Sewer Smell When Running Bathroom Fan

While the principal function of a bathroom fan is to remove hot and moist air from the bathroom, most people agree that odor removal is still a very important secondary function of the system. So, when your bathroom fan runs and a sewer smell subsequently pervades your bathroom, something must be wrong.

Below I present you with several reasons why running your bathroom fan may be associated with a sewer smell. Perhaps one will stand out as the obvious cause in your situation, but if not, you still have a good place to start. Then, as causes without solutions are only half as helpful, we cover solutions as well.

The sewer smell when running the bathroom fan is unlikely to come from the fan itself. Instead, turning on the fan pulls the smell into the bathroom from drains, toilets, and windows due to blockages or incorrect plumbing or venting.

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The Smell Is Not Coming From Your Fan

Your bathroom exhaust fan is part of your home’s ventilation (although it is separate to the HVAC system—it’s classed as local ventilation). The fan is not part of the home plumbing system, so unless something has gone horribly wrong, it will not be the source of the sewer smell.

Instead, the smell is most likely coming from some part of your plumbing system.

I won’t go into too much detail on how a bathroom exhaust fan works; all you need to know for addressing a sewer smell issue is that the fan extracts air from the bathroom, creating a negative pressure gradient (you can also call it a vacuum).

This is resolved naturally by the inflowing movement of new air, mostly from outside or other rooms.

While bathroom plumbing is designed to prevent unpleasant odors from creeping their way back into the bathroom air, there are specific circumstances under which the mechanical action of the exhaust fan can pull these odors out of your drains, toilet, or vents.

Possible Causes of a Sewer Smell in the Bathroom

Blocked Drains

One of the most common reasons for the sewer smell in your bathroom is a buildup of waste and water in the pipes of basins, bathtubs, showers, and toilets.

The predominant reason for this buildup is a combination of dirt, hair, skin flakes, soap, and or waste products that can get caught on the drain stopper or stick together and collect on the inner surfaces of drainpipes.

Moreover, mold can sometimes grow within a drain, and this can also cause or contribute to blockages.

As you can imagine, the longer these materials sit stagnant in a drainpipe, the more likely they are to start rotting, growing bacteria, and stinking!

When you turn your bathroom fan on and the negative pressure system is created, then it can pull some of this foul-smelling air out of your blocked drain and into the general bathroom environment, leading to that distinct sewer smell.

9 Reasons Sinks Drain Slow Even When Not Clogged

Airtight Building Construction

Airtightness is a fundamental design feature of homes.

We humans crave control over the temperature, humidity, and airflow rate in our houses so that we can tailor these conditions to our preferences.

Airtightness also has very functional purposes, including conservation of energy, preservation of our home’s structural integrity, and promoting the good health of ourselves and our loved ones.

However, if your bathroom is so airtight that when the door is closed and the fan is running, the major sources of “new” air are your drains and toilet, then you could end up pulling sewer-smelling gases into your bathroom.

This can easily be remedied by installing a vent into the bathroom door.

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Faulty Plumbing Design

Remember when I said that bathroom plumbing is designed to trap unpleasant odors? Well, this only applies to properly plumbed bathrooms.

If your basin, bathtub, shower, or toilet have been improperly plumbed or if there was a fault in one of the plumbing components, then it may mean that these gases can more easily escape or flow the wrong way. Then, when you turn your fan on, it accelerates the upward movement of gases.

This is more likely to be your issue if you also tend to notice a sewer smell after the bathroom door has been closed for a few hours, even if the fan was not running.

Plumbing issues that can cause this include the following:

A leak in bathroom fixture water traps. The water trap is the bent pipe at the fixture-end, e.g., the U-shaped pipe beneath a basin. Its function is to hold some water, which blocks drain gases from coming up through the plughole or toilet bowl. If there’s a leak, the gases are not trapped.

Loose pipe connections. If the pipe connections are not properly secured, then air can escape at these points and enter the bathroom air.

An improperly sealed toilet. If your toilet is not secured to the drainpipe correctly with a well-fitting flange, or if the wax ring has been compromised, it can cause similar problems as a loose pipe connection. You will also likely notice a bit of leaking around your toilet if this is the case.

The wrong size plumbing vent. There are plumbing vents in your bathroom that help to keep the pressure in the pipes optimal for function. If these are the wrong size, then the pressure is affected, and the air may not move away from the open part of the fixture as it should.

Improper Positioning of Plumbing Exhaust Vent

Plumbing exhaust vents are required by the International Residential Code (IRC). According to Chapter 31, Section P3102.1 of the IRC:

“The vent system serving each building drain shall have not less than one vent pipe that extends to the outdoors.”

