Well, there is such a thing as temperature inversions, which cause gases that would rise into the atmosphere to move downward. But, my guess is that you have a small leak from the bathroom drains (especially wax ring on toilet) and that it is entering the floor space, rising and ending up in the next room.

As heat is introduced, a volume of gas will expand. When it seeks the path of least resistance, that may be where the problem appears. A clogged vent is not the cause of this kind of problem. In fact, if the drains are intact, sewer gas will be in the sewer, isolated from the living space.

Pinoke

wouldn't I smell it in the bathroom then???
Also wouldn't I see a leak somewhere?
Thanks,
Renee

Also, it happens most often when I use the shower. I don't think it happens if I just use the toilet. Ohh, in addition to this we redid the tub faucet that is in the wall behind the tub and this wall cavity did not smell. The toilet plumbing is not by that wall but the wall is all connected (no insulation on interior walls) so if it were leaking from the toilet wouldn't we smell it in the wall there also? I just don't get it!

-renee

First Thing I would do as Pinoke mentioned is change the wax rings under the toilets. My parents had a similar problem and i changed out the wax ring with no success,still had the occasional smell. This went on for months. Then one day they had a leak from the drain pipe. It turned out to be a rotted out drain line from the bathroom sink to the main drain stack. The drain line was a copper pipe that drain cleaner chemical finally ate through. replaced it and the smell went away.

The above is a worse case situation. The smell went away for a few years until this summer and came back. First thing I did was pull the toilet. The wax ring had melted! The only room in the upstairs that has air conditioning is their bedroom. It had gotten so hot over the years that the ring melted. Replaced it and the smell went away.

Easiest way to check for a bad ring is stick your nose at the base of the toilet and see if you smell sewer gas. If you do thats the problem.

I did the toilet smell thing and never smelled anything.
I will check again today but I have never had it smell there so I am not sure if that is the problem.

a year ago we replaced the drain lines (it smelled before) and we still have the smell. :(

Do you think we can just call a stupid rotor rooter service??

Sewer gas is a serious problem and one that should not be trusted to "Stupid" people no matter what their business letterhead or personal job title might be.

Sewer gas is a product of the decomposition of organic waste material and primarily contains a number of different gasses, which include but is not limited to: Hydrogen Sulfide, Ammonia, Methane, Carbon Dioxide, Nitrous Oxide and may contain remnants of household solvents, chemicals, oil, gasoline or any other liquid that may have been discharged into the drain system.

Sewer gas can be very elusive because the gasses are present in differing amounts from one moment in time to the next and some of the gasses or vapors are lighter than air while others are heavier than air so they may be rising in the vent system at one time and falling at another time. In addition, if the sewer gasses should happen to leak out of the vent system into the living space they may stratify in layers in the room air, near the ceiling and upper level of the room at one time and near the floor where they are less noticable at other times. It must also be noted that the rate at which these gasses rise or fall will vary in proportion to changes in the normal atmosphere barometric pressure. To make matters more complex some of the gasses present in sewer gas have a very strong odor such as Hydrogen sulfide (rotten egg smell) or Ammonia, while some of the gasses such as nitrous oxide or carbon dioxide are odorless. Again, because these gasses are present in differing amounts they may smell strong at one time but go totally unnoticed at others. To make matters even more complex when hydrogen sulfide is present in higher levels it actually paralyzes the body's olfactory sensors in your nose so you do not smell it.

Having said all that, let me also say that although it is not impossible, it is extremely unlikely that you could ever get a sewer gas buidup in a residence that would even approach dangerous levels.

Now to locate the source. As Pinoke already pointed out, the most common source is a defective wax ring under the toilet.

It is possible that it could result from an obstruction in the vent stack but generally you will see other indicators of a plugged stack.

1.The at rest water level in the toilet bowl lower than usual.
2.Slight movement of the water or a gurgling sound in the toilet bowl when a sink or tub is drained.
3.Dry traps-take a flashlight and visually look down the sink drain to see if there is water in the trap.

The Uniform Plumbing Code requires all vents to go through the roof however if you live in a region that is under the International Residential Code you should be aware that the code permits terminating auxillary vents in the attic space.
When doing so the code requires the vent opening to be at least 6" above the level of an insulation present but often people are unaware of the vents and add additional insulation covering the vent openings. When a vent is terminated in the attic space it is quite possible that the heavier than air components of the sewer gas will then permeat through any cracks or crevices and come into the living space.

Check under the lavatory to to see if you have an air admittance valve (Studor Vent) on the drain line. If so, the diaphram may be stuck in the open position. The proper fix is to replace the AAV. AAV's are threaded on the bottom and screwed into a female thread adapter so they can easily be changed.

If you have chrome plated brass tailpieces or traps check the brass carefully for signs of corrosion. Even if the surface of the pipe only feels rough it may be corroded sufficiently to allow gasses to escape. The only solution here is to replace the defective components.

If you have checked all of these points and still have not located the source of the problem you should contact a plumber or drain cleaning service that can run a camera down the vent stack to inspect it. In a worst case scenario the can also make a small access hole in the sheetrock and run the camera inside the wall which would preclude making a major tear out until you are sure you have identified the problem.

sorry I didn't mean to offend anyone with my comment. I just meant a service to clean my vent pipe.

Our house does not have everything up to code because of it's age. Things were "grandfathered" in and therefore not everything is what is expected.

all piping is of the pvc type and water in-lines are copper. The sewer pipe is cast iron.

there are no gurgling sounds and I don't think water is getting sucked from the traps. Once in a blue moon I will hear gurgling downstairs when the washer is running but it doesn't happen all the time.

we prefer to fix things ourselves as repair men we have had do not seem to respect our house and we are always dissapointed with their work. I was hoping to isolate the problem so I could figure out where to look first. I was hoping it was a simple clogged vent fix however I guess we will have to open the wall and see what is happening.

we can't use a AAV becuase I believe the vent in question is the main stack vent.