Photo of tranite cement-asbestos material used for air ducts in a slab over a sewer pipe (C) Daniel Friedman and Conrad HVAC Duct Sewer Odors Diagnosis
How sewage or septic smells may originate in or be transported by building air conditioning & heating ducts

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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.

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Septic & Sewer Gases & Odors from Ductwork

Photo of tranite cement-asbestos material used for air ducts in a slab over a sewer pipe (C) Daniel Friedman and ConradReader 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 buiding. 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,
  • 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.

  • 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 HVAC Ductwork: 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
  • 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 [METHANE 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).

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.

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.


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

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


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.


Continue reading at SEWER GAS ODOR REMEDIES or select a topic from the More Reading links shown below.

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