Sewer or septic gases through conduit or ducts: this article describes the detection and repair for sewer or septic gases & smells that are traced to passage through electrical conduit, open piping, or HVAC duct systems.

We describe how odors may be traced back to electrical wiring, conduits, or panels & sub panels, and how people stop that odor passage.

Septic & Sewer Gases & Odors from Ductwork

Reader Question: Question: There is a Septic Tank Odor Coming from a Vent Under My Air Conditioner: Who do I Call?

I live in a condo on the second floor, of three floors. I often smell a septic tank type of odor coming from a vent under the air conditioner. When I opened the vent there is nothing but open space.

When I go into the bathroom and open the cabinet under the bathroom sink I could smell the same smell.

The bathroom sink and the air conditioner and vent share a common wall. They are next to each other only separated by a wall. I don't know who to call to help fix the problem. I'm not sure if it's a plumbing problem or if it is an air conditioning problem. HELP!!!! - N.A.

Reply: Air Conditioners Themselves Don't Produce Sewer Gas but They Might Move Gases and Smells Around in a Building

A competent onsite inspection by an expert usually finds additional clues that help accurately track down an odor or smell problem. That said, here are some things to consider:

• A nearby odor source (bad drain line or leaky plumbing vent pipe inside the building wall) may be mistaken as coming from the air condtioning system, or the odor might be increased when the A/C is running, due to air movement or room air pressure changes.
• Air conditioning equipment itself would not produce a septic odor but the system might pick up and redistribute an odor or gas from somewhere else, or in certain conditions negative air pressure in a room might cause sewer gas to backdraft out of a drain system. Air conditioning refrigerants are odorless.
• Air Ducts Draw In Gases: We have found instances of drain pipe leaks or plumbing vent leaks that happened to be near an air intake return register at central heating or central air conditioning systems.
  
  The air handler easily picked up a smell from one source and blew it into another part of the building. But usually, if the odor is being "relocated" by the HVAC system, the building or a floor in the building are all using a common HVAC duct system, the smell will appear in multiple rooms, smelling strongest at the room closest to the air handler blower unit.
  
  Other examples of how sewer gases can be picked up and transported for distances inside of ductwork-like conduits,
  
  see SEWER SEPTIC GAS CONDUIT LEAKS

• Other Septic Tank-Like Odors: A less obvious connection between air conditioning systems and sulphur or septic like smells is the Chinese Drywall outgassing problem we discuss
  
  at CHINESE DRYWALL HAZARDS. Below we also warn about very dangerous natural gas or LP gas leaks.
  
  Also see SULPHUR & SEWER GAS SMELL SOURCES.
• Sewer vs Septic Tank Odors: You didn't say whether or not your property is served by municipal sewer or a private septic tank. But there could be a "septic tank" odor at a property even where there is no septic tank - since sewer gases from the building drain-waste-vent piping system, leaks at toilets, or backups from a municipal sewer produce very similar gases and smells as a private septic tank.
• Sewer pipe leaks can send gases into the HVAC duct system, as we illustrate
  
  at SEWER GAS ODORS in HEATING or COOLING DUCTS: sewage gas may appear in HVAC ductwork, being picked up from a plumbing drain or waste line gas leak near a building air return, or sewer gas may be entering in-floor-slab ductwork from a nearby leaky sewer or septic drain line. More illustrations of this leaky sewer line that sent sewer gases into the building's heating duct system can be seen
  
  at TRANSITE PIPE AIR DUCTS
  and
  
  at CAST IRON DRAIN PIPING.
• Whom are you Gonna Call? - Stink busters!: You should probably start with two approaches: a general inspection outdoors and around the building for obvious odors or likely odor sources.
  
