....have a floor drain in an old house. it has a plate on it and doesn't smell unless the plate is removed to drain something. it is a very old house and i don't think the line has a trap under the floor. is there a fitting/grate that could let water drain but prevent odors from coming out? i guess it would be a check valve or something similar. anyone? i believe it is a 4 inch diameter. any such animal that would solve the situation? outside of diggin' up the floor and putting in a trap?