We have a three story house. On the third floor, imagine an open sitting area in the middle with two attic spaces on the right and left sides of the house, both of which have a nice exterior style door. In the attic spaces, the walls are covered with peg-board and the floors plywood. We have a faint smell on the second floor in a Jack and Jill bath and a strong odor in one of the attic spaces. Note the smell in the attic is not the attic that has the vent pipe through the roof. I have searched very diligently and there are no plumbing vents on that side attic. There is no smell at all from the other side attic, which contains the vent through the roof. There is no smell on the first floor. The wax ring was replaced in the Jack and Jill bath and it was not installed properly when the tile was installed back when the house was built. I though this would fix the problem. I was wrong. All sinks in the house have studor vents but their is no smell emitting from within the cabinetry so I have ruled them out. The bathroom is used daily by my two sons so the traps should not be dry. The only thing in the attic space where the smell is so strong is the heat unit. There is a drain line on it, but I am not sure exactly where it is piped to yet. The smell is very strong on one side of the room which is the opposite side of the heat unit. I have tried to be thorough in my explanation and hope someone knows what else I can check. I did smoke test the system and found no leaks. Could be in a wall, but I would have though I would find some smoke somewhere if that where the case.

it is possible that the drain for the heat unit goes into its own trap and that may be dry ??
dump some water down the heat unit drain to see if that does anything to help ?

You are a genius. I had not considered that. I will try that this evening.

So the drain for goes out of the house and does not tie into the plumbing at all. The smell is horrible today in the attic. I am to the point of pulling the plywood flooring up to see if there is some type of plumbing hidden there. I really don't think there is, but I am out of other ideas.

Long shot: The smell started "faint," and is rapidly getting worse. Could it be a dead animal in that side of the attic? If the smell slowly fades over the next few days, then eventually disappears, that would be consistent with a dead animal. If you see any flies soon, that's a dead give away. But no flies does not mean no dead animal.

Dead rodents can occur within wall cavities and under floors. Search the entire perimeter of the stinky side where the floor and ceiling meet the walls, looking for gaps a rodent could enter. 1/4" is enough for a mouse, 1/2" is enough for a rat.

I found the problem. There was a pipe in the wall behind the shower that was wide open. I put a studor vent on it today. Evidently the sheetrock guys came in before the plumber finished everything and covered it up. Fortunately there is a closet behind the shower and I was able to locate the pipe, put the vent on, and put a vent cover over it so it can get the proper amount of airflow. The reason the smell was so bad in the attic is because their was no sheetrock or plywood covering the hole from the topside. This was just venting into the attic. It was in a tight spot over toward the edge of the roof-line and it was stupid luck that I pulled the insulation away in the right area and found the area with the help of a light and a mirror ( it was really a tight spot). Thanks for all the suggestions.



Edited 1 times.