  Inside the building, ask an experienced plumber to take a look at your building drain/waste/vent piping, pressure test it if necessary, and also check for a loose or leaky toilet.
• Our odor track-down suggestions in this article, and just below
  
  at FIRST STEPS in Septic or Sewer SMELL DIAGNOSIS might be helpful. Let me know what you're finding and we can pursue this further.
• Other methane gas sources: you may also want to check
  
  METHANE GAS SOURCES - other sources of methane gas in and around buildings, and Depending on the sewer gas source and other factors such as humidity and building and weather conditions, mold spores may also be present in sewer gases
  
  See METHANE & SEWER GAS HAZARDS

Watch out: we warn in all sewer or septic gas odor articles that because sewer gas contains methane gas (CH₄) there is a risk of an explosion hazard or even fatal asphyxiation.

Watch out: also sewer gases also probably contain hydrogen sulfide gas (H₂S). In addition some writers opine that there are possible health hazards from sewer gas exposure, such as a bacterial infection of the sinuses (which can occur due to any sinus irritation).

See HYDROGEN SULFIDE GAS

Also see ODORS in AIR HANDLERS & DUCT WORK for other sources of HVAC air duct odors & smells & how these are tracked to their source & cured.

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-11-30 by (mod) - other possible sources of sewage smell when AC is running

Defective or missing condensate drain trap

On 2020-11-30 by Denise Kiewel

There is a sewage smell that only occurs when the AC is running...not the heater. A plumber came and smoke tested the plumbing and there were no issues. What else could it be?

On 2020-11-25 - by (mod) -

Anon

YOu might want to start with a check that the toilet is not loose

AND

that it remains properly sealed to the waste pipe flange.

I AM MAKING THIS EXAMPLE UP: but suppose someone removed a toilet in order to tile a bathroom floor. When the toilet is re-installed, a new wax ring is needed to be sure that the toilet will have enough material thickness (including that added height above the original floor) to seal to the waste pipe flange.

If a tiler, who doesn't know plumbing, just left the old wax ring in place, maybe smushed around and stuck on the bottom of the toilet or stuck to the waste flange, THEN when the toilet is bolted back down it's higher off the original floor by the thickness of the new floor tile job.

And it'll leak or smell.

On 2020-11-25 by Anonymous

We had new roof installed around the same time we tiled our bathroom. We had no issue before but ever since then, we have this strong smell, a mildew / sewer like smell that starts in the bathroom and spreads through the A/C would we need to call a plumber or A/C technician?

On 2020-06-14 - by (mod) -

Anon:

That in-slab duct is unsanitary and not entirely functional. I'd abandon it.

See details at SLAB DUCTWORK https://inspectapedia.com/aircond/Ducts_in_Slabs.php

On 2020-06-14 - by (mod) -

KL

Gee I cannot make a useful guess at when a sewage odor will dissipate from the duct system. It depends on a number of variables like indoor humidity, length of exposure to sewer gas, duct materials (fiberglass vs. metal), how clean the duct interior is.

I would have the duct system inspected for mold; if it's metal it can be cleaned; else you may need to replace it.

On 2020-06-13 by KL

Sewage smell coming from AC alerted me to sewer pipe leak which since has been fixed. Smell is slowly dissipating post work but AC smell is still very strong. How long until smell is completely gone will AC stop reaking?

On 2020-01-09 by Anonymous

Need help figuring out possible slow sewage leak into heating ducts. I have lots of photos and videos. Just somebody please help. We have a house on a concrete slab. No crawl spaces. In order to see into the vent we had to rip out the whole furnace (which we did already see added photo.)

We got a new furnace installed. The last 3 months, we have been trying to figure out why water has been pooling in a specific vent. Fruit flies or sewage flies have been breeding down there as well.

On 2019-12-18 by Elaine

I have one kitchen heating vent (forced air gas) randomly emitting sewer smell. It's about 15 feet away from powder room, 5 feet away from sink and dishwasher. I've tested whether it happens when I flush toilet, use sink, use dishwasher...can't determine it.

Happens using heat or A/C. Sometimes mild sometimes horrific. Happens sometimes daily, sometimes weekly, sometimes monthly for 12 years now. Not rotting animal smell. Help. Me. Please.

On 2019-10-19 by (mod) -

Do you have gas heat? If so it may be leaking and unsafe.

On 2019-10-18 by addie

Why do I get sewage smell when AC is on but not when Heater is on... ducts are clean. Home on on slab and ducts are in ground.

