Hello, and thanks for your question!
What vents is the smell located ?? plumbing?
The vents should be located outside the house. Is the smell coming from sinks, toilet's or drains ??
The smell appeared when we first turned on the furnace and is coming through the registers inside the house. Some days it appears stronger than others. It doesn't appear to be coming from the sinks, toilets or drains, so I'm thinking it has something to do with the furnace. I does get really bad on some days.
If it appeared only after turning on the heating system then I dont thing its sewage.
You need to clean the duct work, I think you may have had a small animal that could have dies in the furnace or ducts.
I would have a ac/heating company come check the duct work and furnace. Its not uncommon for small animals and such nest in the duct work. They may not even be there any longer and smell is the old nest.
The other option is if you have floor registers, and small kids, Check them good as kids like to stick things in the registers.
I hope this answers your question, if so please click the green accept button. If not im still here to assist you.