The Telltale Signs of Poorly Vented Plumbing Drain lines

A plumbing vent is a vital component of your house’s drain-waste-vent system (DWVS). It however has to be properly installed if you are go get its maximum benefits.

So, how do you know if your plumbing is properly vented? Or better still, what are the signs of a 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.

If your plumbing is properly vented, your fixtures will drain as fast as they are supposed to and the toilets will flush strongly. No sewer odors will come up through the drains unless the plumbing vent or main drainpipe is clogged.

Venting is not an easy task. It needs to be done by a professional who understands the building code. The size of the plumbing vent matters as well as the number of fixtures being vented and their distance from the vent stack.

In your house, the plumbing vent also known as a vent stack is the vertical pipe which runs through the roof of the house. It is usually connected to the main drain stack just above the fixture on the highest floor level.

All the water fixtures (toilets, sinks, showers, tubs and washers) have independent drainpipes which are then connected to the main drain stack and vented through the vent stack. This means that if the venting is poorly installed, all the fixtures will be affected.

Sometimes you may be dealing with a clogged plumbing vent and not necessarily poorly vented drainpipes. Poorly vented drains exhibit the same signs as drains whose vent stack is clogged.

If a drain is poorly vented, fixtures will drain slowly, water will rise inside a toilet bowl then drain out slowly during flushing, drains will gurgle when others are draining, and a sewage smell will fill the room. The toilet will also start bubbling when showering or draining a sink.

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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