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Why Does My Toilet Bubble When the Tub Drains? Reasons & Prevention

water bubbling in toilet

A fully functional plumbing system is essential in every household. However, plumbing-related issues often occur, and most people don’t even know how to diagnose or fix them. One such issue is bubbling in the toilet after you drain your tub.

The gurgling or bubbling phenomena may be common in most households; however, you should be concerned when your toilet starts making weird noises after you drain your bathtub. This is usually a sign of a more serious underlying issue. Often, the bubbling effect is caused by air in the drain, issues with the P trap, and air coming out of the drain. So, when air that should be going to the vent comes up through the toilet drain, bubbles will be present in your toilet bowl.

Depending on how serious the gurgling sound is, you can troubleshoot this issue on your own. But first, you have to find out if the bubbling effect is present in one particular toilet or multiple fixtures. If only present in one toilet, this indicates clogging in a specific drain. However, if gurgling is present in multiple bathroom fixtures, your main sewer line is probably blocked. You might also experience water-backups in the tub when your septic tank is full.

No matter the situation, this article will guide you on how to diagnose and get rid of bubbles in your toilet. We will also look into the main reasons why your toilet is bubbling when you drain your bathtub. Read on to learn more.

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Reasons Your Toilet Bubbles When You Drain the Bathtub

Bubbling in the toilet is usually a result of wet venting. All states have plumbing codes that require all the drains in a house to be vented separately. This helps release sewer gasses outside as well as reduce pressure build-up in the drain. But when bathroom fixtures are too close to each other, one vent may handle multiple drains, including the sink, toilet, and tub.

So, when there is clogging in this system, air will enter and exit through one fixture while using the other. This will end up creating a gurgling effect when the tub, toilet, or sink drains waste water. Also, the bathroom fixtures are likely to drain slower when clogging occurs in a wet vented configuration.

bathtub with water
Photo Credit By: ivan_kislitsin, Shutterstock

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Other Reasons

As earlier mentioned, the main reason the toilet gurgles is because the air is leaving the toilet drain due to the pressure difference from your tub. So, are there other reasons for the pressure difference in the plumbing system other than a clogged vent? Indeed, there are.

Bad Drain Pipes

If your plumbing system is connected with bad pipes, you will inevitably experience toilet issues over time. Toilet gurgling is one such problem. Most pipes in a drainage system are made from PVC materials. These pipes should also be an appropriate size for the efficient disposal of wastewater from the house fixtures. Yet, if you under or over-approximate the pipe size, air will get stuck in the pipes and create low pressure.

In such a case, pressure will be insufficient to push down the air and water through the pipes. Eventually, the entire plumbing system will collapse. The pipe size is also an important factor because a lot of wastewater will flow through the tub drain, which can cause air to leave the drainage system through your toilet bowl.

If you have narrow pipes that are too small, water flow through them will be stronger and faster due to the high pressure created in the small drain pipes. This will also prompt air to be released inside the toilet, hence the bubbling phenomena.

Poor Pipe Connection

In case you have appropriately sized pipes but still experience gurgling in your toilet, chances are that there is a poor connection between the pipes. This connection is normally made between the toilet and bathtub to the drain pipe that directs waste water down to the sewer system. So, if the bathtub and toilet pipe are loosely connected, your toilet will experience a bubbling effect.

This can be attributed to the fact that a bathtub is the lowest fixture in a bathroom. The toilet follows, and finally, the sink, which is the highest fixture you can find in most bathrooms. Negative air pressure is particularly prevalent when the drainpipe connection between the toilet and bathtub is significantly shorter.

plumber repairing toilet flush
Photo Credit By: Africa Studio, Shutterstock

Poorly Installed Bathtub Drain

The bathtub usually drains water faster than the sink or toilet. This simply means that the bathtub will be pumping water at a higher pressure which can lead to bubbling inside your toilet bowl. Another common reason for gurgling is the installation of a faulty bathtub drain.

Many homeowners usually install a bathtub drain as an afterthought. What this means is that most often, the tub drain is usually connected to the nearest waste pipe in the bathroom. When homeowners neglect to install a P trap, water will be sucked away from the bathtub when it drains. Another common reason is using a too-short or too-tall standpipe in the tub drain.

If there isn’t enough air pressure in the pipes to drain all the wastewater, the bathtub will likely borrow air from the toilet to help push water down the sewer system. Consequently, this will create a bubbling effect in the toilet. Gurgling from a poorly installed bathtub drain will likely cause more problems in the future. This includes overflow issues and sending wastewater back into the bathtub.

