Chances are your furnace fresh air intake is drawing in the odor from the vent pipe. The reason you only smell it at the end of the heating cycle is because when the burner is running the air coming through the intake is being used for combustion. But once the burner shuts down, and the blower continues to run for 2-3 minutes, then that air is being sucked in and not burnt off, so the smell continues out through your registers. Solutions include making sure your drain traps are refreshed, filling and priming your furnace trap, and reducing the amount of time the blower is on after combustion ends.

I don't see how any toilet - save one on a boat or R/V - could possibly function without a vent stack. "but the vent goes to the garage" would not be approved by any building inspector anywhere in this country. Perhaps it was a "SPOT" - stupid previous owner trick. There is one possibility - other than installing a toilet and vent properly - which apparently was not done. The vent or soil pipe could have been pierced by a drill, nail or screw, especially if PVC was used. As an appraiser. I've seen that happen before. And yes, I *almost* made that rookie mistake myself, installing a new cabinet over the toilet. Since this is a 1938 house with plaster on lath walls, I was using a stud finder and drilling pilot holes. One was in an unexpected location, but it was *perfectly* placed. But it wouldn't drill...and that's when I realized it was the cast iron vent stack. No harm done....

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.

Did anyone inspect the vent pipes to make sure they were installed correctly? Try also posting this on the Plumbing forum. I found this on another web discussion site: "It may be that you have a leak in the sewer stack, and that the expansion with the hot weather is whats causing it. Cold weather contracts various things and may seal it up again. I too am the type that I can smell things in parts per billion (reason why a lot of times, the Superintendent would ask me how the plant was doing, because I was the first to be able to detect things at the Waste Water Plant). Another possibility is that something is in your attic and its coming down through cracks in the ceiling or through the walls, in your bathroom, (dead squirrels or some critter). Now, make sure, to check your TRAPS in the shower or bath. its possible, that you have a plugged up soil stack, (though the plumber should have found this, if he checked the roof), and that when the temperature gets warm, or something happens outside, it sucks down the water from the traps in your bathrooms, (like when you use your sink in the kitchen and it creates a vacuum and draws down the water from the traps if the pipes aren't big enough...)"