I have a forced hot air heating and cooling system in my house and continue to get dark shadows in my living-room and bathroom. The dark shadows grow darker over time and can be cleaned with any common cleaning solution. I am trying to find out why the dark shadows continue to return as I am tired of reprinting the interior of the house every 2 years. I have changed the entire heating and cooling system and included a hepa filter to captured any dust or dirt in an attempt to put an end to the dark shadows to no avail. Anyone have a similar problem with their forced air system that would like to share with me how they corrected this condition? Please help, I am tired of painting.