if the smell is coming out of a duct, then maybe you have somethign dying in teh attic close to teh duct and that is where the smell comes from. it does sound like something dying since it goes away in about a week.

"Every so often, we get a very bad smell always coming from the same place. It seems to be in an unused upstairs bathroom. It comes on suddenly every few months"

I seriously doubt that animals come into the house every few months to the exact same wall for the purpose of dying. Way too coincidental for me....

How is the bathroom vent and the toilet's wax seal?

Western PA Luann (I am in w pa too!)

If the wax seal is bad

1) how would I know?
2) why would it sometimes have a smell but usually not?
3) what would you do as a first step to find out if it is the problem?

The reason I guessed it might be animals is 1) it does not smell like sewer to me 2) it will go away whether I do anything or not 3) it seems to come from the wall that is against the garage and might be open to the garage above the ceiling.

The toilet is cleaned and flushed once a week. Inbetween it is not (ie never) used.

There is an unused shower that everyone blamed but I pour water down it and it makes no difference. The sink is used off and on.

The problem is most likely the little used bathroom. We had the problem in an office with a shower we never used. We just solved the problem by running water and a lot of it down the shower. It doesn't clear up immediately, and it does go and come. I am not sure what solutions we used but the fact that we started running it every other day made the difference.

I 'll try Marge. That's what everyone tells me. Maybe I expect it to clear up too fast. I do my ironing there and hang clothes on the rod so I don't find it convenient to run it every day but I will try again. Actually I will try anything at this point. Looking for the Chlorox.

Before you tear holes in your drywall (like we did), check any outlets in the area. After finding nothing in the opened up wall (thankfully a bedroom closet), a final 'nose' check pinpointed the problem at one wall outlet in the bedroom. Upon removing the outlet cover, we found a mouse had somehow squeezed in there and died wrapped around the innards of the outlet. Once removed, we cleaned and deodorized.

P.S. For whatever reason, the smell increased when my son used the shower in the adjacent bathroom. I researched and read about drain issues, sewer gasses, etc.

sukis makes a good point. Nose-check everything, even the base of the toilet as you flush it.

DS purchased a home 2 years ago and the smell of dead mouse would come from one of the FR's walls about ever 4-6 months. He finally isolated it to an area where an outlet and the vacuum connection were located. We climbed under the house and saw signs from both mouse and chipmunk visits directly under the outlet area.

He removed both outlet covers and loosened the elec. box enough so that he could spray expanding foam in the wall. He hasn't had any odor problems in the wall since.

The wax seal is under the toilet... you need to lift it to check the seal.

I've had plenty of dead mice in walls, under floors, etc.

Usually, the smell is bad for a while, but it gradually goes away completely. A single small animal like a mouse or a bird decomposes very quickly. If it's a larger animal, or multiple small ones, decomposition would take longer and the odor would persist longer, obviously.