How To Fix It

Why Is There a Bad Smell Coming From One Vent in The House?

When there is a bad smell coming from the vent in your house, there are quite a few things that can cause this problem. Mold, mildew and various pests might be to blame, as well as mechanical and plumbing issues.

The good news is that the majority of these smells have a distinct odor that you can easily detect and then link to a cause.

Of course, it is important to eliminate not only the actual smell but also the thing that is causing it. That’s why, at times, calling a professional team might be the best decision.

So, what exactly does the odor that is coming with the airflow remind you of?

11 Different HVAC Odors. How to Determine?

Rotten Eggs

Usually, such a smell indicates a gas leak. So, if you heat with natural gas and you started smelling rotten eggs, you should act immediately – turn off the gas, open all of your windows, call a professional, and leave the house, if you can.

Fact: natural gas is actually odorless, but a lot of brands have decided to add a special chemical with a strong smell that will warn the homeowners in case there is a leak.

And what if you can smell rotten eggs, but you do not use gas? Dead rodents that have started to rot can emit such an odor as well.

Electrical Odors

It looks like there is an issue with the wiring or the motor system. You should definitely switch the device off, in such a case, as the whole system might end up overheating.

Tip: if you haven’t been using your air conditioner or furnace for quite a while, it may have such a smell as soon as you switch it on after a break. Give the system some time and the smell will, most likely, go away in 20-30 minutes.

Rotting Garbage

Yet another odor that can indicate that your ductwork once had rodents…but now doesn’t. Needless to say, that you should try to get rid of the remains as soon as possible.

Mold (A Musty Smell)

While it might be tricky to spot the actual mold growing deep in the HVAC system, a musty smell is a great indicator of the problem. This is, perhaps, the most common smell-related problem with air conditioning systems.

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When the AC is running, the inside of the unit can develop a lot of condensation. If the moisture doesn’t get a chance to dry properly, it will cause mold growth.

Warning! The chances are high that your HVAC would still be able to work perfectly well with mold in it, but the thing affects the quality of the air you breathe. In fact, the spores that come from mold can increase the risk of respiratory infections.

Related post: Why does my air conditioner smell musty?


Just like with electrical odors, it is absolutely normal to hear a burning smell, if you have switched the furnace on for the first time in more than a few months. The dust that has gathered on the surfaces of the device will burn off in a matter of minutes, so the smell will soon go away.

Warning! There is a difference between a slight burning smell and a unit that smells like there might be an overheating issue. In such a case, turn the system off right away and wait for an HVAC professional to arrive.

Dirty Socks/Stinky Feet

Such smells indicate the presence of mold and mildew. If you have found an area in your house where the odor is stronger, then you might have located the source.

You can try using a clean cloth to wipe a part of the ductwork that you have access to. If you notice patches of mildew and mold on the cloth, then calling a professional might be the only option.

Tip: check the ductwork in your attic, crawlspace, and any other unconditioned areas – these are the parts of the system that are most prone to mold growth.


This one is pretty much straightforward. If there is a smell of raw sewage coming from the air ducts, then the chances are high that a crack in the plumbing pipe has caused the smell.

Fresh-Struck Matches/Sulfur

A mechanical failure inside of the heater might be to blame. However, if your house had undergone renovations anywhere between 2001 and 2009, then the contractor might have used the infamous Chinese drywall. After people started reporting concerns, various tests were conducted. The analysis revealed that plenty of drywall samples imported from China during those years have sulfur in them.


The first thing that you need to do is look for an oil leak. You might have to simply tighten a fitting in a certain area, in order to fix the problem.

If there is no leak, then your oil burner might not be working the way it should be. This may be because of a bad fuel pump or a clogged burner. Anyway, you should let a professional technician take care of the problem.


If you smell urine, it is, most likely, because rodents have made your ductwork their home. Of course, mice do defecate and urinate in places where they nest. If it’s winter, then you might find them living around the air handler.


Does it smell like someone is removing their nail polish? It might be because there is a refrigerant leak in your HVAC system. Older units and the ones that have not been properly installed can develop such a problem.

Of course, such a leak will decrease the system’s efficiency, but the substance is also extremely flammable, so you should definitely call a technician to deal with the leak as soon as possible.

How to Deal with The Bad Smell Coming from Your Vents?

In a nutshell, your vents are not going to have a bad smell if the ductwork is clean and is functioning well.

Of course, the unit has to undergo regular maintenance and you should try to keep the areas that you can reach nice and clean.

Clean the vent and duct cover

You can use warm water and soap to clean the cover. A scrubbing tool or a brush can come in handy if there is more persistent residue. To go the extra mile, wipe down the actual duct.

You can also use a vacuum cleaner on the cover and the duct passage. Try to vacuum up any webs, dust, and dirt that you will see.

Related post: Duct cleaning after renovation. Do I need it?

Clean the evaporator coil on AC

This part of the system is responsible for much of the cooling process. But it tends to get dirty and smelly quite fast.

To clean the evaporator coil, remove the access panel, vacuum up the area, and spray-clean the coils. You can buy a special cleaner or go for a DIY option – a 10% bleach solution in water will do the job.

Use baking soda in the ductwork

This is an easy way to cover up the annoying smells. Simply place the baking soda in the duct, before putting the cover back on. Put the substance in a bowl or a special box made for refrigerators.

Soda will not deal with the root cause of the problem, but it can make the smell a bit more bearable while you’ll waiting for a professional to eliminate the actual issue.

Change the filters

There might be something stinky stuck on the filter. See if a fresh filter will make a difference. And don’t forget to change it every few months.

Use duct filter pads

You can install duct filter pads. Even though the filter pads are usually used in commercial buildings, schools, manufacturing sites, and so on, you can take advantage of them, as well.

The pads capture airborne contaminants and will not only make the smell less annoying but will also protect your HVAC system from the particles.

Call a professional

Unfortunately, in the absolute majority of cases, you will be able to only deodorize the smell. If you want to get rid of the odor for good, you should consider hiring a professional.

The expert will find the root cause of the problem and eliminate it with the help of specialized tools.

To Sum Up

Why there is a bad smell coming from one vent in house?

Now you know that various things can cause different odors.

  • A gas leak can smell like rotten eggs.
  • Dead rodents can also smell like rotten eggs or rotting garbage. If the creatures are still alive, then you might notice a urine smell coming from the vent.
  • If the unit has problems with the wiring or motor system, then you’ll detect electrical odors or a burning smell in the house.
  • Smells like dirty socks? Mildew and mold might be to blame. They can also have a distinct musty smell.

Of course, dealing with these causes on your own might be too challenging. So, in a lot of cases, hiring a team of professionals that is going to clean your HVAC system is the best possible solution.