We have a similar problem in our house. Since late October (as soon as the weather chilled and we were no longer using the central air conditioner), we have had a horrible sewer gas smell intermittently in our house. We've had an usused floor drain in the basement capped, but the smell came back. We've had two different plumbing companies come and look at all of the plumbing to see whether any problems reside in the pipes, etc. They've found nothing. We moved into the house in August of this year and noticed no smell from then 'til late October. Any ideas?