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Plumbing Vent | How to Tell if It Is Clogged

Plumbing vents regulate the air pressure in a plumbing system. They do this by allowing air into the plumbing system.

Clogged vents prevent the drain from functioning optimally and cause a range of plumbing issues. I’ve put together the key indicators of a clogged plumbing vent to help you diagnose a clog in your vent.

The following are indications of clogged plumbing vents:
  • Draining is slow
  • Noisy draining
  • Backflow
  • Odors from drains and toilet
  • Low or dry toilets
  • Water backs up when poured into stack vent

Draining Takes a Long Time

Slow draining is one of the most common plumbing issues households experience.

The primary cause of slow draining in plumbing fixtures is blockage or pressure buildup in pipes (although this is not always the case). 

Clogged sink full of dirty water with bubbles

The plumbing vent regulates the air pressure in your drain system. When the vent is clogged, it is no longer able to regulate the air pressure in the pipes because the air cannot reach beyond the blockage. 

This leads to pressure imbalance within the drainpipe, which prevents water from draining properly. 

So, if your bathtubs and sinks are not draining as fast as they used to, and you have checked that the fault isn’t from your drain, then it is probably a sign that you are battling a blockage in your plumbing vent. 


Slow draining in plumbing fixtures is an annoying inconvenience.

It can cause you to spend longer than necessary completing simple tasks and activities. 

For example, if you are trying to get through a stack of dirty dishes or clean the last few before you leave for vacation, then a kitchen sink that takes forever to drain can become a cause of stress and frustration.

Alternatively, if you are supposed to take turns using the bath, the delay in draining would force you or other members of your family to wait until the water drains or forfeit using the bath altogether. 

Also, depending on how long the water takes to drain, the standing water can breed insects and cause odors. 

Noisy Draining

Plumbing vents remove air and gas from the drain line and allow fresh air to enter the line as well when this is required to balance the pressures. 

When there is a clog in the vent, air is not able to leave the drain as it should, and when this happens, negative air pressure develops in the line. 

The negative pressure in the drain line traps air in the drain. Water leaving the drain would be forced to fight against this air pressure, which results in a sputtering or gurgling noise. 


Besides disturbing the quiet in your home, a noisy drain doesn’t have any serious implications. 

How much you are affected by the noise would depend on how loud it is and when it happened. 

This might not be a problem during the day unless you have a baby who wakes easily. But loud gurgling noise is definitely not something you want to hear at night. 

The sound could disrupt your sleep or that of other people living in the house. This can be a problem for light sleepers or people who find it difficult to fall back to sleep. 

You might also experience noises from a drain as water exits from another fixture. For example, the toilet can gurgle when the sink or shower drain is in use. This can be startling or even eerie, especially at night. 


Backflow is the unwanted flow of water in the opposite direction than is intended. The reversal in the flow of water is usually caused by a change in pressure within the pipes. 

Backflow isn’t always caused by a clogged vent. Other causes of backflow include:

  • Clogs in the sink trap
  • Collapsed drainpipe
  • Blockage in pipelines or drain
  • Blockage in washing machine pipe
Blonde man in a bun fixing the drain pipe of the faucet

Plumbing lines are designed to flow in a single direction. In the case of a drain, wastewater flows out of a unit or fixture to a disposal location.

Ventilation is necessary for the free flow of water within the drain line. Whenever there is a clog, air would be unable to move in and out of the drain. This causes water to stay stagnant in the line and flow back to your sink, toilet, or tub. 

You can experience the backup of wastewater to your sinks, dishwasher, washer, showers, or tubs and sewage can backup to your toilets. 

The effect of backflow is worse if fixtures aren’t protected by a backflow preventer


Backflow is a terrible plumbing issue that can have an adverse effect on your fixtures, appliances, and even your health. 

Whenever contaminated water flows back into your sink or tub, your house is exposed to the bacteria in the water. 

If the water isn’t removed immediately, it can breed insects and give the room an offensive smell. You might also be faced with the chore of cleaning the mess whenever wastewater spills on the floor. 

