I've searched through numerous posts over the years without an answer to the problem I'm having so I thought if there were some generous plumbers out there or people that know the answer to this it would be very helpful.
I have a simple home in the country. The plumbing is only on the main level and in the basement. There is 1 toilet, 1 bathroom sink, 1 kitchen sink, and a tub drain. A water softening system is installed.
My bathroom has a septic smell in the winter time which appears to come from the toilet. No smell comes from the tub, or either sink.
The toilet is functioning well other than the winter smell. It doesn't appear to have any leaks at the drain seal, because I see no water on the bathroom floor around it, or seeping through the floor and into the basement.
I thought the water in the toilet acted as a trap? Is this correct or should there be something else. There is no smell in the summer, but from what I've read most people experience this problem in the winter.
A septic vent pipe goes from the toilet drain pipe (about 3" up from the only 90 degree bend) which you cannot see in the picture. The vent goes through the roof and a smell comes from it so it is not clogged.
The rest of the simple plumbing setup is shown and labelled in the 2 pictures.
Any ideas about what is causing this? I can only think that the seal on the toilet is not perfect and is letting gas escape from under the toilet along the bathroom floor but then I should see a water leak, which I don't.
Any help greatly appreciated.
Pictures are here: