Thanks for all the replies. Just so others may benefit I'll address
some of the possibilities. It's not a vent problem with winter freeze
because the vent used to go into the attic only when I first bought
the house (ie. no freezing or ice build up there) and the problem
still existed. I put the vent out properly through the roof as soon as
possible of course. I don't use propane in the house so the smell
isn't that, and the dryer is properly vented (although the previous
owners had it vented to the basement - venting wizards these folks
were!!).
I'm going to go with the assumption that the toilet seal is leaking. I
didn't realize that gas could leak from the seal but there still be no
water leakage. The toilet does rock if you lean on it heavily enough
so it's probably a bad seal. I'm replacing the seal and seeing what
happens.

Thanks to all....

Regards,
Marc

"Michael Baugh" wrote in message ...
Unsupported claim from the original poster.

Leaks at the drain seal (wax ring) are not defined by
whether there is water leaking in around it. It is indicated
by whether you're getting sewer gas around it. The presence
of sewer gas from a leak around the ring is not yet excluded.

With the design of the toilet, and the ring, it would oftentimes
be necessary for the water to go uphill to manage to leak around
the ring. But sewer gas can make its way past very nicely,
especially when the house is tight, and the furnace is pulling it in
to its air return duct. Go to the FAQ's in Toiletology at
http://www.toiletology.com/frequent.shtml

Jim Elbrecht wrote in message
...
(Marc) wrote:

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.

Is there a window in the bathroom that gets tightened up in the
winter? My first thought would be to eliminate the possibility that
it is really a rear-round problem that is only noticed in the winter.

Next I would suspect the vent getting snow, debris or sitting water
that is causing a plug when it freezes in the winter.

Do you use propane gas in the house. I've seen folks confuse the
smell with a septic tank. A poorly vented dryer, hot water heater or
space heater could stink up the place.

Jim