In Section P3103.4, the location of the vent terminal is regulated as follows:

“An open vent terminal from a drainage system shall not be located less than 4 feet (1219 mm) directly beneath any door, openable window or other air intake opening of the building or of an adjacent building, nor shall any such vent terminal be within 10 feet (3048 mm) horizontally of such an opening unless it is not less than 3 feet (914 mm) above the top of such an opening.”

If your plumbing exhaust vent has not been located according to code, then when you turn on your bathroom fan, the “fresh” air it is pulling in from outside the bathroom will carry with it a sewer smell.

Neighbor Has a Plumbing Problem

In rare cases, it might actually be your neighbor that has the plumbing issue and not you.

A faulty damper in a multiunit dwelling may result in the sewer smell actually coming through your bathroom fan while it is off or being pulled in from other vents in the system while the fan is running.

I have also read about an even rarer (and quite horrifying) case in which a person’s neighbor was the one with the plumbing problem, but the sewer smell pervaded the first person’s bathroom because there was no continuous firewall in their shared bathroom wall! You can read the full thread here.

But before you start panicking, this is a worst-case scenario and is highly unlikely to be the cause of your sewer smell.

Improper Exhaust Fan Venting

If your bathroom exhaust fan vents into the plumbing vent, then this can readily result in a sewer smell whenever you run the bathroom fan. You can read more in my article on the Reasons Why a Bathroom Fan Should Not Be Vented Into a Plumbing Vent.

How Do You Get Rid of Sewer Smells in the Bathroom?

Unclog Bathroom Drains

As I mentioned previously, a clogged drain is one of the most common issues resulting in a sewer smell when your bathroom fan is running. If you have a clogged drain, then you just need to unclog it!

To a certain extent, this is easier said than done. Some clogs can be cleared by pouring boiling water down the drain a couple of times. Others require chemicals to break down the matter. Still, others will only be dislodged with brute force.

Start with the more common techniques, like boiling water, then move onto the chemicals. However, you have to be so careful that you do not end up mixing the wrong chemicals together and creating a plumbing bomb, which will result in severe damage to your pipes and possibly even personal harm.

If you are unsure or worried, a clogged drain is not something that you have to deal with on your own—call a plumber!

Add an Air Supply Vent

If your problem is airtightness, you can consider opening the window a crack while the fan is running, but this defeats the point of airtightness.

As an alternative, you can install an air supply vent, which will pump fresh air into your bathroom so that the fan-induced negative pressure system does not have to pull air from the drains to stabilize.

This is different to an open window because you have a certain level of control over the condition of the air entering.

This video by This Old House provides a great demonstration of how an air supply vent would help you. Although their example involves a range hood in a kitchen, the principle is the same.

Correct the Plumbing Design

If your sewer smell is the result of a plumbing design problem, then you will need to address it according to what the issue is.

A leaking sink trap can be easily replaced as you can see in the below video from The Home Depot.

Loose pipe connections may just need to be tightened (all you would need is a toolbox and some elbow grease).

Replacing your toilet’s flange or wax ring is also something that a committed DIY-er can do, as you can see by the following short instructional video.

If you are hesitant to address the plumbing design issue alone (it does need to be done correctly to prevent further problems and ensure code compliance), then you can hire a professional plumber for the job.

Additionally, if you are uncertain as to what the exact problem is but you have ruled out all the alternatives, then you can also call a plumber out to do an assessment.

Relocate the Plumbing Exhaust Vent

If your plumbing exhaust vent has been installed in the incorrect location, then you will probably need to call a plumber in to do the relocation.

It will involve opening up your bathroom walls to some extent and you also want to make sure that the job is definitely done according to code the second time around!

Inspect the Building Design

If you or your plumber suspect that the problem lies in your neighbor’s bathroom, then you will need to go through the proper channels to get the repairs or alterations attended to.

This is typically a problem with multiunit dwellings, which are managed by a third party (i.e., someone other than you or your neighbor). Ventilation systems in these buildings often fall under common space, so an issue will have to be addressed through the building manager or super.

Correct the Bathroom Fan Venting

Should your problem lie in the fact that your bathroom fan is hooked up to your plumbing vents, it is in your best interests to get the fan re-vented so that the air is exhausted to the outside.

Is Sewage Smell in a Bathroom Dangerous?

The sewage smell in your bathroom is not likely to be extremely or imminently dangerous to your health.

However, the smell is caused by sewer gases and if the problem is severe, then the amount of gas that is in the bathroom air and which you can inhale can reach toxic levels.

In other cases, prolonged sewer gas exposure, even for minor leaks, can also lead to adverse health effects.

If the gases contain hydrogen sulfide, this can turn into a fatal problem!

Furthermore, there are airborne pathogens that can also be released into the bathroom air with the gases, which can lead to other health concerns.

As I said, yours is not likely to be an urgent issue, especially if the smell quickly dissipates once the fan has been turned off. But it is always best and safest practice to address the issue of a sewer smell in your bathroom as soon as possible.