Signs of Clogged Plumbing Vent and How to Unclog It

A plumbing vent, also known as a vent stack has 2 main functions. It helps to remove sewer smells from the drainage system, and also allows air into the drainpipe so that fixtures in the house can drain easily.

Vent stacks are the vertical pipes which run from the main drainpipe/sewer line through the roof of the house. They are usually installed away from windows, doors and HVAC system to prevent them from compromising the quality of air coming into the house.

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With time, plumbing vents get clogged, usually by leaves, dead birds and rodents, tennis/baseballs, bird nests and sometimes even snow. A clogged plumbing vent will have a direct impact on the performance of your drains.

Slow draining fixtures, gurgling drains and a sewer gas smell in the house are telltale signs of a clogged plumbing vent. If your toilet all of sudden has a weak flush, bubbles when flushed or when the bathtub/shower drains, your vent stack is clearly clogged.

It is important to know the difference between a clogged vent stack and a clogged main house drainpipe as they sometimes exhibit the same symptoms. A partially clogged drainpipe will have your fixtures draining slowly, and gurgling sounds can be heard from other fixtures when another is draining.

How a Plumbing Vent Works

Every fixture in your house has its own independent drainpipe. All these drainpipes are then all connected to the main house drainpipe, which turn drains to the city’s sewer lines or septic tank.

To have a good understanding of how the drainpipe network in your house look like, think of it like a tree. A tree has 1 trunk and many branches. The trunk represents the main house drainpipe while the branches represent each of the different fixture’s drainpipe.

The plumbing vent is connected from the main house drainpipe and runs through the roof of the house. That way it is able to channel sewer gases out of the drainpipe, and bring in air, therefore balancing the pressure inside the drainpipe.

In order for fixtures to drain properly or flush toilets, there needs to be air in the drainpipe. If the vent stack is clogged such that air cannot flow in and out of the drainpipe, there will be negative air pressure in the drainpipe and your fixtures will be unable to drain properly.

To help you see the impact of a vent in a drainpipe, fill a water bottle with water, lift it up and starting pouring out the water. Look at what is happening inside the bottle as the water pours out.

Now use a knife or nail to make a small hole at the bottom of the bottle, block it with a finger and fill it with water. Invert the bottle, remove your finger from the hole and let the water pour out.

You will notice that in the first instance water inside the bottle will bubble and gurgle as it pours out while in the second scenario it flows out faster and smoothly. That is the effect of a plumbing vent in a drainpipe.

Signs of a Clogged Plumbing Vent

A clogged vent stack communicates through the drains. If you find a pattern from your kitchen sink, bathroom sink, toilet, shower or bathtub drain to the washing machine then most you likely have a clog in your vent.

The following are the telltale signs of a clogged plumbing vent:

1. Gurgling Drains

With gurgling drains, you might notice one or all of the following:

  • Bathtub/shower drain gurgling when the toilet if flushed.
  • Sink drain gurgles when the toilet is flushed.
  • Toilet bubbling when flushed.
  • Sink drain gurgles when the washing machine or tub is draining.

Gurgling from drains is an indication of 2 things. Either air is flowing out of the P-trap, or out of the P-trap. Every drain in your house has a P-trap.

A P-trap is the U-shaped part of the drain, which also looks like an inverted P that is installed below each fixture with a drain. If you don’t know how a P-trap looks like underneath your bathroom/kitchen sink or at the bottom of the toilet from the side.

P-traps allows for a little amount of water to be held, which acts as a barrier preventing sewer gases from coming up into the house. If the vent stack is clogged, air pressure builds up inside the drainpipe and is forced out of the drain traps in form of air bubbles.

The gurgling sound is usually caused by the trapped air/gases as they forces their way through the water in P-trap.

When water is dumped through a drainpipe with a clogged vent suddenly, it creates a vacuum inside the drainpipe as it flows out. Since a vacuum cannot exist in the pipe the water in the closest drain P-trap is sucked inside the drainpipe creating the gurgling sound.

2. Slow Draining Fixtures

When your bathtub, sink or washing machine takes too long to drain, it is a sign that there is a restriction in the drainpipe. This restriction is usually caused by a partial clog in the drainpipe, or negative air pressure as a result of a clogged plumbing vent.

A weak flushing toilet is also another symptom of a clogged vent. Just like the example with the bottle, a vent is supposed to introduce air into the drainpipe, which helps it drain quicker. When that does not happen, your fixtures will drain sluggishly, often causing other drains to gurgle in the process.

Check out how to fix a slow draining bathtub here and a slow draining sink here.

3. Sewer Smell in the House

A sewage smell in the house (usually in the bathroom or kitchen) is usually as a result of an empty P-trap. When a P-trap is empty, sewer gases flows through it unrestricted since the barrier created by the water has already been broken.

When for instance you flush the toilet and you have a clogged vent stack, a vacuum will be created inside the drainpipe which will result in the water in the bathtub drain trap being sucked in. While that is happening, a gurgling sound will be heard.

The above can also happen with the bathroom sink when you drain the bathtub/shower drain, or when you flush the toilet.

If there is a sewage smell in your house and you can hear the tub/shower drain gurgle when you flush the toilet then you surely do have a clogged vent stack. Sometimes you have all the 3 signs (slow drains, sewer gas smell and gurgling drains) at the same time.

Read more on how to get rid of sewer smell in the house in this post.

How to Unclog a Plumbing Vent

Unclogging a plumbing vent is quite easy, if you are not afraid of heights. Since you will need to climb to the roof of your house, it is important to make sure that you are working safely, by placing the ladder on a level surface, and at and angle to the house.

It is also a good idea to have someone hold down the ladder for you as you climb to the roof of the building. Unclogging a vent stack actually is better done by 2 people.

To unclog a vent, you will need a garden hose, plumber’s snake, screwdriver and flashlight. Here is how to proceed:

  • Climb to the roof of the house using a ladder.
  • Locate the vent. In newer houses the vent is made of a 3-inch PVC pipe but in older houses it could be made of steel.
  • Remove the cap at the top of the vent using the screwdriver.
  • Using the flashlight, try to see if you can see the clog from the top of the vent.
  • Insert the garden hose in the vent until you encounter the clog. Probe the clog with the garden hose, in a bid to break it down. Have someone turn on the water to the garden hose to help dislodge and wash down the clog.
  • If the clog won’t budge, upgrade to the plumber’s snake. Feed the cable down the vent until you encounter resistance. Lock the cable and start cranking the handle. When the cable is really tight and even starts to twist, you have grabbed the clog. Pull the cable straight out and remove the clog. Push the cable down the vent once again just to be sure there are no more clogs.
  • Have someone flush a toilet in the house. If the toilet is flushes strongly without bubbling and gurgling noises from the bathtub drain then the plumbing vent is unclogged.
  • Put the vent cap back on and climb down safely.

If you would like to watch a You Tube video on how to clear a clogged plumbing vent then find it here.

And that is how to unclog a plumbing vent. If your plumbing vent is not clogged but the problems remains, you either have a clogged sewer line, or the drains are not vented properly.

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