Toilet and Bathtub Are Connected in the Same Vent Stack

Some plumbing systems are installed in a way that the bathtub and toilet are connected to a single vertical vent stack. This is a common reason why pressure builds up, causing a bubbling sound in the toilet bowl when the bathtub drains.

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How to Prevent Your Toilet from Gurgling When the Tub Drains

The following are possible solutions to bubbling issues in the toilet. The fix you will need to implement will depend on what brought on the issue in the first place.

Plunge the Toilet

If you want to stop your toilet from gurgling after draining the tub, you need to remove everything that may be blocking the pipes. This will allow all the wastewater to drain freely and flow into the intended destination. To do this, you can simply start by plunging into your toilet. This solution works to clear out the partial or moderate blockage.

However, before you plunge into your toilet, you should make sure that the drains are first tightly sealed off. You can use duct tape to seal the drain but make sure to put stoppers in the sink and bathtub. If you forget to put the tub and sink stoppers, you will likely have to deal with a bigger mess than anticipated.

Once you have ascertained the drains are sealed and the stoppers have been placed, plunge the toilet to loosen any clogging in the ventilation system. Make sure that the end of the plunger is completely covered by the water in the toilet bowl for optimum results.

Repeat the process for a few minutes then flush the toilet to confirm whether the bubbling effect is still present after you drain your tub. If gurgling persists, plunge the toilet a few more times until all obstructions clear from the drains.

Clean the Ventilation System

If plunging doesn’t clear out debris from the drains, check whether there are obstructions in the plumbing vent lines. Usually, vent issues often occur near the main stack exit on the roof.

It is fairly common in many households for bubbling to occur in the toilet due to debris falling inside the vent stuck opening. These obstructions often collect in the elbow system, barring air from being released into the environment. On rare occasions, even rodents and birds get into the vents and build nests.

To fix this problem, you’ll have to clear out all the debris from the vent stack. A plumbing snake or a garden hose should help clean the ventilation system. Just make sure they are long enough to access the debris obstructing the vent stack located further down the ventilation pipe.

In addition, the covering on the vent stack might be fully obstructed by dust particles and grime. In such a case, you have to clean the vent screen to resolve ventilation issues.

plumber working with toilet leak
Image Credit: New Africa, Shutterstock

Change the Pipe Diameter

If the bubbling phenomenon is a result of inappropriately sized drain pipes, you might have to change the pipe diameter. This may seem like a daunting and expensive solution, especially if you lack the necessary plumbing experience, but it is worth the trouble. Fortunately, you can easily purchase different pipe sizes from your local hardware store. However, you should consult a certified plumber to assess whether the bubbling in your toilet bowl was caused by inappropriately sized drain pipes.

Consult Your Neighbors

Sometimes, the bubbling issue in your toilet may originate from the community line shared by neighbors instead of an individual home. In such a situation, it is best to talk to your neighbors to find out if they are experiencing the same issue in their toilets.

If the issue is common among your neighbors, you can report the problem to your municipal sewer authority. They will send out one of their professional plumbers to assess the situation. If the issue is present, they will fix it without incurring costs to you or your neighbors.

Fix Your Tubs Drain Pipe

If the gurgling noise stems from your bathtub drain pipe, it is pertinent that you fix it. The bathtub drain usually gets clogged the most in bathrooms, creating toilet bubbling issues. So, you will need to clear out soap build-ups and hair that got stuck inside the tub drain.

For diagnosis purposes, check if the tub drains slower than expected. If the problem is present, consider installing a P-trap in your bathtub drain before joining it to the pipe that leads to the drain. You can also install an air admittance valve near the P-trap to compensate for insufficient venting. However, you should first make sure that local plumbing regulations allow it.

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When you hear a bubbling sound coming from inside your toilet bowl, chances are high that there is a problem with your plumbing system. Therefore, you should fix it as soon as possible. The bubbling sound is usually caused by obstructions in the vent stack, lack of a P-trap, and issues in the drain pipes.

To get rid of the annoying bubbling after flushing, you first have to determine the root cause. Then, proceed to apply the fixes we have recommended such as plunging for moderate clogs and changing the diameter of your pipes if the ones used create issues in your toilet.

If the bubbling issue is experienced by your neighbors too, report the problem to your local municipal sewer authority who will send someone over to fix it. If the clogging is more pronounced, hire a professional plumber who will recommend the best solution depending on the root cause.

See Also: There’s a Worm in My Toilet, What’s Going On?

Featured Image Credit: Irik Bik, Shutterstock


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