You must understand that you are asking us to resolve a problem sight unseen and with very limited information to work from even though you have already had a plumber on site and he/she was unable to find the problem. In my previous post I was not making any assumptions, but rather I was pointing out a potential source of your problem based upon current code approved construction methods. Having now ruled out attic vent termination we must then turn to other potential, although seldom noticed sources. The four principal gasses that make up sewer gas are Hydrogen-sulfide, Sulphur-dioxide, methane and carbon dioxide. Of these four gasses carbon dioxide is odorless and while hydrogen-sulfide has a very distinct odor, it also causes an anesthitation of the olefactory nerve in rather low doses. This means that while hydrogen-sulfide is readily noticeable at very low concentrations, as the level of concentration increases it will shut down your ability to smell it rather quickly. It must also be noted that hyrdogen-sulfide and methane are heavier than air gasses so they tend to stratify at low levels in any given space. This means that it could be in a rather high concentration at the floor level in your basement, yet you will not smell it when standing erect in the room. The problem here is that most HVAC units have an updraft air flow, which means the air intake is rather close to the floor where the sewer gasses would concentrate. Once the gasses are pulled into the HVAC unit they will then be distributed throughout the structure by the normal HVAC air flow. You also stated that this structure is only 5 years old so it would be safe to assume that it was constructed with a very tight envelope and knowing state of the art construction it is highly unlikely that there is a source of outside air into the basement. Under these conditions when an HVAC unit is running they will often cause a slight drop in normal atmospheric pressure in the basement, especially if the return air ducting is sized critically or if any opening to the return air duct is even partially obstructed by furnishings or personal belongings. (one prime indictation of a reduction in air pressure in the basement when the unit is running is a basement door that is left open but slams shut when the HVAC unit runs). We must also keep in mind that normal atmospheric air pressure is 14.96psi at sea level and nature abhors a vacuum. Any reduction in atmospheric pressure, no matter how slight, will immediately cause air to infiltrate in from any possible source. Further understanding that all sewer and drain lines are vented to atmosphere it would then stand that if the air pressure in the basement drops even slightly air might be infiltrating into the basement through floor drains, laundry standpipes or utility sink drains. The rate of infiltration through these drains may not be enough to even effect the water levels in the traps, other than to cause them to dry out quicker than usual. One way to test this problem is to keep the basement door open a couple days and see if the problem still persists. If not then the permanent solution would be to install a small makeup air intake into the basement.

Could I have a clogged roof vent? Possibly Backed up septic (pumped 3 years ago, live alone)? Probably not since it drains fine. Bad valves? Possibly If you have roof vents why do you need the AAVs? I get the feeling the AAVs are sticking open. Either bad design or need cleaned and lubed or something. This is one reason to avoid AAVs they require maintenance and are another thing to break or not work.

Thank you David! When having this house renovated post purchase, much of the bathroom was gutted and the sink I had installed is one I bought new some years before and was waiting to use (American Standard Standard Collection Console sink with oval glass shelf.) Because the works underneath are totally exposed I bought the best chrome P-trap and fittings a local plumbing supply house had. In a month or so I'll have my plumber back to install a pump for my utility sink, and he will need to add another vent stack I believe, so I'll have him see if the sink is connected to the bath vent, and if the vent has any blockage. I'll ask him about the air admittance valve - thank you so much for that information!

We recently had a similar issue after installing a new septic system. Plummer came a few weeks ago now and suggested that before drastic measures like modified venting, we start by treating all the sinks, toilets, showers and tubs upstairs (area with smell) overnight with BioOne, a blue liquid. So far, so good. We don’t use them all regularly and apparently that can be how the issue starts.