On 2019-07-03 by (mod) - odor from dead heat pump

If your duct system is picking up a sewer gas odor from a sewer line or other nearby odor Source I don't see the changing the heat pump itself would affect that at all. Watch out: sewer gas is potentially unhealthy and also explosive. You need to find the source and corrected promptly.

On 2019-06-26 by Anonymous

@Tommy, Our heat pump died and is not running. Odor is worse now. Having unit replaced, but doubt the odor will go away once unit is replaced

On 2019-06-26 by Tommy

Single family home has just one duct that emits odor (like rotting flesh or sewage). We are on a septic tank, but no odors coming from sink, showers, or toilets. I'm thinking maybe an animal died in this one section of duct work. No other ducts give off this odor. Any ideas on what to do?

On 2019-01-31 by (mod) - sewer odors in Hud subsidized apartment

Carolyn

It sounds to me as if the building's plumbing vent system may not be working, sending potentially dangerous (explosive) sewer gases into the building. If that's the case it could be a serious hazard and something worth bringing to the attention of the landlord.

Plumbers - real ones - know how to pressure-test the vent system to see if it's leaking, as well as to track down plumbing vent or sewer vent piping leaks.

You or the landlord might also ask for help from the health department.

On 2019-01-31 by Carolyn Johnson

I live in a Hud subsidize apartment I've had a terrible odor emitting from the toilet. The landlord finally had the wax ring replaced but it did not eliminate the odor entirely by replacing the ring the smell is not as strong.

The problem now is the smell is coming through the central air vents especially when using heat. Plumbers were sent out to I thought to find the cause or make an effort to find the cause

. Their reason for coming was just to replace the wax ring which had already been done. When I told them the smell was coming through the vents, " that is not what we do,

He needs to get someone else" I don't know but something is irritating my nose and throat. I feel sick along with headaches. Is there a test to determine if my apt is yet contaminated? Who can I contact to address this issue, track down an odor or smell? I would appreciate your response. Thank you C. Johnson

On 2018-09-26 y Greg

I think it is sewer gas, but could be a dead rodent or something. But it still comes and goes. I don't know how to nail down quite what the smell is. But if it is sewer gas, I really want it found. We hired a new house cleaner and she asked if there was a sewer gas problem.

Will have another look for open traps. We bought the house some months ago, but there was extensive remodel a few years ago and I'm wondering if there's an open pipe or damaged vent somewhere in the walls. If it is an open trap, how is that odor getting into the AC?

There is an area under the air handler (where the return air comes together) with an access door. When the smell is happening in the kitchen and I stop the air and put my nose in the access door, I don't sense a strong smell. Nothing like the smell at the output vent in the kitchen. So I'm wondering if there's some way to isolate this to the output side of the system.

If I engage a plumber, will they all have the ability to do vent pressure tests, etc?

Thanks again, Greg.

Greg

On 2018-09-26 16:31:28.621836 by (mod) -

If you're pretty sure it's a sewer gas and if you've checked the obvious sources then yes a plumber can look for leaks in the vent system or other issues.

Be sure you checked around the air returns for a dry plumbing trap or drain or sewer line leaks.

Also remember that there can be other gas leaks -LP gas, propane, or sewer gas leaks are all dangerous.

On 2018-09-26 by Greg

Thanks for the tip. There are two air returns. When it happens again, I'll try blocking one return at a time to see if I can isolate to one of them.

Then I guess try for a plumber for more help? Any questions I should ask a plumber to see if he's the right person for this job?

Thanks again, Greg.

On 2018-09-26 by (mod) -

In addition to inspecting the entire length of the duct system with a appropriate camera, you probably want to look at the air returns to see if you can be picking up sewer gases at that location or those locations.

On 2018-09-25 by Greg

I have a 2 story house on slab with 3 HVAC systems (near Phoenix, so gets hot). One of the HVAC systems with the blower/heat-exchanger portion in the garage occasionally sends out noxious air. I think it smells like sewer gas. This side of the house in this zone has the kitchen and family room and one half-bathroom in the HVAC zone.

