We have what is called a split foyer home with a full basement. We are on a city sewer system. We get the sewer odor when we run the washer & dayer in the basement, or when we use the whole house fan. The odor seems to come from inside the wall between the washer/dryer and the adjoining downstairs bath. It is also in the bathroom sink upstairs. We use water in all drains to stop the odor but it doesnt always work. Is there a check valve in the main drain which leaves the house? Could that be the problem? Thanks, please respond. Don