Backflows in toilets are a lot more dangerous because sewer pipes house an array of bacteria that cause a variety of health issues, including allergies, peptic ulcers, and gastrointestinal problems. 

Besides the obvious mess an overflowing toilet would create, they also release a horrible stench in the room, which sticks long after you have gotten rid of the mess. 

Devices like dishwashers and washing machines can also be affected by backflow. When this happens, contaminated water will settle in the bottom of the device. 

If you run the washing machine without draining the water, your clothes might get stained and they wouldn’t wash properly. 

The presence of contaminated water in dishwashers creates a health hazard, and if you run the dishwasher with dirty water in the unit, you expose yourself to sickness-causing bacteria.

The standing water in the appliance would also cause the appliance to run longer than normal, causing an increase in energy expenses. 

Bad Smells From Drains and Toilet

Sewer gas is the primary cause of the offensive stench that is released by toilets and drains whenever the plumbing vent is clogged. 

Besides controlling the air pressure in the drain line, plumbing vents also remove sewer gas and odors from your drain. 

A clogged vent doesn’t allow air in or out of the drain line. This means that sewer gas and noxious odors remain trapped inside the drain line. 

Woman wearing yellow gloves cleaning the toilet sink covering her nose due to foul smell

The buildup of gas and the lack of airflow would cause gas and odors to eventually find their way into your home. 

Plumbing drains have a trap that retains a small amount of water each time the drain is used. This water creates a seal that prevents gases from entering your home. 

The negative pressure in the drain would force the water in the trap to empty out, allowing toxic gas to flow freely into your home. 


If the issue isn’t addressed swiftly, the stench will flow to other parts of your house and affect the overall air quality in your home. 

If sewer gas enters your house, it releases toxic gases like hydrogen sulfide, ammonia, and methane into your home. 

Prolonged exposure to sewer gas can lead to health issues like headaches, dizziness, nausea, and fatigue.  

Hydrogen sulfide is responsible for the rotten egg odor that’s associated with sewers, and even at low concentrations, the gas would give off the offensive stench.

Using air fresheners or scented candles isn’t going to help with the effects of badly vented drains. The only way to tackle the issue is to address the root of the problem. 

Low or Dry Toilets

The water at the bottom of the toilet bowl isn’t for show. It serves as a protective barrier that prevents sewer gas from entering your home.

It also aids the free flow of waste whenever you flush the toilet. 

The S shape of the toilet trap makes it so that there is always water in the bowl. A dry toilet bowl or one with a low water level is usually an indication of a plumbing issue. The common causes include:

  • Damaged fill tube or fill valve
  • Clogged sewer vent pipe
  • Dirty or clogged rim jets
  • A partial or full clog in the toilet trap
Common causes of dry toilet bowl: damaged fill tube or fill valve, clogged sewer vent pipe, dirty or clogged rim jets and a partial or full clog in the toilet trap

If you have a clogged vent, the negative pressure caused by the blockage can create a partial vacuum in toilet pipes and this might suck water out of the toilet, causing the water level to be lower than normal or to completely dry out. 


The water at the bottom of the toilet is essential for the free passage of waste into the drain. If the water is low or dry, you might experience difficulties while flushing. 

Depending on how low the water level is, waste might not flow into the drain as fast as it should or it wouldn’t go through at all. Worse, the waste can back up and lead to an overflowing toilet. 

Also, the water provides a seal against the entry of sewer gas and when it is not in place, sewer gas can enter your house, causing your home to smell funky

This exposes you and members of your family to toxic gases like methane and ammonia, which can cause a range of health challenges. 

Do the Water Test

The opening of the vent stack (which is known as the stack vent) is usually located on the roof (although it can be in the soffit or sidewall). 

Illustration of a plumbing stack vent

To perform the test, get someone to help you and climb to the roof with a garden hose. 

When you are next to the vent, place the hose in the vent opening and have the other person turn on the water. 

If you pour water into the vent opening and it backs up, that’s an indication that the vent is blocked. If it doesn’t, you might have to explore other possible causes. 


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