We moved into our house (no septic) approximately 6 months ago and right away had some issues. The house had been rehabed/flipped and about 2 weeks after we moved in water started coming up all the drains in the bathrooms (which are over the craw space). We called a plumber who checked the sewer cap (is that the right terminology?) leading to the outside of the house in the crawl space and somehow he determined there was a clog inside the house. He snaked the inside of the house from one of the toilets and all was fine. We also had to have our vent stack tied into the roof because it was venting to the attic. This was all done in the first 2 weeks of moving in. I mention this just in case it's related.
Well it's been approximately 6 months with no problems and now that it's getting colder, we are noticing a strong sewage type smell in the basement. The smell gets stronger as you come closer to the crawl (floor drain and sump pump are all there too right outside the crawl). You can smell it anytime you are in the basement. It also smells upstairs only in the bathrooms that are above the crawl and seems to be coming up from the heating vents in the floor. You can smell it in the mornings (don't know if it's worse from water use or from the heat kicking in). It doesn't smell while the heat is blowing, only after. Weird... Any ideas?
I forgot to say that I searched this topic and couldn't find anything the 'exactly' fit this issue. Thanks in advance!

It sounds like your home flipper cut a few corners and possibly wasn't strong in his plumbing skills. Vents terminating in the attic is very basic and totally wrong and I'm surprised you didn't have the home inspected. I'd get into the crawl with a good flashlight and check the drain line for leaks. I'd also inspect the vent for cracks or clogs. Rod the vent from the roof. Keep in mind that your new home was probably in horrible shape just a few months ago and many flippers are too superficial in their flip. Did they replace the toilets? Check the wax rings if the toilets were taken up for new flooring. A corner cutting flipper might have just reseated them without replacing the ring with new.

I'm sorry I probably didn't give enough clarification. We did have the vent stack vented through the roof. The fact that it was venting in the attic was caught during a home inspection. That shouldn't be the issue... I was just trying to give background on our plumbing issues just in case there are relating factors.

The wax rings were also replaced when they rodded the house to fix the issue I mentioned at the top of my first post.

Stephh, I had assumed that you corrected the bad venting, just trying to draw attention to the possible poor job by the remodeler. I would still reinspect the drain pipes in the crawl. I think what is happening is the furnace causes a slight negative pressure in the house while the flame is lighted, this can allow outside or sewer gas to be drawn in if there is any leak. The furnace blower stirs the air so the smell isn't immediately apparent. Once the furnace shuts down the sewer gas settles to the low spot in the house. The toilet issue isn't 100% out of the question just yet. The fact that the rings werer replaced doesn't mean they aren't leaking. I have replaced hundreds but once in a blue moon I did not get a good seal and had to start over.