I think if the trap is broken, then a sewer gas smell could easily be the culprit
My mom has a gas furnace in her home as her main source of heating… She called myself and others up the other afternoon & told myself and others that whenever the furnace kicks on now, there’s a weird smell that seems to be coming up through the ductworks. She said that it doesn’t smell like a dead animal, but that it does smell a little bit skunky to her. She already has a smart control unit method & a detector for carbon monoxide so that shouldn’t genuinely be the concern for her; Her home doesn’t have a basement & it’s easily sitting on a stone slab so I don’t guess that anything could easily be under the home causing the smell, however the ductworks for her heating & cooling method run under the floor on the lower level of her house, though; I’m start to wonder if there isn’t a skunk or some other kind of stinky animal somehow residing in her ductworks. I think that your ventilation method is a closed circuit, but maybe there’s a way that an animal could get in there. I was asking around for possible reasons, & a associate of mine at work said that he thinks it’s entirely something with the trap on the air conditioner condensate pipe in her Heating, Ventilation & A/C system. He said that if the pipe goes to the drain then the drain can smell & get into the ductworks & cause a smell. I think if the trap is broken, then a sewer gas smell could easily be the culprit. I am going to tell my mom to call a plumber & if he can’t repair it, then she should call up her Heating, Ventilation & A/C corporation.