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Sewer smell in basement

jtlloyd | Posted in General Questions on

We recently finished our walk out basement. In the process of finishing the basement we added a new bathroom and laundry. So the plumbing contractor installed new drain lines and repaired a collapsed floor drain. Air admittance valves were installed.

My question is can these valves fail. Because there are 3 installed, one in the laundry room, one in the bathroom under the sink and another at the intersection of the main waste line and new toilet waste line in the utility area by the furnace. Since these valves were installed, I now get a strong sewer smell in the utility area. However the smell is only present when the air conditioning has been running for extended periods of time. The rest of the time, there is no smell, even when the furnace is running. And I would not assume it would be from the floor drain because we have a radon tight drain cover.

Any thoughts on why this is happening would be greatly appreciated.

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  1. Christopher Briley | | #1

    All the plumbing contractors I know hate to use those things (AAVs). It might be you've got some cheap ones, and a simple upgrade might fix them? OR perhaps you have something like a shower or other fixture with a trap that dose not get used in the summer and the AC helps to dry out the trap and so sewer gas can escape.

    I don't feel like I'm nailing the answer here. I'm an architect, and will defer to a mechanical engineer/contractor on this one, but I thought I'd help start the brainstorming.

  2. Garth Sproule | | #2

    Check to see if the drain line from the A-coil on the furnace is properly connected to the sewer system. It should either drain into a trap or it should have a loop in it to create a trap if it is connected directly into the sewer line...

  3. Bill Friend CHI, CMI | | #3

    What type of A/C system and how is the condensate drain line run? Yes, the Island vents (what we call them here) could be failing. An open trap, crushed or cracked waste line, missing clean out cover, are the areas I would check first. Some of the houses I have inspected in the past 21 years have installed the condensate drain straight into a sewer drain line. Some of them were installed in such a way as to allow sewer gas to run back into the air handler. Hard to guess without seeing the basement and plumbing. Could be any of the items mentioned above or a combination of a few. Good luck. Let us know when you find it.

  4. Edward D. Nikles CGP CAPS GMB CAASH CGB CGR CR | | #4

    We've encountered similar smells in the past in two other basement areas . First we have septic pump & alarm wires running in conduit from the home to septic pump tank . We have to caulk between the wires & the conduit or the sewer gas smell will back up into the basement . Second we have had smell coming out from around the laundry tub drain where the undersized plastic wasteline hose enters the PVC wasteline . We usually stuff a rag around the wasteline hose to close off the space .

  5. HDendy | | #5

    Does the furnace run for 'extended periods of time'? Or are the cycles much shorter? where are supply ducts located etc.? another possibility is depressurization of the basement drawing fumes in. a small supply leak outside the thermal envelope could lead to a significant depressurization over an 'extended' run time. ideally p-traps would block all gasses but it is possible for a plumbing layout to allow gases in under pressure, sometimes a back-draft type effect from vent stacks. the suggestions mentioned before might be easiest to trouble shoot first, but this is something else to add to your list of possibilities. you would need to get a blower door and/or duct blaster in to locate and diagnose the leaks. good luck!

  6. Josh | | #6

    Well first of all the smell is only in the utility area. The new bathroom and laundry room are separated by a wall. Those rooms typically do not have the smell.

    As for the ducts. The entire duct system is run in the basement. I have sealed most of the ducts at this point. With the exception of the filter housing which is an open slot to slide the filter in. This opening is about 4' from the AAV in question.

    Next time I notice the smell I will have to test the pressure difference in the utility area. But I don't see why running the A/C would cause more of a depressurization in the area than actually running the furnace which gets all of it's combustion air from that room. And like I said, we only get the sewer smell in the summer months when running the A/C.

  7. Danny Kelly | | #7

    The hole for the filter can cause a huge negative pressure - especially being so close to the fan. They make magnetic strips that can cover this gap or simply install duct tape over top of it - will have to remove it each time you change the filter, but does not take very long. Sounds like the negative pressure is sucking the sewer gas either through a dried out trap or a connection lacking a trap. Not sure how those studor vents work - a negative pressure in the room may be able to open one up, will have to ask a plumber about that.

    As to why it would only happen during AC and not heat is a mystery to me but could be that the blower runs at a slower speed for the heat or maybe whatever the issue is needs a long cycle to build up enough pressure or something. Hard to guess without being able to see each trap and connection.
    Good Luck.

  8. homedesign | | #8

    Is the air handler/closet "IN" the Utility room?
    I toured a modular house where the air handler closet and the "one and only return" was in the laundry room.
    When I closed the laundry room door.. I could feel the pressure on the laundry room door.

    (This was a Major modular Home manufacturer)

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