Why Is There a Sewer Smell When My Bathroom Fan Is On?


Bathroom plumbing can be a bit of a mystery sometimes. And when you turn on your bathroom fan and it smells like a sewer, that’s a mystery that definitely needs to be solved.

A sewer smell isn’t the outcome you’d normally expect when you turn your bathroom fan on, so what could the cause be?

If there is a sewer smell when your bathroom fan turns on, the problem is not with the fan. The negative pressure caused in the bathroom by the fan extracting air from the room is causing odors from a waste pipe to be pulled into the bathroom and can indicate a problem with one of these outlets.

The role of the bathroom fan is to draw air and moisture out of the bathroom, so why would the fan cause a bad smell in the bathroom?

There are several causes for a bathroom fan resulting in this unpleasant effect, and the fault is not with the fan.

modern bathroom

Does Your Bathroom Fan Cause A Sewer Smell When It is On?

A bathroom fan is necessary for most bathrooms, especially bathrooms with no windows and poor airflow.

Although a bathroom fan does extract bad odors from your bathroom, that is not its primary function. The main role of the bathroom fan is to extract moisture from the room that would otherwise result in the development of mold in the bathroom.

The good news is that the fan itself is not producing the sewer smell in the bathroom, but rather it is giving you a “heads-up” that there is a problem with your plumbing.

How to Find The Cause of the Problem

To solve this stinky bathroom mystery, you need some understanding of your bathroom plumbing to eliminate suspects.

Bathrooms have two types of pipes; inlet pipes and waste outlet pipes.

We know the problem does not lie with the inlet pipes because this system brings clean water into the bathroom.

The problem, therefore, must lie with one of the waste outlet pipes.

Causes For The Sewer Smell In Your Bathroom

We have eliminated the input pipes as a cause for problem odors in your bathroom, so now we need to look at the waste outlet system.

Waste Outlet Pipes

Depending on your bathroom configuration, you have two, three, or four waste outlet pipes.

The bath, shower, hand basin, toilet, and bidet (if you have one) will each have a waste outlet pipe to take dirty water out of the bathroom and into the sewer system.

The waste pipes attached to these bathroom fixtures have a P-Trap pipe with a U-shaped bend.

The purpose of the U-bend is to trap a section of water in the bottom of the pipe to prevent sewer-smelling air and gasses from entering your bathroom through the waste pipes.

The P-Trap keeps the bad smells in the waste pipe in the pipe system and allows it to vent through the vent pipe that is usually above the roof of your home.

This system generally works well to keep the odors at bay, so if you are smelling unpleasant sewage smells, one of the waste pipes P-Traps is not functioning properly. The cause is usually a blockage of some sort, a damaged seal, or a broken pipe.

When the bathroom fan kicks in and extracts moist air from the bathroom, it creates a negative air pressure in the room.

If the P-Trap on one of the water pipes is not working correctly, the negative air pressure can pull foul-smelling air from the waste pipes into the room.

It is important to give this problem attention since the odors not only smell bad, but the associated gases can pose a health risk.

Solving The Sewer Smell Problem In Your Bathroom

Before you can solve the sewer smell in your bathroom, you need to eliminate more of the suspects.

This will require searching for some evidence that the culprit leaves behind.

Your nose is the best detection tool in this case, so if you get your nose close to the bath, shower, and basin waste plug, you should be able to detect where the smell is strongest.

You do not need to get your nose close to the toilet; there will be other clues to find the cause.

If the toilet bowl water rises too high when you flush, it can indicate a clogged pipe or blockage, or if there is water on the floor behind the toilet, it could indicate a cracked seal or pipe.

Once you have found the offending waste pipe, you need to establish the final piece of the mystery.

bathroom drain

Is the Pipe Blocked, or is it Broken?

  • Waste pipe blockage. A blockage in the P-Trap of the waste pipe can be the source of the smell. The blockage may be on the bathroom side of the pipe rather than the waste side, allowing smells to emanate from the pipe.
  • A cracked waste pipe. A waste pipe that is cracked in the waste side of the P-Trap will allow smells to pass through the pipe at the crack. If the crack is at the P-trap, the water could drain from this part o the pipe, allowing smells from the waste system to enter the bathroom.
  • Broken or loose seals. This is especially a problem on toilets, where the seal around the P-Trap may have deteriorated, allowing odors and gases to pass through the seal.
man holding pipe in bathroom

How To Resolve A Sewer Smell In Your Bathroom

Resolving the problem of a sewer smell will depend on the cause of the problem.

If the problem is a blockage, the blockage must be cleared, and if the problem is a cracked or damaged pipe or seal, it needs to be replaced.

If you consider yourself a DIY type of person, you can tackle many of these problems yourself.

Many of the blockages in shower, bath, and basin waste pipes are caused by hair buildup, which accumulates in the P-Trap and eventually blocks the pipe.

This can be cleared with a plunger or a pressurized drain cleaner canister, or a drain cleaner designed to dissolve this type of blockage.

If the P-Trap pipe on the waste pipe of the bath, shower, or basin is easily accessible, it is normally quite an easy DIY job to replace this section of pipe.

Below is a video that shows you how to do this:

If you suspect that the issue is a blocked toilet, you can try using liquid to unclog your toilet or a good old-fashioned plunger.

However, if that doesn’t work, it may be a more involved job (translate that as messy and unpleasant) and is best left to a plumber.

Of course, if you do not consider yourself a DIYer of any shape or form, then you can call a plumber to sort out these issues in your bathroom.

If you can at least do some of the diagnosis parts of the process, it could save some time for the plumber, resulting in some savings on costs to repair the problem.


If you are experiencing a sewer smell in your bathroom when the fan switches on, you need to know that this is not normal.

It is usually an indication of a problem with one of the waste pipes in the bathroom, and you should give it attention as soon as possible.

The fan is not normally the issue in this case, but it indicates where the problem lies in the bathroom.

Leaving the problem can result in an escalating situation that could cost more to repair, and the noxious gases leaking into your bathroom from the waste pipes can become a health hazard.

If you are uncertain how to proceed with resolving the problem, you should contact a local plumber to conduct the repairs.

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