater and you want it to drain out faster? You open a hot water faucet. By opening the faucet, air will be introduced into the water heater allowing it to drain faster, by equalizing air outside and inside the heater.

That is the same role played by a plumbing vent. In order for fixtures to drain faster, there needs to be air behind the fixtures drain opening.

If a drain line is poorly vented, there will be what we call negative air pressure inside the drain lines. Negative air pressure refers to a situation whereby the pressure of the air is lower that the atmospheric pressure.

The negative air pressure inside the drainpipes restricts the movement of waste down the drain line, and hence the slow draining fixtures.

2. Weak Flushing Toilets

A toilet functions slightly different from other drains in your house. In order for a toilet to flush powerfully, a siphon needs to be created at the toilet trap. A siphon is the suction force which pulls waste from the bowl and down into the drainpipe.

To have a powerful siphon inside the toilet trap, there needs to be air inside the drainpipe, whose pressure equals the atmospheric pressure. Negative air pressure will result in a poor/weak flushing toilet.

It is the same concept when you are siphoning a drink from a glass or bottle using a straw. If you pull forcefully, the siphon effect will be stronger resulting in more drink in your mouth, meaning the bottle/glass is being drained out faster.

If you flush your toilet and notice that the toilet won’t flush but water rises inside the toilet bowl then drains out slowly, you most likely have a poorly vented drain line. This problem could however also be caused by a clogged drainpipe or vent stack.

3. Gurgling Drains

Are you experiencing the following in your house?

  • Shower/tub drain gurgling when the toilet is flushed.
  • Bathroom sink gurgling when the toilet is flushed.
  • Toilet bubbling when the shower/tub is draining.
  • Kitchen sink gurgling when the washing machine is draining.

Why is there a gurgling sound from your drains?  The cause is poorly vented plumbing drain lines or a clogged plumbing vent.

If you look underneath your bathroom or kitchen sink, you will notice (or you already know) that part of the drain line is U-shaped. That part of the drain is called a P-trap.

A P-trap is usually full of water. Just like the one at the bottom of your toilet bowl. Shower/tub drains as well as washing machines have P-traps only that you can’t actually see them.

The water inside the P-trap acts as a barrier preventing sewer gases from coming up through the drains.  That however doesn’t hold if the drain lines are poorly vented.

As I had mentioned when a drain line is poorly vented, air will not enter the drain during draining resulting in the creation of a vacuum. Since a vacuum fights very hard to exist, it will try to pull air from the surrounding.

There is a column of water (in the trap) standing between the vacuum and the surrounding air. To access the air, the water in the P-trap will be siphoned inside the drain line and air will enter. The siphoning of water from the P-trap is the cause of the gurgling sound in drains.

4. Sewer Odors

Another function of a plumbing vent is to expel sewer gases from the drain lines. With a poorly vented drain line, sewer gases will accumulate inside the drain lines as they have nowhere to go.

As we have just seen in point number 3, siphoning of water from the P-trap will leave it empty, allowing sewer odors to flow out of the drains without any restrictions. If there is a sewage smell in your house or bathroom, you could very well be dealing with poorly vented drain lines.

In the case of a toilet, you will notice it bubbling after flushing it or draining other fixtures. The bubbling is as a result of sewer gases being expelled from the drain line.

5. Empty Toilet Bowl

Toilets are always gurgling during flushing and therefore their gurgle can be easily ignored unlike gurgling shower/tub drains. The same thing happening to the water in the shower, sinks and washer P-traps can also happen to the water at the bottom of the toilet bowl.

If you flush a toilet and the water level in the toilet bowl falls significantly, it means that there is a vacuum inside the drain line siphoning out the water. Again, this is sign of a poorly vented drain line.

What to Do about Poorly Vented Drain Lines

If indeed your drain lines are poorly vented, the best thing you can do is to call in a professional plumber to assess the situation and advice you on the best course of action.  There is however one thing you can do on your own.

An air admittance valve (AAV) is a short pipe with a valve at the top that allows air to flow inside the drain line while draining but will not allow sewer gases to flow out through it.


This valve is usually closed and will only open when there is negative air pressure inside the drain line to allow in air. You can easily install one under your kitchen or bathroom sink but installing it elsewhere will most likely need a professional who understands the building code.

One thing that you however should do is check whether you have an S-trap instead of a P-trap under your sink. S-traps are banned and will give you problems with the same signs as a poorly vented drain line

Check out the differences between S-traps and P-traps in this post.

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