Our furnace is still taking air and odor from the basement and pusishing it up stairs. We sealed all the openings around the furnace and there is no vent on the furnace. Our 5 intake vents are on the main floor what could be the cause of this. Thanks in advance

Are you sure the return is pulling smells in from the basement? Something could be in your ductwork causing the smell, there are companies that clean ductwork if you can`t do it