I've tried using my nose to find the real source of the smell, like under the kitchen sink, dishwasher, toilet, but don't find anything like the sewer smell that comes out of the vents. The three output vents are high in the air, so it's hard to tell if all of them give off the same smell, but I think they do. The one closest to the ground definitely has a pungent odor, when the odor occurs.

That's another thing - the smell comes and goes. I've tried to figure out if it happens when there is a lot of toilet use, shower use, dishwasher or laundry, but none of that really rings true. It seems to happen more in the afternoon, so thinking that warmer weather is the key, but in this area, it can be hot most of the day, although sunlight gets on the west side (the side in question) of the house, so perhaps that's a factor.

One air return is directly in the floor and seems to be buried in the slab. I stuck a camera down there that I bought on amazon and snaked it down the duct about 20 feet or so. It seemed dusty but no signs of water that I could see.

I also ran the camera from the air handler intake out under the house, but was less successful getting it to snake any distance. It also looked dusty but not otherwise noteworthy. No signs of water, past or present.

I ran the camera from the smelly air vent as well. It is a flexible hose for part of the run and then joins a solid duct. I ran for about 10 or 15 feet and didn't see anything. Was hoping to find a dead rat or something that might explain the smell.

What do you think the next logical step is to find the source of the smell? Should I get a plumber? Or?

Thanks, Greg.

Question: Sewage ejector pump system causing sewage odor picked up by furnace air intake

I have a septic system. I also have an ejector sump pump and pit that receive drain water from my master bathroom sinks and bathtub. This water is ejected through a 2” check valve into a 4” drain that goes to the septic tank.

I also have 2 in the ground basement catch basins flush with the basement floor and hooked up in tandem. They receive ground water from around, and under the house. The water is discharged into a common 2” line to outside the house where it increases to 4” and runs under ground for a long distance to a holding area where it is discharged. All that goes through these lines is ground water, no septic.

I sometimes have a septic odor coming from these 2 tanks when the tanks are emptied. The odor is then picked up by my furnace intake air duct and distributed throughout the house, and the bathroom toilet water also has odor problems. We have been putting chlorine bleach into the sumps while they are stagnant and not in operation, and this has helped, but I was wondering if you have any ideas as to how to eliminate these odors? - T.B.

Reply:

TB something is very wrong, unpleasant and even potentially dangerous if your warm air heating system is picking up sewer gases. I'm guessing that you are referring to smells around the sewage ejector pump tanks, not any outdoor components.

I've inspected quite a few sewage ejector pump installations. Where sewage odors were present I usually find as the cause one of the following [of course there could be a different defect in any case]:

• There is a drain or ejector pump tank leak that leaks sewage or effluent around the tank or even under the tank and into the space below the floor slab, sometimes entering a nearby basement de-watering drain and sump system
• There is a defective check valve at the sewage ejector pump tank and piping system, backflowing and even overflowing or overloading the ejector pump system (look for too-frequent ejector pump motor cycling on and off)
• There is a defect or blockage in the building vent piping system that is failing to properly vent sewer gases above the building.

See details about sewage ejector pump inspection, diagnosis, repair and maintenance

at SEWAGE EJECTOR / GRINDER PUMPS

Question: rotten egg smell from bathrooms and HVAC air handler

We are currently renting a condo and we have only been here a month. We started smelling a rotten egg smell coming from the bathrooms and the air handler.

We had the Fire department come out and they did not take two steps and they could smell what we were smelling and immediately said that it was sewer gas. Now we have had the wax rings changed in all the bathrooms and we still are experiencing this smell. Are there any gas experts in Jacksonville, Fl. ? - Helpless in Jacksonville

Reply:

Helpless in Jacksonville:

Local home inspectors equipped with a TIF8800 combustible gas analyzer or industrial hygienists who have experience in residential plumbing problem solving can look at this, but frankly I'd start with an experienced plumber. Could be you have a venting problem, clogged drain, blocked plumbing vent, or even open pipe connections in the walls or ceilings.

Notify your landlord in orally and in writing of these problems immediately because there are potential health and worse, explosion hazards if it's really sewer gas.

...

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