My mom has a gas furnace in her house as her main source of heating. She called me up the other day and told me that whenever the furnace kicks on now, there’s a weird smell that seems to be coming up through the air ducts. 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 thermostat system and a detector for carbon monoxide so that shouldn’t really be the problem for her. Her house doesn’t have a basement and it’s actually sitting on a cement slab so I don’t think that anything could actually be under the house causing the smell. The air ducts for her heating and cooling system run under the floor on the lower level of her house, though. I’m beginning to wonder if there isn’t a skunk or some other kind of stinky animal somehow living in her air ducts. I know that your ventilation system is a closed circuit, but maybe there’s a way that an animal could get in there. I was asking around for possible reasons, and a friend of mine at work said that he thinks it’s probably something with the trap on the air conditioning condensate pipe in her HVAC system. He said that if the pipe goes to the drain then the drain can smell and get into the air ducts and cause a smell. I guess if the trap is broken, then a sewer gas smell could definitely be the culprit. I am going to tell my mom to call a plumber and if he can’t fix it, then she should call up her HVAC company.