Here we have a bathroom fan vented into the drain waste vent pipe…a wet vent pipe. When the bathroom fan is not running the occupants get to smell sewer and methane gases, 24/7.
Different house, we have the bathroom fan vented into the kitchen range hood. Picture if you will the range hood fan is off. Someone smelling up the bathroom running the fan has now sent that smell into the kitchen for everyone to enjoy.
Solution
When in doubt, if you don’t know what you’re doing hire a professional, in the end you’ll save money.
