Huge house fly problem can someone help:?

This time of year flies are trying to find someplace warm to spend the winter. They crawl into spaces under siding, around windows and anywhere else they can get into. There will not be a nest of maggots as suggested or you would have something dead and decaying in your house and would sure be able to smell it. Close your chimney tightly and check for air leaks but it will be hard to find exactly where they are getting in.