They may not be seeing the forrest for the trees...LOL

You stated, "(just attic, where it was released),"

This would lead me to think that you are under the International Residential Code which permits terminating auxillary vents in the attic space. If that is the case, that is also your problem. Two of the four principal gasses that make up sewer gas are heavier than air. When the sewer gas is released into the attic space it can then settle down through a pipe chase or ceiling materials and settle in the living space.

If you have any vent openings in the attic space I would insist that they be put through the roof were they can vent to atmosphere outside the house.

Thanks, but it's not correct to assume that our vents terminate in the attic. They do not. The plumbers had to cut into the pipes because our house is tall and the roof pitch requires a safety harness if you were to drop from the top of the pipe, e.g. from above the roofline. The pipes were re-sealed after the test and were capped off during the test, so the scent could potentially permeate if there were an intrusion into the pipe or a crack.

You say the plumber was on the roof, cut pipes and resealed them. Have you gone into the attic to make sure he sealed the vent stack properly? If you did...

Are you sure what you are smelling is sewage or just a, not really sewage and not really musty smell? Just an off smell?

Our house is only 4 years old but on really hot days, when I would wake up in the morning and come downstairs there was a smell that wasn't quite either. The A/C drains into the ejector pit so I thought that was it. I opened the ejector lid and it didn't smell. I then opened the Sump lid and there was that smell!

I pulled the sump pump and figured out how to adjust the float. After trial and error I moved it so the pump comes on a little sooner, about 1" of water or less than it was. Now when I smell it, it is only because it hasn't rained, or I haven't watered in a long while. And the yard is SCREAMING for a drink.

I smelled it yesterday morning and spent the day moving the sprinklers around, one hour in each spot to the give the yard a drink. When I went downstairs this morning the smell was gone, making coffee I heard the sump turn on and getting water for coffee I saw that thanks to the water, the grass is still brown but the weeds perked up and it is too hot out for weed killer. LOL

Thanks for your reply. :)

We did verify that the vent stack was resealed properly in the attic (the plumber did not climb onto the roof, but rather cut the pipe inside the attic; the pipe continues up out through the roof.)

The smell is absolutely, positively, 100% not "musty" or "moldy" -- it's sewer. On a good day, it smells like an outhouse. On a bad day, well...it's worse. We can't have people over now because it's so powerful (and really embarrassing.) The smell did not get better or worse after the pipes were cut, if that indicates anything.

Our sump pump rarely runs; we've maybe heard it run a few times in the 5 years we've been here.

Check any shower and tub drains for hair build up.
Check bathroom sink overflows for any buildup.

Either of these will produce a lot of smell since debris can collect before the trap.

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.

Have you tried Smoke Testing? YOur problem might just be mis-aligned joints or something. Maybe the Plumber who installed your pipes/drains did not do a good enough job and there is a leakage somewhere. By smoke testing the smoke is put down the pipes and the plumbers looks for the smoke which hence indicates a leakage. Otherwise maybe using CCTV equipment to have look down your sewer pipes for any holes or tree roots etc.

the OP already had a similar test done.
And FYI Videoing sewer lines is not the diagnosis or solution for every problem. It has it's uses and benefits, but it is costly.

Here again alexbar is talking through his hat.

"Maybe the Plumber who installed your pipes/drains did not do a good enough job and there is a leakage somewhere."

The original post explicitly says the house is 5 years old. From that fact we can further assume that the DWV system is either PVC or ABS and all joints were glued at the time of installion. It would also stand that the installation was inspected. Pray tell, where are these supposed "Mis-aligned joints"?.

I think lazypup that all your 'Gas" talk has gone to your head!!! I am sure as a very experience plumber u make out to be you have come across jobs of other plumbers that u have had to re-do due to them not being done right the first time. You assume too much